history of solar system essay

The Origins of the Solar System Essay

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Introduction

The nebular hypothesis, origin of the molecular cloud, runaway star hypothesis, formation of the sun and planets, creation of the earth, formation of the oceans: comet/proto-planet impact theory, reference list.

The origin of the Sun and its orbiting planets has been a point of hypothesis and conjecture ever since man looked upon the stars and planets and wondered about their origins. For the ancient Greek and Roman civilization the celestial bodies they observed in the sky were thought of as Gods and Goddesses, looking down up the Earth from some form of godlike platform. Today, it is an established fact that the heavenly bodies we see in the night sky are composed of planets and stars, celestial bodies of rock, gas and varying forms of elements that were formed billions of years ago. Even though such objects have been observed for hundreds of years it is only within the last 200 that humanity has begun to understand their unique qualities. While there have been conjectures, varying hypothesis and age old established theories what must be understood is that as the science of astronomy evolves humanity begins to slowly adapt to new information, new discoveries and subsequent re-evaluations of what we knew of as fact. For example, early studies of astronomy adopted the geocentric model in that they believed that the sun, planets, moon and stars revolved around the Earth, not only that there was also the belief that the Earth was in fact flat (Copernicus, 2009: 83). It is based on this that when examining the established theories on the origins of the solar system one must do so with both an open yet skeptical mind, taking into account the given data and observations yet not clearly adhering to any one theory as being definitive proof.

Another interesting topic that should be taken note of is the origin of the Earth itself for just as there have been numerous theories as to the origin of the solar system there have been a plethora of theories which have attempted to determine the origin of the Earth itself. Our home planet is unique in that it is the only planet within our solar system that has sufficiently developed to be able to support life. While there have been varying accounts of how life came to be on Earth, with religion and science vying for attention, the fact remains that the uniqueness of our planet should not be underestimated and as such bodes a certain degree of curiosity as to the origins of the unique circumstances that enabled Earth to become what it is today. It is based on the various questions presented that this paper will explore the origins of the solar system and of Earth itself in order to attain a clear picture of where it came from and what its possible end could be.

Currently, one of the most widely accepted theories regarding the formation of the solar system is that of the nebular hypothesis which states that the solar system originated from a molecular cloud wherein through the introduction of an external force caused a gravitational collapse of the fragment resulting in the creation of A pre-solar nebula that would eventually become our solar system (Glassmeier, 2006: 1 – 5). While there has been no definitive evidence as to the exact origin of the external force that caused a section of the molecular cloud to collapse rather than dispersing it into space it is theorized that the energy from a nearby supernova produced sufficient enough force to cause the collapse and help trigger the necessary events needed to create the solar system. While few studies dispute the nebular hypothesis several do call into question the theory that a supernova caused the initial collapse. Studies such as those by Woolfson (2010) state that the energies from a supernova instead of causing a section of the molecular cloud to collapse would have actually dispersed a majority of the cloud into space thus preventing the formation of the solar system (Woolfson, 2000: 1 – 15). Furthermore, while the nebular hypothesis has been well established as a guiding concept in understanding the creation of celestial bodies little is known as to the precise origins of the molecular cloud that gave birth to the solar system itself. Several scientists such as Lognonne et al. (2007) state that origin of the Sun and its surrounding planets was a molecular cloud and go to great lengths explaining how it led to the creation of the solar system yet a lot of studies neglect to mention how the molecular cloud came to be in the first place (Lognonne et al., 2007: 1 -3)

While this paper has so far expounded on the nebular theory involving the Solar system’s origins as coming from a giant molecular cloud a rather interesting question comes to mind, “if the origin of the solar system is that of a giant molecular cloud where did the molecular cloud come from?”. Studies such as those by Sorrell (2008) explain that while our own sun is 4.5 billion years old the age of the universe itself has been estimated at roughly 13.75 billion years (estimate subject to change due to varying accounts as to the proper calculation) (Sorrell, 2008: 45 – 49). Furthermore it must be noted that our sun is not the oldest sun in the universe let alone in our galaxy and in fact can be considered in the prime of its “youth” as a main sequence star (Naylor, 2009: 432). It has been theorized by researchers such as Freire (2008) that a few billion years after the Big Bang, Super Massive stars, many times the temperature of our current sun and several times its size, were among the first stars to form within the universe (Freire, 2008: 459-460). These celestial bodies were able to grow to such great size due to less “competition” for available materials in order to coalesce into stars; it must be noted though that at this point in time planets were unable to form due to the lack of heavier elements in which a sufficient enough solid mass could coalesce into a planet (Dessart, 2010: 2113-2125).

Rather interestingly, it was actually due to the inherent instability of Super Massive stars that the universe became what it is today; this is due to the theory that as a direct result of their internal instability most of the original Super Massive stars became supernovas which actually caused the original molecular clouds in the universe to form (Dessart et al., 2010: 2120 – 2125). The original state of the universe was actually more “pure” in the sense that there was a distinct lack of heavier elements, as such the question of “where did the heavier elements come from?” comes to mind. This is actually resolved by looking at the activity of our own sun wherein through a process called stellar nucleosynthesis in which the nuclear reactions within the sun itself is able to help build the nuclei of elements that are heavier than hydrogen (Chiosi, 2010).

In relation to the explanation of the origins of the molecular cloud as coming from the debris from Super Massive stars Courtland (2010) presents a new theory that details exactly how the molecular cloud that spawned the solar system came to be. In her study which involved the examination of various meteorites she discovered that sealed within the rock were calcium-aluminum rich incisions (Al-26) that could only have been formed by stars that were at least 10 times as massive as the sun (Courtland, 2010: 8). Due to the fact that Super Massive stars usually form within clusters with Al 26 usually decaying rapidly due to the intense heat within such clusters it is hypothesized by Courtland (2010) that a run away must have been tossed out of its orbit as a direct result of either an explosion of a nearby Super Massive star or due to combined gravitational push by its sibling stars within the cluster (Courtland, 2010: 8). Due to Super Massive stars having a relatively short life cycle when the star became a supernova the dispersed molecules and elements became the molecular cloud that we know of today as being the primary basis of the nebular hypothesis.

Creation of the Sun

Since this paper has now established the various theories which attempt to explain the origins of the molecular cloud that brought about the creation of the solar it is now necessary to explain the current prevailing theory on how the planets and the creation of the sun came about. As mentioned earlier, in the section detailing the nebular theory, it was explained that as a direct result of a gravitational collapse of a section of the molecular cloud this precipitated the creation of the solar system (Boeyens, 2009: 493-499). A better explanation of this would be that as section of the nebula collapsed this produced a certain degree of angular momentum wherein the nebula actually began to spin faster as it collapsed in on itself. This spinning combined within the collapse produced a great deal of kinetic energy within the core of the molecular cloud until the result was a contraction of the center of the molecular cloud, which had now become a disc shaped object, into what is known as a proto-star, namely a star that has yet to have hydrogen fusion occur at its core (Boeyens, 2009: 493-499). Within 50 million years the internal temperature and pressure of the core itself was able to build to sufficient levels resulting in the start of hydrogen fusion marking the entry of the sun into its life as a main sequence star (Boeyens, 2009: 493-499)

Theory of Accretion

The theory of accretion is currently the most widely accepted theory proposing the creation of the planets, in it the theory indicates that the leftover material from the sun’s creation continued to spin around the sun slowly clumping together piece by piece until larger dust shaped particles were created (Ogihara et al., 2007: 522-530). Gradually these dust particles also began clumping together resulting in the creation of larger and larger objects until finally the entire solar system was composed of literally dozens of moon sized objects that crashed into each over a period of several million years (Ogihara et al., 2007: 522-530). It must be noted that the reason why such a process didn’t just create a system of bits and pieces of rock is due to the fact that these moon sized objects actually had viscous outer cores in the sense that their composition was similar to lava due to the high temperatures of the sun at the time and the process of accretion itself. As such when the objects collided what resulted was not a titanic clash that mutually shattered the objects but rather a process where both objects combined to form a larger structure or surfaces were “swapped” in the sense that certain parts of either proto-planet’s surface accreted to the colliding object (Ogihara et al., 2007: 522-530).

Originally the Earth was a proto-planet no bigger than the moon yet over several million years the process of accretion was able to slowly build up the Earth to its present shape. It must be noted though that the early outer core of the planet was fluid in that due to the intense heat present at the time metals that had accumulated on the planet’s surface slowly submerged into the inner core creating the metallic core that is present today (Robin, 2008: 4061 -4075). Within 150 million years of the planet reaching its current mass the surface sufficiently cooled resulting in the creation of a primitive crust, yet unlike today the surface of the Earth is estimated by studies as being roughly 1600 degrees Celsius with numerous volcanoes dotting the landscape releasing gases into the atmosphere which formed the initial atmosphere of the planet which was kept in place by Earth’s inherent gravity (Robin, 2008: 4061 -4075).

Most scientists agree that the presence of water on the Earth was the pivotal necessity necessary in order for life to start on the planet. When examining the process of Earth’s creation though there seems to be few indicators of water actually forming directly from the process of creation or within the Earth itself (Robin, 2008: 4061 -4075). One theory that attempts to explain this is the comet/proto-planet impact theory which states that proto-planets, planetoids and comets that were composed of ice were actually prevalent in the inner system during the later stages of the process of accretion. (Robin, 2008: 4061 -4075) As such as the Earth continued to orbit around the sun it supposedly impact millions of comets along with several icy proto-planets to create the water that can be seen in the oceans today. In fact, 4.4 billion years after the creation of the sun the Earth had actually sufficiently cooled enough to actually create clouds, rain, and the even oceans on the planets surface (Robin, 2008: 4061 -4075). This particular period marks the creation of the atmosphere that is present in the world today which is a combination of oxygen, carbon dioxide and other gases.

By the end of this paper it has become apparent that the process of creation of our solar system and even of our planet has been an accumulation of fortunate incidents that culminated in humanity evolving into its present state. When examining the theories explaining the creation of the molecular cloud, how Courtland (2010) presented the notion that the molecular cloud our present system came from originated from a rogue Super Massive star that coincidentally was shot out of its group by gravitational forces, that it was able to travel far enough to an area ideal enough for uninterrupted growth, that the creation of our planet was in the right place, at the right time with readily available water literally crashing into the planet in order to support life; a combination of all of these completely coincidental factors almost leads one to believe that the creation of humanity itself was no accident but on purpose. On the other hand there are quite literally billions upon billions of solar systems within the universe and it might actually be the case that the process that created the Earth is not so coincidental and that somewhere out there life similarly exists on thousands of planetary systems with the exact same composition as that of humanity yet far away enough that we cannot see the similarities at the present.

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‘Copernicus’ 2009, American Heritage Student Science Dictionary , p. 83, Science Reference Center.

Courtland, R 2010, ‘Runaway star may have spawned the solar system’, New Scientist , 205, 2754, p. 8, Academic Search Premier.

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Dessart, L, Livne, E, & Waldman, R 2010, ‘Shock-heating of stellar envelopes: a possible common mechanism at the origin of explosions and eruptions in massive stars’, Monthly Notices of the Royal Astronomical Society , 405, 4, pp. 2113-2131, Academic Search Premier.

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Lognonne, P, Des Marais, D, Raulin, F, & Fishbaugh, K 2007, ‘Epilogue: The Origins of Life in the Solar System and Future Exploration’, Space Science Reviews , 129, 1-3, pp. 301-304, Academic Search Premier.

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Naylor, T 2009, ‘Are pre-main-sequence stars older than we thought?’, Monthly Notices of the Royal Astronomical Society , 399, 1, pp. 432-442, Academic Search Premier.

N.I.. (2010). The Creation of the Earth. Web.

Ogihara, M, Ida, S, & Morbidelli, A 2007, ‘Accretion of terrestrial planets from oligarchs in a turbulent disk’, ICARUS , 188, 2, pp. 522-534, Academic Search Premier.

Photo Journal. (2007). Pia09967: water’s early journey in a solar system (artist concept) . Web.

Robin M., C 2008, ‘Accretion of the Earth’, Philosophical Transactions of the Royal Society A: Mathematical, Physical & Engineering Sciences , 366, 1883, pp. 4061-4075, Academic Search Premier.

Sorrell, WH 2008, ‘The cosmic age crisis and the Hubble constant in a non-expanding universe’, Astrophysics & Space Science , 317, 1/2, pp. 45-58, Academic Search Premier.

Woolfson, M 2000, ‘The origin and evolution of the solar system’, Astronomy & Geophysics , 41, 1, pp. 1.12-1.19, Academic Search Premier.

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Solar System Essay for Students and Children

500+ words essay on solar system.

Our solar system consists of eight planets that revolve around the Sun, which is central to our solar system . These planets have broadly been classified into two categories that are inner planets and outer planets. Mercury, Venus, Earth, and Mars are called inner planets. The inner planets are closer to the Sun and they are smaller in size as compared to the outer planets. These are also referred to as the Terrestrial planets. And the other four Jupiter, Saturn, Uranus, and Neptune are termed as the outer planets. These four are massive in size and are often referred to as Giant planets.

The smallest planet in our solar system is Mercury, which is also closest to the Sun. The geological features of Mercury consist of lobed ridges and impact craters. Being closest to the Sun the Mercury’s temperature sores extremely high during the day time. Mercury can go as high as 450 degree Celsius but surprisingly the nights here are freezing cold. Mercury has a diameter of 4,878 km and Mercury does not have any natural satellite like Earth.

Venus is also said to be the hottest planet of our solar system. It has a toxic atmosphere that always traps heat. Venus is also the brightest planet and it is visible to the naked eye. Venus has a thick silicate layer around an iron core which is also similar to that of Earth. Astronomers have seen traces of internal geological activity on Venus planet. Venus has a diameter of 12,104 km and it is just like Mars. Venus also does not have any natural satellite like Earth.

Earth is the largest inner planet. It is covered two-third with water. Earth is the only planet in our solar system where life is possible. Earth’s atmosphere which is rich in nitrogen and oxygen makes it fit for the survival of various species of flora and fauna. However human activities are negatively impacting its atmosphere. Earth has a diameter of 12,760 km and Earth has one natural satellite that is the moon.

Get the huge list of more than 500 Essay Topics and Ideas

Mars is the fourth planet from the Sun and it is often referred to as the Red Planet. This planet has a reddish appeal because of the iron oxide present on this planet. Mars planet is a cold planet and it has geological features similar to that of Earth. This is the only reason why it has captured the interest of astronomers like no other planet. This planet has traces of frozen ice caps and it has been found on the planet. Mars has a diameter of 6,787 km and it has two natural satellites.

It is the largest planet in our solar system. Jupiter has a strong magnetic field . Jupiter largely consists of helium and hydrogen. It has a Great Red Spot and cloud bands. The giant storm is believed to have raged here for hundreds of years. Jupiter has a diameter of 139,822 km and it has as many as 79 natural satellites which are much more than of Earth and Mars.

Saturn is the sixth planet from the Sun. It is also known for its ring system and these rings are made of tiny particles of ice and rock. Saturn’s atmosphere is quite like that of Jupiter because it is also largely composed of hydrogen and helium. Saturn has a diameter of 120,500 km and It has 62 natural satellites that are mainly composed of ice. As compare with Jupiter it has less satellite.

Uranus is the seventh planet from the Sun. It is the lightest of all the giant and outer planets. Presence of Methane in the atmosphere this Uranus planet has a blue tint. Uranus core is colder than the other giant planets and the planet orbits on its side. Uranus has a diameter of 51,120 km and it has 27 natural satellites.

Neptune is the last planet in our solar system. It is also the coldest of all the planets. Neptune is around the same size as the Uranus. And it is much more massive and dense. Neptune’s atmosphere is composed of helium, hydrogen, methane, and ammonia and it experiences extremely strong winds. It is the only planet in our solar system which is found by mathematical prediction. Neptune has a diameter of 49,530 km and it has 14 natural satellites which are more than of Earth and Mars.

Scientists and astronomers have been studying our solar system for centuries and then after they will findings are quite interesting. Various planets that form a part of our solar system have their own unique geological features and all are different from each other in several ways.

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Jatan Mehta • Jan 14, 2021

Solar System History 101

Our solar system is a wondrous place. Countless worlds lie spread across billions of kilometers of space, each dragged around the galaxy by our Sun like an elaborate clockwork.

The smaller, inner planets are rocky, and at least one has life on it. The giant outer planets are shrouded in gas and ice; miniature solar systems in their own right that boast intricate rings and moons. Scattered throughout the solar system are small worlds like lumpy asteroids and comets and complex dwarf planets like Pluto and Ceres.

How did our solar system come to be? Why are these objects where they are now? Here is the series of events that made and shaped our solar system, to the best of our knowledge, pieced together from space missions, Earth-based observations, and complex simulations by scientists trying to figure out our place in space.

Solar System Timeline

A condensed timeline of the events that shaped our solar system.

The Sun Shines

The Big Bang brought the Universe into existence 13.8 billion years ago. Our solar system formed much later, about 4.6 billion years ago. It began as a gigantic cloud of dust and gas created by leftover supernova debris—the death of other stars created our own. The cloud, which orbited the center of our galaxy, was mostly hydrogen with some helium and traces of heavier elements forged by prior stars.

Over the next 100,000 years, the cloud collapsed under its own gravity to form hot, dense protostars, one of which was our Sun . Our baby Sun kept accumulating material for 50 million years, at which point temperatures and pressures in the core became so intense that hydrogen began fusing into helium.

And then there was light. Hydrogen fusion released tremendous amounts of energy that countered the Sun’s gravity, stabilizing the young star and keeping it from accumulating more material out of the rotating disk of leftover debris around it. The Sun entered the longest phase of its life, becoming a main sequence star. It is still in this phase today and will remain so for about 5 billion years.

Within 500 million years, the Sun separated from its stellar siblings and continued orbiting our galaxy’s center as a lone star.

The Planets Form

While the infant Sun was still collecting material to start fusing hydrogen, tiny dust particles in the disk around it randomly collided and stuck to each other , growing in just a few years to objects hundreds of meters across. This process continued for several thousands of years, forming kilometer-sized objects big enough to gravitationally attract each other. This led to more collisions and accretions, forming Moon-sized protoplanets in less than a million years.

In the inner, hotter part of the solar disk, the planets grew primarily from rocks and metals because it was too warm for water and other volatiles—substances that evaporate at room temperature—to condense. Up to hundreds of these worlds collided and combined in the inner solar system for about 100 million years until only four large bodies remained: Mercury , Venus , Earth , and Mars . The inner planets didn’t get as big as the outer planets because the percentage of rocks and metals available in the Universe—and thus our solar system’s starting materials—is lower compared to hydrogen, helium and volatiles like water ice.

Just after this point we think a Mars-sized planet collided with Earth . The resulting debris coalesced to form the Moon . Mercury may have experienced a high-speed collision with another planet that stripped off Mercury’s outer layer, which would explain why the planet’s core makes up so much of its volume. The resulting debris may have spread out into space instead of forming a moon.

In the outer, cooler part of the disk, gases and water ice were dominant. The Sun’s weaker gravitational influence in this region, combined with the presence of significantly more material, meant protoplanets there grew faster and became large enough to attract light elements like hydrogen and helium. Jupiter formed less than 3 million years after the birth of the solar system, making it the eldest planet.

Saturn formed shortly after, amassing less material since Jupiter gobbled such a large portion of the outer disk. With little hydrogen and helium left, the next planets to form–– Uranus and Neptune ––accumulated more ices like water and ammonia. This is why we call them ice giants. Some simulations show that additional ice giants may have formed that were later kicked out of our solar system.

Jupiter didn’t allow planets to form in the asteroid belt as its gravity pulled on dozens of Moon-and Mars-sized baby planets there, causing them to either collide and shatter with other bodies or leave the region. This process took a few ten million years after Jupiter’s formation, leaving the asteroid belt with only small bodies of rock, ice and metal that collectively weigh less than 1% of Earth’s mass. Ceres, the largest object in the asteroid belt, is considered an outlier because it has plenty of organics and water ice , which means it likely formed farther away and then migrated into the belt.

Small Worlds Stick Together

While the inner terrestrial planets were forming, baby planets beyond Neptune were colliding and sticking together to form planet-like worlds like Pluto and lumpy, icy bodies like Arrokoth . These objects formed what we now know as the Kuiper belt, though the belt was much denser than it is today. Just as Earth’s Moon formed after a collision between Earth and another world, similar smashups in the Kuiper belt created moons, some of which are relatively large. This may have been the case with Pluto and Charon.

Jupiter’s huge mass attracted a dense disk of material that eventually coalesced into 4 planet-like moons : Io, Europa, Ganymede, and Callisto. Saturn’s moon Titan formed the same way. Some outer planet moons like Triton at Neptune may have been independent worlds captured by the giant planets’ gravity fields.

That, as far as we know, was the end of the beginning. Planets and other small worlds didn’t grow any further as the young Sun’s strong solar wind blew most of the leftover dust and gas into interstellar space.

Giant Planets Wreck Havoc

The giant planets formed closer to the Sun than where they are now. There wasn’t enough material in the solar disk for Uranus and Neptune to form where they currently orbit, 19 and 30 times farther from the Sun than Earth, respectively. The Kuiper belt also likely formed closer in, roughly spanning the current orbital distances of Uranus and Neptune.

Simulations suggest that the orbits of the giant planets shifted about 4.1 billion years ago. Gravity from the numerous Kuiper belt objects nudged Jupiter and Saturn into a 2:1 resonance, meaning Jupiter orbited the Sun twice for every Saturn orbit. This periodically brought the two planets close together, causing wide-ranging gravitational effects.

Uranus and Neptune got pushed further away from the Sun, ploughing through the Kuiper belt, scattering most of its objects either inward or outward over the next millions of years. Any additional ice giants that had formed were kicked out of the solar system entirely. The outwardly scattered worlds formed today’s sparsely populated Kuiper belt and the farther-away sphere of icy bodies we call the Oort cloud. This is where most comets come from.

The inwardly scattered worlds raced through the inner solar system, smashing into the worlds there and creating basins as large as a thousand kilometers or more on Mercury, Venus, Earth, the Moon, and Mars. Scientists call this event the Late Heavy Bombardment .

Destruction and Life

Blistering impacts during the Late Heavy Bombardment heated the inner planets and our Moon, which had barely cooled after their formations. Widespread volcanism resulted and continued for about 500 million years. The Late Heavy Bombardment is thought to have brought water and possibly organic materials—essential ingredients for life as we know it—to the inner planets, which had otherwise lost most of their water after being internally heated during their formation.

Mercury and the Moon couldn’t hold on to much of this imported water due to their weaker gravities, except the fractional amount that froze inside permanently shadowed regions . Some volcanic activity continued on the Moon and Mercury until one billion years ago , when their interiors cooled enough to stop it.

Venus’ surface may have held onto liquid water for two billion years, until something turned this potentially Earth-like world into the hellscape it is today. It is still geologically active.

Mars was habitable for at least some periods of time around 3 to 4 billion years ago, with lakes and river-like channels of water on its surface. But without a protective magnetic field, solar radiation stripped off most of Mars’ atmosphere and water 3 billion years ago and the planet turned into a cold, dry desert. The Martian moons Phobos and Deimos are either asteroids captured by Mars around the time of the Late Heavy Bombardment or they coalesced from debris ejected by an asteroid that collided with the red planet. Observations of Mars show that the planet was volcanically active just a few million years ago .

Fortunately for us, the water on Earth stuck around. The oldest unambiguous evidence of life on Earth is from 3.5 billion years ago, after the Late Heavy Bombardment. Photosynthetic organisms evolved 2.5 billion years ago and started pumping oxygen into our atmosphere, helping create the blend of gases we breathe today. Our planet is still geologically active.

A Calmer Place?

After the Late Heavy Bombardment, the solar system became a calmer place. Asteroid impacts still happen but their frequency and the sizes of impacts have reduced drastically. There is still cause for concern as some relatively large impacts have happened in the last 100 million years:

An asteroid or comet impacted the Moon and formed the 86-kilometer-wide Tycho crater 108 million years ago, which you can see from Earth . The dinosaurs would have been alive and thriving to witness this event. During the same geological era, Saturn’s iconic rings formed .

A five-to-fifteen-kilometer-wide asteroid impacted Earth 66 million years ago, causing global climate change . This caused the extinction of three quarters of life on Earth, including the dinosaurs.

Comet Shoemaker-Levy 9 crashed into Jupiter in 1994 in a spectacular but sobering event witnessed by telescopes around the world . Even more recently in 2013, an asteroid exploded over the Russian city of Chelyabinsk, damaging buildings and sending more than a hundred people to area hospitals. Our planet remains at risk for dangerous asteroid impacts, highlighting the need for planetary defense .

Space Exploration Teaches the Timeline

How do we know all this? Space exploration missions, Earth-based observations, and other scientific activities help us piece together our past. While we can’t look back and see how our solar system was born, we have observed similar baby star systems using observatories like the Hubble Space Telescope , which has imaged young stars in the Orion nebula surrounded by rotating disks that will evolve into star systems like our own. We know the Sun formed alongside other stars in the same cloud complex because the orbits of some of the farthest objects in our solar system can only be explained if other stars once came close enough to gravitationally nudge them.

We know the solar system’s age thanks to multiple lines of evidence. At some point in their orbits around the Sun, several small rocks from the original disk that formed the solar system have fallen on Earth as meteorites. Using extensive laboratory analysis, scientists found the oldest to have formed 4.57 billion years ago. The oldest Moon rocks brought back to Earth by the Apollo missions have an age of 4.46 billion years . We’ve found well-preserved sedimentary rocks in Australia that contain grains that are 4.4 billion years old .

There is much we don’t know about the Late Heavy Bombardment. Volcanism, geologic processes, and weathering wiped out much of the evidence on Venus, Earth, and Mars. Fortunately, the airless Moon still bears scars from those days that you can see from Earth . Moon samples brought back from Apollo missions revealed the ages of some of its largest basins to be between 4.1 to 3.8 billion years old, which is how we even inferred the possibility of such an event taking place.

Missions to asteroids and comets like Rosetta, Hayabusa2 , and OSIRIS-REx continue teaching us about small worlds and how much of Earth’s water and organics they delivered here. In 2019, New Horizons visited Arrokoth , giving us our first close look at the structure and composition of one of the most primitive solar system objects.

Future space missions will tell us even more. There are two clumps of asteroids called Trojans that share Jupiter’s orbit around the Sun. Jupiter’s journey from its original location may be imprinted on these asteroids as they moved along with Jupier. A NASA mission called Lucy is slated for launch in October 2021 to visit 7 of Jupiter’s Trojans for the first time, helping us figure out what really happened in the early solar system.

Japan’s upcoming MMX mission will try to determine the origin of Mars’ two moons by bringing samples of Phobos back to Earth. New missions to Uranus and Neptune are needed to help us understand where the ice giants were born and how they evolved. NASA, international space agencies, and private companies are sending robotic and human missions to the Moon . Some of these missions will collect samples from more diverse areas than the Apollo missions, which will help us understand the Moon and Earth’s past.

We still have much to learn, and because science is an iterative process some of our assumptions will change with new discoveries and future missions.

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How Was the Solar System Formed? – The Nebular Hypothesis

Since time immemorial, humans have been searching for the answer of how the Universe came to be. However, it has only been within the past few centuries, with the Scientific Revolution, that the predominant theories have been empirical in nature. It was during this time, from the 16th to 18th centuries, that astronomers and physicists began to formulate evidence-based explanations of how our Sun, the planets, and the Universe began.

When it comes to the formation of our Solar System, the most widely accepted view is known as the Nebular Hypothesis . In essence, this theory states that the Sun, the planets, and all other objects in the Solar System formed from nebulous material billions of years ago. Originally proposed to explain the origin of the Solar System, this theory has gone on to become a widely accepted view of how all star systems came to be.

Nebular Hypothesis:

According to this theory, the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust. Then, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.

From this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more and more matter, conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up. Most of the material ended up in a ball at the center while the rest of the matter flattened out into disk that circled around it. While the ball at the center formed the Sun, the rest of the material would form into the protoplanetary disc .

The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies. Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury , Venus , Earth , and Mars . Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In contrast, the giant planets ( Jupiter , Saturn , Uranus , and Neptune ) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost Line ). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the Asteroid Belt , Kuiper Belt , and Oort Cloud .

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved. At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.

History of the Nebular Hypothesis:

The idea that the Solar System originated from a nebula was first proposed in 1734 by Swedish scientist and theologian Emanual Swedenborg. Immanuel Kant, who was familiar with Swedenborg’s work, developed the theory further and published it in his Universal Natural History and Theory of the Heavens  (1755). In this treatise, he argued that gaseous clouds (nebulae) slowly rotate, gradually collapsing and flattening due to gravity and forming stars and planets.

A similar but smaller and more detailed model was proposed by Pierre-Simon Laplace in his treatise Exposition du system du monde (Exposition of the system of the world), which he released in 1796. Laplace theorized that the Sun originally had an extended hot atmosphere throughout the Solar System, and that this “protostar cloud” cooled and contracted. As the cloud spun more rapidly, it threw off material that eventually condensed to form the planets.

The Laplacian nebular model was widely accepted during the 19th century, but it had some rather pronounced difficulties. The main issue was angular momentum distribution between the Sun and planets, which the nebular model could not explain. In addition, Scottish scientist James Clerk Maxwell (1831 – 1879) asserted that different rotational velocities between the inner and outer parts of a ring could not allow for condensation of material.

It was also rejected by astronomer Sir David Brewster (1781 – 1868), who stated that:

“those who believe in the Nebular Theory consider it as certain that our Earth derived its solid matter and its atmosphere from a ring thrown from the Solar atmosphere, which afterwards contracted into a solid terraqueous sphere, from which the Moon was thrown off by the same process… [Under such a view] the Moon must necessarily have carried off water and air from the watery and aerial parts of the Earth and must have an atmosphere.”

By the early 20th century, the Laplacian model had fallen out of favor, prompting scientists to seek out new theories. However, it was not until the 1970s that the modern and most widely accepted variant of the nebular hypothesis – the solar nebular disk model (SNDM) – emerged. Credit for this goes to Soviet astronomer Victor Safronov and his book Evolution of the protoplanetary cloud and formation of the Earth and the planets (1972) . In this book, almost all major problems of the planetary formation process were formulated and many were solved.

For example, the SNDM model has been successful in explaining the appearance of accretion discs around young stellar objects. Various simulations have also demonstrated that the accretion of material in these discs leads to the formation of a few Earth-sized bodies. Thus the origin of terrestrial planets is now considered to be an almost solved problem.

While originally applied only to the Solar System, the SNDM was subsequently thought by theorists to be at work throughout the Universe, and has been used to explain the formation of many of the exoplanets that have been discovered throughout our galaxy.

Although the nebular theory is widely accepted, there are still problems with it that astronomers have not been able to resolve. For example, there is the problem of tilted axes. According to the nebular theory, all planets around a star should be tilted the same way relative to the ecliptic. But as we have learned, the inner planets and outer planets have radically different axial tilts.

Whereas the inner planets range from almost 0 degree tilt, others (like Earth and Mars) are tilted significantly (23.4° and 25°, respectively), outer planets have tilts that range from Jupiter’s minor tilt of 3.13°, to Saturn and Neptune’s more pronounced tilts (26.73° and 28.32°), to Uranus’ extreme tilt of 97.77°, in which its poles are consistently facing towards the Sun.

Also, the study of extrasolar planets have allowed scientists to notice irregularities that cast doubt on the nebular hypothesis. Some of these irregularities have to do with the existence of “hot Jupiters” that orbit closely to their stars with periods of just a few days. Astronomers have adjusted the nebular hypothesis to account for some of these problems, but have yet to address all outlying questions.

Alas, it seems that it questions that have to do with origins that are the toughest to answer. Just when we think we have a satisfactory explanation, there remain those troublesome issues it just can’t account for. However, between our current models of star and planet formation, and the birth of our Universe, we have come a long way. As we learn more about neighboring star systems and explore more of the cosmos, our models are likely to mature further.

We have written many articles about the Solar System here at Universe Today. Here’s The Solar System , Did our Solar System Start with a Little Bang? , and What was Here Before the Solar System?

For more information, be sure to check out the origin of the Solar System and how the Sun and planets formed .

Astronomy Cast also has an episode on the subject – Episode 12: Where do Baby Stars Come From?

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5 Replies to “How Was the Solar System Formed? – The Nebular Hypothesis”

So… the transition from the geocentric view and eternal state the way things are evolved with appreciation of dinosaurs and plate tectonics too… and then refining the nebular idea… the Nice model… the Grand Tack model… alittle more? Now maybe the Grand Tack with the assumption of mantle breaking impacts in the early days – those first 10 millions years were heady times!

And the whole idea of “solar siblings” has been busy the last few years…

Nice overview, and I learned a lot. However, there are some salient points that I think I have picked up earlier:

“something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.”

The study of star forming molecular clouds shows that same early, large stars form that way. In the most elaborate model which makes Earth isotope measurements easiest to predict, by free coupling the processes, the 1st generation of super massive stars would go supernova in 1-10 million years.

That blows a 1st geeration of large bubbles with massive, compressed shells that are seeded with supernova elements, as we see Earth started out with. The shells would lead to a more frequent 2nd generation of massive stars with a lifetime of 10-100 million years or so. These stars have powerful solar winds.

That blows a 2nd generation of large bubbles with massive, compressed shells, The shells would lead to a 3d generation of ~ 500 – 1000 stars of Sun size or less. In the case of the Sun the resulting mass was not enough to lead to a closed star cluster as we can see circling the Milky Way, but an open star cluster where the stars would mix with other stars over the ~ 20 orbits we have done around the MW.

“The ices that formed these planets were more plentiful”.

The astronomy course I attended looked at the core collapse model of large planets. (ASs well as the direct collapse scenario.) The core grew large rapidly and triggered gas collapse onto the planet from the disk, a large factor being the stickiness of ices at the grain stage. The terrestrial planets grow by slower accretion, and the material may have started to be cleared from the disk. by star infall or radiation pressure flow outwards, before they are finished.

An interesting problem for terrestrial planets is the “meter size problem” (IIRC the name). It was considered hard to grow grains above a cm, and when they grow they rapidly brake and fall onto the star.

Now scientists have come up with grain collapse scenarios, where grains start to follow each other for reasons of gravity and viscous properties of the disk, I think. All sorts of bodies up to protoplanets can be grown quickly and, when over the problematic size, will start to clear the disk rather than being braked by it.

“But as we have learned, the inner planets and outer planets have radically different axial tilts.”

Jupiter can be considered a clue, too massive to tilt by outside forces. The general explanation tend to be the accretion process, where the tilt would be randomized. (Venus may be an exception, since some claim it is becoming tidally locked to the Sun – Mercury is instead locked in a 3:2 resonance – and it is in fact now retrograde with a putative near axis lock.) Possible Mercury bit at least Earth and Mars (and Moon) show late great impacts.

A recent paper show that terrestrial planets would suffer impacts on the great impact scale, between 1 to 8 as norm with an average of 3. These would not be able to clear out an Earth mass atmosphere or ocean, so if Earth suffered one such impact after having volatiles delivered by late accretion/early bombardment, the Moon could result.

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What is the solar system?

The solar system comprises 8 planets , approximately 170 natural planetary satellites (moons), and countless asteroids , meteorites , and comets .

There are eight planets in the solar system. The four inner terrestrial planets are Mercury , Venus , Earth , and Mars , all of which consist mainly of rock. The four outer planets are Jupiter , Saturn , Neptune , and Uranus , giant planets that consist mainly of either gases or ice. Pluto was considered the ninth planet until 2006, when the International Astronomical Union voted to classify Pluto as a dwarf planet instead.

Where is the solar system?

The solar system is situated within the Orion-Cygnus Arm of the Milky Way Galaxy . Alpha Centauri , made up of the stars Proxima Centauri, Alpha Centauri A, and Alpha Centauri B, is the closest star system to the solar system.

Scientists have multiple theories that explain how the solar system formed. The favoured theory proposes that the solar system formed from a solar nebula , where the Sun was born out of a concentration of kinetic energy and heat at the centre, while debris rotating the nebula collided to create the planets .

Is there life in the solar system aside from on Earth?

Europa and Enceladus , moons of Jupiter and Saturn respectively, are ice-covered rocky objects that scientists think may harbour life in the water beneath the surface. Some geological evidence points to the possibility of microorganisms on Mars .

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solar system , assemblage consisting of the Sun —an average star in the Milky Way Galaxy —and those bodies orbiting around it: 8 (formerly 9) planets with more than 210 known planetary satellites (moons); many asteroids , some with their own satellites; comets and other icy bodies; and vast reaches of highly tenuous gas and dust known as the interplanetary medium . The solar system is part of the " observable universe ," the region of space that humans can actually or theoretically observe with the aid of technology. Unlike the observable universe, the universe is possibly infinite .

The Sun, Moon , and brightest planets were visible to the naked eyes of ancient astronomers, and their observations and calculations of the movements of these bodies gave rise to the science of astronomy . Today the amount of information on the motions, properties, and compositions of the planets and smaller bodies has grown to immense proportions, and the range of observational instruments has extended far beyond the solar system to other galaxies and the edge of the known universe. Yet the solar system and its immediate outer boundary still represent the limit of our physical reach, and they remain the core of our theoretical understanding of the cosmos as well. Earth -launched space probes and landers have gathered data on planets, moons, asteroids, and other bodies, and this data has been added to the measurements collected with telescopes and other instruments from below and above Earth’s atmosphere and to the information extracted from meteorites and from Moon rocks returned by astronauts. All this information is scrutinized in attempts to understand in detail the origin and evolution of the solar system—a goal toward which astronomers continue to make great strides.

Composition of the solar system

Located at the centre of the solar system and influencing the motion of all the other bodies through its gravitational force is the Sun , which in itself contains more than 99 percent of the mass of the system. The planets, in order of their distance outward from the Sun, are Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , and Neptune . Four planets—Jupiter through Neptune—have ring systems, and all but Mercury and Venus have one or more moons. Pluto had been officially listed among the planets since it was discovered in 1930 orbiting beyond Neptune, but in 1992 an icy object was discovered still farther from the Sun than Pluto. Many other such discoveries followed, including an object named Eris that appears to be at least as large as Pluto. It became apparent that Pluto was simply one of the larger members of this new group of objects, collectively known as the Kuiper belt . Accordingly, in August 2006 the International Astronomical Union (IAU), the organization charged by the scientific community with classifying astronomical objects, voted to revoke Pluto’s planetary status and place it under a new classification called dwarf planet . For a discussion of that action and of the definition of planet approved by the IAU, see planet .

Any natural solar system object other than the Sun, a planet, a dwarf planet, or a moon is called a small body ; these include asteroids , meteoroids , and comets . Most of the more than one million asteroids, or minor planets, orbit between Mars and Jupiter in a nearly flat ring called the asteroid belt. The myriad fragments of asteroids and other small pieces of solid matter (smaller than a few tens of metres across) that populate interplanetary space are often termed meteoroids to distinguish them from the larger asteroidal bodies.

The solar system’s several billion comets are found mainly in two distinct reservoirs. The more-distant one, called the Oort cloud , is a spherical shell surrounding the solar system at a distance of approximately 50,000 astronomical units (AU)—more than 1,000 times the distance of Pluto’s orbit. The other reservoir, the Kuiper belt , is a thick disk-shaped zone whose main concentration extends 30–50 AU from the Sun, beyond the orbit of Neptune but including a portion of the orbit of Pluto. (One astronomical unit is the average distance from Earth to the Sun—about 150 million km [93 million miles].) Just as asteroids can be regarded as rocky debris left over from the formation of the inner planets, Pluto, its moon Charon , Eris, and the myriad other Kuiper belt objects can be seen as surviving representatives of the icy bodies that accreted to form the cores of Neptune and Uranus. As such, Pluto and Charon may also be considered to be very large comet nuclei. The Centaur objects , a population of comet nuclei having diameters as large as 200 km (125 miles), orbit the Sun between Jupiter and Neptune, probably having been gravitationally perturbed inward from the Kuiper belt. The interplanetary medium —an exceedingly tenuous plasma (ionized gas) laced with concentrations of dust particles —extends outward from the Sun to about 123 AU.

The solar system even contains objects from interstellar space that are just passing through. Two such interstellar objects have been observed. ‘Oumuamua had an unusual cigarlike or pancakelike shape and was possibly composed of nitrogen ice. Comet Borisov was much like the comets of the solar system but with a much higher abundance of carbon monoxide .

All the planets and dwarf planets, the rocky asteroids, and the icy bodies in the Kuiper belt move around the Sun in elliptical orbits in the same direction that the Sun rotates. This motion is termed prograde, or direct, motion. Looking down on the system from a vantage point above Earth’s North Pole , an observer would find that all these orbital motions are in a counterclockwise direction. In striking contrast, the comet nuclei in the Oort cloud are in orbits having random directions, corresponding to their spherical distribution around the plane of the planets.

The shape of an object’s orbit is defined in terms of its eccentricity . For a perfectly circular orbit, the eccentricity is 0; with increasing elongation of the orbit’s shape, the eccentricity increases toward a value of 1, the eccentricity of a parabola. Of the eight major planets, Venus and Neptune have the most circular orbits around the Sun, with eccentricities of 0.007 and 0.009, respectively. Mercury, the closest planet, has the highest eccentricity, with 0.21; the dwarf planet Pluto, with 0.25, is even more eccentric . Another defining attribute of an object’s orbit around the Sun is its inclination , which is the angle that it makes with the plane of Earth’s orbit—the ecliptic plane. Again, of the planets, Mercury’s has the greatest inclination, its orbit lying at 7° to the ecliptic; Pluto’s orbit, by comparison, is much more steeply inclined, at 17.1°. The orbits of the small bodies generally have both higher eccentricities and higher inclinations than those of the planets. Some comets from the Oort cloud have inclinations greater than 90°; their motion around the Sun is thus opposite that of the Sun’s rotation, or retrograde.

September 29, 1917

17 min read

The Origin of the Solar System

An Outline of the Three Principal Hypotheses

By Harold Jeffreys

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THE question of the origin of the solar system is one that has been a source of speculation for over a hundred years; but, in spite of the attention that has been devoted to it, no really satisfactory answer has yet been obtained. There are at present three principal hypotheses that appear to contain a large element of truth, as measured by the closeness of the approximation of their consequences to the facts of the present state of the system, but none of them is wholly satisfactory. These are the Nebular Hypothesis of Laplace, the Planetesimal Hypothesis of Chamberlin and Moulton, and the Capture Theory of See. Darwings theory of Tidal Friction is scarcely a distinct hypothesis, but is mentioned separately on account of its application to all of the others. The main features of these hypotheses will be outlined in the present paper. The Hypothesis of Laplace.According to Laplace, the solar system formerly consisted of a very much flattened mass of gas, extending beyond the orbit of Neptune, and rotating like a rigid body. In consequence of radiation of energy this slowly contracted, and in so doing gained so much in angular velocity that the centrifugal force at the equator became greater than gravity, and a ring of matter was left behind along the equator. Further contraction would detach a series of rings. These were then expected to break up in such a way that each produced a gaseous planet. This might later evolve in the same way as the original nebula, thus producing satellites. The criticisms of this hypothesis in its original form are very well known, and will only be summarized here. Forest ranger beating out a fire in one of the National Forests in Oregon FIGHTING FOREST FIRES [See page 200] The angular momentum of the system when the gaseous central body extended to the orbit of any planet can be calculated, and is not nearly sufficient to cause detachment of matter. Poincare showed that this objection could be met if the nebula were initially highly heterogeneous, with all but gAtj of its mass in the central body. The matter left behind would not form definite rings; for a gas has no cohesion, and consequently the separation of matter along the equator would be continuous and lead to another gaseous nebula, not rotating like a rigid body. A ring could not condense into a planet. According to the latest work of Jeans, viscosity is inadequate to make a mass of gas as large as a Lapla- cian nebula rotate like a rigid body. No satellite could revolve in a shorter time than it takes its primary to rotate: this condition is violated by Phobos, the inner satellite of Mars, and by the particles constituting the inner edge of Saturn's ring. All satellites should revolve in the same direction as their primaries rotate: this condition is violated by one satellite of Saturn and two of Jupiter. The second, third, and fourth objections seem quite unanswerable at present. The theory of Gravitational Instability, due to Jeans, is an attempt to pass directly from the symmetrical nebula to an unsymmetrical one with a secondary nucleus, without the ring as an intermediate stage. It will be noticed that Laplace's hypothesis implies that all the planets were formerly gaseous, and hence must have been liquid before they became solid. The question of the course of evolution of a gaseous mass initially heterogeneous with several strong secondary condensations has not hitherto been considered; such a mass would be free from at least the first four of the objections offered to the standard forms of Laplace's hypothesis, and its history would serve as a hypothesis intermediate between this and the Planetesimal Hypothesis. The Planetesimal Hypothesis.This hypothesis has been formulated by Chamberlin and Moulton1 to avoid the serious defects of the Nebular Hypothesis. It really consists of two separate assumptions, either of which could be discarded without necessarily invalidating the other. The first of these involves the close approach of some wandering star to the sun. This would raise two tidal projections at opposite sides of the sun, and if the disturbance was sufficiently violent, streams of matter would be expelled from them. On account of the perturbations of their paths by the second body, these would not fall back into the sun, but would go on revolving round it as a system of secondary nuclei, with a large number of very fine particles also revolving round the sun; each particle, however small, would revolve independently, so that the system would in this respect resemble the heterogeneous nebula mentioned at the close of the last paragraph. The mathematical investigation of this hypothesis would be extremely difficult, but there seems to be no obvious objection to it. It will be seen that the nuclei would be initially liquid or gaseous, having been expelled from the sun. Thus this hypothesis implies a formerly molten earth. The smaller particles would soon become solid, but the gaseous part initially expelled and not under the influence of a secondary nucleus would remain gaseous, although its density would be very small. The orbits would be highly eccentric. The second part of the hypothesis deals with the latef- evolution of the secondary nuclei. Its authors believe that these would steadily grow by picking up the smaller particles, which are called planetesimals, and in the process they would have the eccentricities of their orbits reduced. That this is qualitatively correct can easily be proved mathematically. There is, however, a serious objection to its quantitative adequacy. Consider any arbitrary planetesimal. Its chance of colliding with another planetesimal in a definite time is proportional to the sum of the surfaces of the planetesimals, while its chance of colliding with a nucleus is proportional to the sum of the surfaces of the nuclei. Further, if the eccentricities of the planetary orbits are to be considerably affected by accretion, the mass picked up by each planet must be at least as great as the original mass of the planet. Now the more finely divided the matter is, the more surface it exposes, and hence before accretion the mass picked up must have presented a much larger surface than the planet did. Hence collisions between planetesimals must have been far commoner than collisions between planets and planetesimals. Further, as the velocity of impact must have been comparable with an orbital velocity on account of the high eccentricity of the orbits, the colliding planetesimals must in nearly all cases have turned to gas; for it is known that meteors entering the earth's atmosphere at such velocities are volatized. Hence nearly all of the planetesimals must have turned to gas before the nuclei could be much affected by accretion. We are thus back to the heterogeneous gaseous nebula. If the planetesimals moved initially in nearly circular orbits this objection does not arise, but it can then be shown that the product of the mass and the orbital eccentricity of each nucleus would diminish with the time. It can thus be seen that Jupiter could never have been smaller than Uranus is now. There is no obvious objection to this form of the hypothesis, but there is no reason to suppose that solid planetesimals did originally move in nearly circular orbits.2 A further hypothesis that has come to be associated with the present one, although not an essential part of it, is the belief that the earth has always been solid. There are many serious difficulties in the way of this. The mode of formation of the nuclei described in the first part of the Planestesimal Hypothesis implies that they were initially liquid or gaseous. This is not, however, a direct objection; one part of the hypothesis might be true and the other false, as they are not interdependent. Only one satisfactory explanation of the elevation of mountains by the folding of the earth's crust has been offered; this attributes it to a horizontal compression at the surface. Now, if a solid earth grew by the addition of small particles from outside, these would be deposited in a layer on the surface, in a perfectly unstrained condition. Thus, during the whole process of growth the same surface condition would always hold, namely, that there is no horizontal compression at the surface, however much deformation may take place within. Hence any stresses available for mountain- building must have been accumulated after accretion ceased; if the theory that the earth was formerly molten should be proved to give insufficient surface compression to account for known mountains, then a fortiori the theory of a permanently solid earth gives insufficient compression, as the available fall of temperature is less. 3. It is by no means clear that a solid earth growing by accretion would remain solid. A particle falling from an infinite distance to the earth under the earth's attraction alone would develop a velocity almost enough to volatilize it on impact, and the actual velocities must have been considerably greater than this, as the planetesimals would have a velocity relative to the earth before entering its sphere of influence. If, then, the particles required to form the earth were all brought together at once, the resulting body would be gaseous. On the other hand, if the accretion were spread over a long enough time, heat would be radiated away as fast as it was produced, and the body would remain solid. In the absence of a criterion of the rate of growth it is impossible to state whether an earth growing by accretion could remain solid or not. Holmes3 has found that the hypothesis of a cooling earth, initially in a liquid state, leads to temperatures within the crust capable of accounting for igneous activity, whereas the view that the earth is now in a steady state, its temperature gradient being maintained wholly by radio-activity, is by no means certain to lead to adequate internal temperatures. Assuming the former fluidity of the earth, he has developed a wonderfully consistent theory of the earth's thermal state. The present writer, using Holmes's data, finds4 that the available compression of the crust is of the same order of magnitude as that required to produce the existing mountain-ranges. 2Monthly Notices of R.A.S. vol. lxxvn. 1916. It seems, then, that whatever we may assume about the origin of the earth, the hypothesis that it has at some stage of its existence been liquid or gaseous agrees best with its present state. The hypothesis of Laplace, however modified, implies the former fluidity of the earth, and so does the standard form of the Planetesimal Hypothesis. The Capture Theory of See.hLike the Planetesimal Hypothesis, this has been developed during the present century to avoid the objections that have been offered to that of Laplace. The main features of the two theories are very similar. Both involve the idea of a system of secondary nuclei revolving in independent orbits about the primitive sun, with sparsely distributed small particles between them, and the impacts of the small particles on the nuclei are supposed in course of time to act on the orbits of the latter in the same way as a resisting medium; namely, the eccentricities of the orbits tend to diminish, and satellites tend to approach their primaries. The Capture Theory is not, however, stated in so precise a form as the Planetesimal Theory. It is not definitely stated whether all the small particles would revolve in the same direction or not. If they did, then there would be little or no secular effect on the mean distance of a planet. If, however, they moved indifferently in the direct and retrograde senses, then their collective effect would be the same as that of a medium at rest, and the friction encountered by the planets in their motion would cause them to approach the sun. The fact that such a secular effect is stated by See to occur implies that the particles at any point are not on an average supposed to move with the velocity appropriate to a circular orbit at that point, so that the conditions would be such as to ensure that collisions between them would be violent. The small particles are described by the somewhat vague term of “cosmical dust”; if this means that they were solid, the Capture Theory, like the Planetesimal Theory, fails on the ground that the collisions between the small particles would cause the system to degenerate to a gaseous nebula long before any important effect had been produced on the nuclei. If, on the other hand, they were discrete molecules, then the system would be a heterogeneous gaseous nebula at the commencement, and this objection does not apply. It is clear, however, that the planets cannot have entered the system from outer space, for then their orbital planes would be inclined to one another at large angles, which the subsequent action of the medium could scarcely affect, whereas actually all the major planets keep very close to the ecliptic. All must, then, be regarded as having always been members of the solar system, however much their orbits may have changed. They are supposed to be derived from the secondary nuclei of a soiral nebula. The most important difference between the Planetesimal and Capture theories lies in the history attributed to the satellites. In the former, each satellite is supposed to have always been associated with its present primary, having been near it when originally expelled from the sun. In the Capture Theory, primaries and satellites are both supposed to have initially moved independently round the sun in highly eccentric orbits. If, in the course of its movement”, a small body came sufficiently near a large one, and had a sufficiently small relative velocity, then a permanent change would take place in the character of its orbit, and it is possible that, under the influence of the resisting medium, this would ultimately lead to its becoming a satellite. The mechanism of the process has not been worked out in detail, and, in view of the extremely complicated nature of the problem, it would be very dangerous to predict whether it is feasible. All the satellites in the system are supposed to have been captured in this way by their primaries. In both hypotheses the satellites are considered to have approached their primaries after becoming associated with them owing to the secular effect of the resisting medium. 3”Padio-activity and the Earth's Thermal History,” Geol. Mag. FebruaryMarch 1915, June 1916. *Phil. Mag. vol. xxxii. Dec m':er 1916. *>The Capture Theory of Cosmical Evolution, by T. J. J. See The Theory of Tidal Friction.All the theories so far mentioned agree in the fact that each commences with a particular distribution of matter, and tries to predict the course of the changes that would follow if this were left to itself. The success or failure of such hypotheses to lead to a system resembling the present solar system is the measure of their truth or falsehood. The method is thus essentially one of trial and error, and when a theory is found unsatisfactory, the next step is to modify it in such a way as to avoid the defects that have been detected. In this way a succession of different hypotheses may be Obtained, each giving a better representation of the facts than the previous one. Destructive criticism may thus be of positive value. Such a method must necessarily yield the truth very slowly, and must further involve a large number of assumptions concerning the initial conditions; in addition, the set of initial conditions that leads to the correct final state may not be unique. The Theory of Tidal Friction, due to Sir G. H. Darwin,6 is of a totally different character. It? starts with the present conditions, and by means of a single highly plausible hypothesis obtains relations that the properties of the system must have satisfied at any epoch, provided only that this is not too remote for the calculation to be possible, and that no unknown causes have operated that could invalidate the work. The initial conditions thus obtained are then unique, and the only way of disproving the hypothesis would be to discover some new agency of sufficient magnitude to upset the course of the involution. Whatever hypothesis may ultimately be found to account for the present solar system, the Theory of Tidal Friction must therefore form a part of it. The physical basis of the theory is very simple. The attractive force due to the moon is always greatest on the side of the earth nearest to it, and least on that farthest away, while its value at the center of the earth is intermediate. The center of the earth being regarded as fixed, then, the moon tends to cause the parts of the earth nearest to and farthest from it to protrude, thus forming a bodily tide. If the earth were perfectly elastic, the high tide would always occur with the moon in the zenith or nadir; no energy would be dissipated, and there would be no secular effect. If, however, it is viscous the tides would lag somewhat, and their attractions on the moon would, in general, produce a calculable secular effect on the moon's motion and the rotation of the earth. The only case where viscosity would produce no secular effect is when the deformed body rotates in the same time as the deforming one revolves. The tide then does not move round relatively to the body, but becomes a constant fixed deformation, directly under the deforming body, and ceases to produce a secular effect. In the ultimate steady state of a viscous system, then, the viscous body will always keep the same face turned towards the perturbing one. In the solar system system there are certainly two examples of this condition, and no other explanation of it has been advanced. Mercury always keeps the same face towards the sun, and the moon towards the earth; with less certainty it is believed that the same is true of Venus and the satellites of Jupiter. Now if the viscosity of a substance be zero, that substance is a perfect fluid, and there can be no dissipation of energy inside it. If, on the other hand, it be infinite, then we have the case of perfect elasticity, and again there can be dissipation. If the viscosity steadily increase from 0 to infinity, then the rate of dissipation of energy when the same periodic stress is applied increases to a maximum and then diminishes again to zero. The balance of probability seems to imply that the earth was formerly fluid, and, if this can be granted, the fact that most of it is now almost perfectly elastic at once indicates that dissipation of energy by tidal friction must have been important in the past. On this hypothesis Sir G. H. Darwin traced the system of the earth and moon back to a state where the moon was close to the earth, the two always keeping the same face towards each other, and revolving in some time between three and five hours. The lunar orbit was practically in the plane of the equator; the initial eccentricity is uncertain, as it depends altogether on the actual variation of the viscosity with the time. Scientific Papers, vol. ii. The question that next arises is, what was the condition just before this? The natural suggestion is that the two bodies formed one mass. The cause of the separation is, however, open to some doubt. It has been thought that the rapidity of the rotation would be enough to cause instability, in which case the original body might break up into two parts. Moulton, on the other hand, has shown that the actual rotation could not be so rapid as to make the system unstable. It is more likely that Darwin's original suggestion is correct, namely, that at the epoch considered the period of rotation was nearly double the period of one of the free vibrations of the mass; consequently the amplitude of the semidiurnal tide would be enormous, and might easily lead to fission in a system not possessing much strength. The Prevalence of Direct Motion in the Solar System. On all of the theories of the origin of the solar system that have here been described it is necessary that the planets should revolve in the same direction. On the Planetesimal Theory this would be the direction of the motion of the perturbing body relative to the sun at the time of the initial disruption. In addition to this, however, all the planets except probably Uranus and Neptune have a direct rotation, and all the satellites except those of these two planets and the outer ones of Jupiter and Saturn have a direct revolution. The fact that three satellites revolve in the opposite direction to the rotation of their primaries is in flagrant contradiction to the original form of the Nebular Hypothesis. It was, however, suggested by Darwin that all the planets might have originally had a retrograde rotation, and that the friction of the solar tides has since reversed the rotation of all except the two outermost. Jupiter and Saturn would then be supposed to have produced their outer satellites before the reversal took place, and the others afterwards. An objection to this theory has been raised by Moulton, who points out that the secular retardation of the rotation of Saturn due to solar tides is only about tsooo of that of the earth, so that there probably was not time for this to occur. On the other hand, this retardation is proportional to the seventh power of the diameter of the planets: if we can grant then that these planets were formerly much more distended than at present, the viscosity remaining the same, the available time may be adequate. At the same time, solar tidal friction may be adequate to explain the facts that one of the satellites of Mars and the particles at the inner edge of Saturn's ring revolve more rapidly than their primaries rotate, which would not be the case on the unmodified Nebular Hypothesis. Direct rotation and revolution of satellites on the Planetesimal Theory are shown by Moulton to be probable as a result of a very ingenious argument involving the mode of accretion. Whether it is quantitatively adequate is not proved, and the present writer would prefer to regard these motions as having been direct since the initial disruption. Let us suppose, for instance, that disruption would occur when the disruptive force had reached a definite fraction of surface gravity. It can easily be seen that both are proportional to the diameter of the disturbed body, and hence their ratio is independent of it. Other things being equal, then, a nucleus of any size would be equally likely to be broken up and give a set of dependent nuclei, which would then revolve round it in the direct sense. Secondary nuclei expelled at the same time and close together would remain together, and their relative motion might be in either sense. Thus we should expect both direct and retrograde revolution, but the former would predominate. The fact that the retrograde satellites are on the outside of their systems is to be attributed partly to the greater stability of retrograde orbits of larger size and partly to the fact that they would experience less resistance from the medium. Capture may be possible; in the present state of our knowledge we can neither affirm nor deny it. Direct rotation is presumably to be attributed to the attraction of the disturbing body on the tidal protuberance before and during expulsion, and to secondary nuclei with direct motions falling back into the parent body. Subsequent evolution would take place in a similar way to that indicated by Darwin. The Hypothesis of a Heterogeneous Nebula.A system of nuclei revolving in a tenuous gaseous nebula would experience a viscous resistance from it, and hence would probably evolve in much the same way as See has indicated in the Capture Theory; accretion must probably be almost negligible, so that the original nuclei must have had nearly their present masses. The original eccentricities of the orbits of both planets and satellites would be considerably reduced; the inclination to the plane of the ecliptic would be small at the commencement, and would remain so; if the medium revolved the effect on the major axes of the orbit, and hence on the periods, would probably be small. Direct satellites would approach their primaries, and retrograde ones would ultimately be left on the outskirts of their subsystems. Given suitable initial conditions, then, a system might be developed that would bear a strong resemblance to the existing solar system. The resisting medium itself would gradually degenerate and approach the sun on account of its internal friction; the zodiacal light may be the last remnant of it. It may, however, be regarded as certain that there has been no large amount of resisting matter near the earth's orbit for a very long time; there has probably been ample time for the evolution of the earth and moon to take place from the state that Darwin traced them back to. The moon was then probably formed from the earth by the disruptive action of the solar tides; but, as this would be a resonance effect, increasing in amplitude over thousands of vibrations, whereas the formation of a system of nuclei in the way suggested by Moulton would take place at once, there need be no surprise that the former event led to a single satellite of of the mass of the primary, while the latter formed several, the largest having a mass of tTjjfu of its primary. The unsymmetrical nebula here considered might have been produced in the manner described in the last section. A symmetrical nebula becoming gravitationally unstable would lead to an unsymmetrical one, as was proved by Jeans, but it is difficult to see how the phenomenon of retrograde and direct motions occuring to the same subsystem could occur on this hypothesis. On the whole, then, the most plausible hypothesis seems to be that a gaseous neubla with a system of secondary and tertiary nuclei was formed round the sun by tidal disruption owing to the close passage of another star, and that this has been subsequently modified by gaseous viscosity, and at a later stage by tidal friction. The moon was probably formed from the earth by solar tidal disruption, this method being abnormal in the system, and the later evolution of the earth and moon has been dominated by bodily tidal friction.

The solar system, explained

Our solar system is made up of the sun and all the amazing objects that travel around it.

The universe is filled with billions of star systems. Located inside galaxies, these cosmic arrangements are made up of at least one star and all the objects that travel around it, including planets, dwarf planets, moons, asteroids, comets, and meteoroids. The star system we’re most familiar with, of course, is our own.

Home sweet home

If you were to look at a giant picture of space, zoom in on the Milky Way galaxy , and then zoom in again on one of its outer spiral arms, you’d find the solar system. Astronomers believe it formed about 4.5 billion years ago, when a massive interstellar cloud of gas and dust collapsed on itself, giving rise to the star that anchors our solar system—that big ball of warmth known as the sun.

Along with the sun, our cosmic neighborhood includes the eight major planets. The closest to the sun is Mercury , followed by Venus , Earth, and Mars . These are known as terrestrial planets, because they’re solid and rocky. Beyond the orbit of Mars, you’ll find the main asteroid belt , a region of space rocks left over from the formation of the planets. Next come the much bigger gas giants Jupiter and Saturn , which is known for its large ring systems made of ice, rock, or both. Farther out are the ice giants Uranus and Neptune . Beyond that, a host of smaller icy worlds congregate in an enormous stretch of space called the Kuiper Belt. Perhaps the most famous resident there is Pluto . Once considered the ninth planet, Pluto is now officially classified as a dwarf planet , along with three other Kuiper Belt objects and Ceres in the asteroid belt.

Moons and other matter

More than 150 moons orbit worlds in our solar system. Known as natural satellites, they orbit planets, dwarf planets, asteroids, and other debris. Among the planets, moons are more common in the outer reaches of the solar system. Mercury and Venus are moon-free, Mars has two small moons, and Earth has just one. Meanwhile, Jupiter and Saturn have dozens, and Uranus and Neptune each have more than 10. Even though it’s relatively small, Pluto has five moons, one of which is so close to Pluto in size that some astronomers argue Pluto and this moon, Charon, are a binary system.

Too small to be called planets, asteroids are rocky chunks that also orbit our sun along with the space rocks known as meteoroids. Tens of thousands of asteroids are gathered in the belt that lies between the orbits of Mars and Jupiter. Comets, on the other hand, live inside the Kuiper Belt and even farther out in our solar system in a distant region called the Oort cloud .

Atmospheric conditions

The solar system is enveloped by a huge bubble called the heliosphere . Made of charged particles generated by the sun, the heliosphere shields planets and other objects from high-speed interstellar particles known as cosmic rays. Within the heliosphere, some of the planets are wrapped in their own bubbles—called magnetospheres —that protect them from the most harmful forms of solar radiation. Earth has a very strong magnetosphere, while Mars and Venus have none at all.

Most of the major planets also have atmospheres . Earth’s is composed mainly of nitrogen and oxygen—key for sustaining life. The atmospheres on terrestrial Venus and Mars are mostly carbon dioxide, while the thick atmospheres of Jupiter, Saturn, Uranus, and Neptune are made primarily of hydrogen and helium. Mercury doesn’t have an atmosphere at all. Instead scientists refer to its extremely thin covering of oxygen, hydrogen, sodium, helium, and potassium as an exosphere.

Moons can have atmospheres, too, but Saturn’s largest moon, Titan, is the only one known to have a thick atmosphere, which is made mostly of nitrogen.

Life beyond?

For centuries astronomers believed that Earth was the center of the universe, with the sun and all the other stars revolving around it. But in the 16th century, German mathematician and astronomer Nicolaus Copernicus upended that theory by providing strong evidence that Earth and the other planets travel around the sun.

Today, astronomers are studying other stars in our galaxy that host planets, including some star systems like our own that have multiple planetary companions. Based on the thousands of known worlds spotted so far, scientists estimate that billions of planetary systems must exist in the Milky Way galaxy alone.

So does Earth have a twin somewhere in the universe? With ever-advancing telescopes, robots, and other tools, astronomers of the future are sure to find out.

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Home — Essay Samples — Environment — Solar Energy — The Solar System: An Intricate Cosmic Structure

The Solar System: an Intricate Cosmic Structure

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Introduction, body paragraph, components of the solar system, dynamics and interactions, significance of research.

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Our solar system – revolutionary ideas.

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Since the earliest times, humans have made observations of the night sky. These observations, particularly of the Earth, Moon, Sun and planets (visible to the naked eye), led to the development of models to explain the movement of these natural satellites as seen in the night sky.

With the development of the telescope, more accurate measurements of night sky objects were possible. This, along with the development of a more ‘scientific’ interpretation of the collected evidence, caused a major shift from an Earth-centred view (geocentric) of the Universe to a Sun-centred one (heliocentric).

Ptolemaic model

In the second century CE, Ptolemy, who lived in the Egyptian town of Alexandria, produced a mathematical representation based on observation of the known Solar System. In Ptolemy’s model, the Earth was at the centre of the Universe, with the Sun and planets revolving in a series of circular orbits moving out from the Earth. This model became known as the ‘geocentric’ model.

Copernican model

Nicolas Copernicus (1473–1543) was a Polish scholar who reconstructed Ptolemy’s model of the Universe. Over the 1200 years since Ptolemy’s model was put forward, it had been developed into a complex and cumbersome mathematical system. Copernicus was able to simplify it by switching from an Earth-centred model to a Sun-centred one.

The Roman Catholic Church, whose teachings held firmly to the Ptolemaic model, rejected his ‘heliocentric’ ideas. Copernicus’s work was banned and remained out of favour until 1822.

Tycho Brahe

The Dane Tycho Brahe (1546–1601) was born 3 years after the death of Copernicus. He studied mathematics and astronomy in German and Swiss universities and came to the conclusion that the Copernican model defied God’s word as written in the scriptures. He proposed a model with the Sun revolving around the Earth and the planets orbiting the Sun.

Tycho Brahe observed the motions of stars and planets and recorded their movements. He had an island observatory equipped with the best available instruments of the time (minus the telescope since it had not been invented). Years of careful observation allowed him to catalogue, with an exceptionally high degree of accuracy, the positions of stars and the movement of the visible planets. In 1572, he observed the appearance of a very bright new star. This was controversial because the belief of the time was that the stars were fixed. Tycho had observed a supernova, now named SN 1572.

Kepler – laws of planetary motion

Shortly before his death, Tycho Brahe appointed as his assistant a young German, Johann Kepler (1571–1630). Kepler took over Brahe’s records and edited and extended them with his own observations. Kepler, who was a follower of the Copernican model, realised the orbits of the planets could be elliptical rather than circular.

Using Brahe’s data on the movement of Mars, Kepler developed his laws of planetary motion.

• The orbit of every planet is an ellipse with the Sun at one of the two foci.
• A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.
• The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

These laws added considerable weight to the acceptance of the heliocentric model of the Universe.

Galileo discovers Jupiter’s moons

Galileo (1564–1642) lived at the same time as Kepler but they were not in regular communication. Using the newly discovered telescope, Galileo discovered that Jupiter had four moons. At first, he thought they were stars, but he noticed that, each night, the four points of light appeared to change positions slightly.

Galileo’s discovery of four of Jupiter’s moons was evidence that objects can orbit around other objects. He argued that if moons orbit around other planets, then perhaps the Earth is not the centre of all motion of the stars and planets. Galileo stated that the Earth and the planets must all orbit around the Sun (heliocentric).

1500 years ago, everybody knew the Earth was the centre of the Universe. 500 years ago, everybody knew the Earth was flat… Imagine what you’ll know tomorrow. Agent K (Tommy Lee Jones) in the movie Men in Black

Nature of science

Science ideas and models often develop and change over time. It took many years after the invention of the telescope for people to agree that the Earth was not the centre of the Universe. Galileo’s telescope showed that moons could orbit around planets. This gave evidence to help prove that planets orbit around the Sun.

Newton and gravity

Sir Isaac Newton (1642–1727) was born in the same year that Galileo died. His studies of the motion of objects on Earth and of natural satellites in the night sky resulted in three laws of motion and a law of universal gravitation. He linked universal gravitation to Kepler’s laws of planetary motion. This monumental discovery meant that the heliocentric model of the Solar System was finally accepted by the scientific community

The journey from the geocentric to the heliocentric model was a long and tortuous one. It was the collection of empirical evidence along with mathematical applications in conjunction with insightful deep-thinking minds that finally replaced the geocentric model with the heliocentric one.

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8.2: Origin of the Solar System—The Nebular Hypothesis

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• Page ID 6889

• Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher
• Salt Lake Community College via OpenGeology

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Our solar system formed at the same time as our Sun as described in the nebular hypothesis. The nebular hypothesis is the idea that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk, and became a solar system consisting of a star with orbiting planets [ 12 ]. The spinning nebula collected the vast majority of material in its center, which is why the sun Accounts for over 99% of the mass in our solar system.

Planet Arrangement and Segregation

As our solar system formed, the nebular cloud of dispersed particles developed distinct temperature zones. Temperatures were very high close to the center, only allowing condensation of metals and silicate minerals with high melting points. Farther from the Sun, the temperatures were lower, allowing the condensation of lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water [ 13 ]. This temperature differentiation resulted in the inner four planets of the solar system becoming rocky, and the outer four planets becoming gas giants.

Both rocky and gaseous planets have a similar growth model. Particles of dust, floating in the disc were attracted to each other by static charges and eventually, gravity. As the clumps of dust became bigger, they interacted with each other—colliding, sticking, and forming proto-planets. The planets continued to grow over the course of many thousands or millions of years, as material from the protoplanetary disc was added. Both rocky and gaseous planets started with a solid core. Rocky planets built more rock on that core, while gas planets added gas and ice. Ice giants formed later and on the furthest edges of the disc, accumulating less gas and more ice. That is why the gas-giant planets Jupiter and Saturn are composed of mostly hydrogen and helium gas, more than 90%. The ice giants Uranus and Neptune are composed of mostly methane ices and only about 20% hydrogen and helium gases.

The planetary composition of the gas giants is clearly different from the rocky planets. Their size is also dramatically different for two reasons: First, the original planetary nebula contained more gases and ices than metals and rocks. There was abundant hydrogen, carbon, oxygen, nitrogen, and less silicon and iron, giving the outer planets more building material. Second, the stronger gravitational pull of these giant planets allowed them to collect large quantities of hydrogen and helium, which could not be collected by the weaker gravity of the smaller planets.

Jupiter’s massive gravity further shaped the solar system and growth of the inner rocky planets. As the nebula started to coalesce into planets, Jupiter’s gravity accelerated the movement of nearby materials, generating destructive collisions rather than constructively gluing material together [ 14 ]. These collisions created the asteroid belt, an unfinished planet, located between Mars and Jupiter. This asteroid belt is the source of most meteorites that currently impact the Earth. Study of asteroids and meteorites help geologist to determine the age of Earth and the composition of its core, mantle, and crust. Jupiter’s gravity may also explain Mars’ smaller mass, with the larger planet consuming material as it migrated from the inner to the outer edge of the solar system [ 15 ].

Pluto and Planet Definition

The outermost part of the solar system is known as the Kuiper belt, which is a scattering of rocky and icy bodies. Beyond that is the Oort cloud, a zone filled with small and dispersed ice traces. These two locations are where most comets form and continue to orbit, and objects found here have relatively irregular orbits compared to the rest of the solar system. Pluto, formerly the ninth planet, is located in this region of space. The XXVIth General Assembly of the International Astronomical Union (IAU) stripped Pluto of planetary status in 2006 because scientists discovered an object more massive than Pluto, which they named Eris. The IAU decided against including Eris as a planet, and therefore, excluded Pluto as well. The IAU narrowed the definition of a planet to three criteria:

• Enough mass to have gravitational forces that force it to be rounded
• Not massive enough to create a fusion
• Large enough to be in a cleared orbit, free of other planetesimals that should have been incorporated at the time the planet formed. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets

12. Montmerle T, Augereau J-C, Chaussidon M, et al (2006) Solar System Formation and Early Evolution: the First 100 Million Years. In: From Suns to Life: A Chronological Approach to the History of Life on Earth. Springer New York, pp 39–95

13. Martin RG, Livio M (2012) On the evolution of the snow line in protoplanetary discs. Mon Not R Aston Soc Lett 425:L6–L9

14. Petit J-M, Morbidelli A, Chambers J (2001) The Primordial Excitation and Clearing of the Asteroid Belt. Icarus 153:338–347. https://doi.org/10.1006/icar.2001.6702

15. Walsh KJ, Morbidelli A, Raymond SN, et al (2011) A low mass for Mars from Jupiter’s early gas-driven migration. Nature 475:206–209

Essay on Solar System

The universe is a vast and mysterious place, and at the heart of it lies our solar system—a captivating and wondrous collection of celestial bodies. In this essay, we will explore the incredible beauty and fascinating science of our solar system, revealing why it continues to be a source of wonder and inspiration.

The Sun: Our Radiant Star

At the center of our solar system shines the Sun, a blazing ball of hot gases. It provides us with warmth, light, and the energy needed for life on Earth. Did you know that the Sun is so massive that it makes up 99.86% of the solar system’s total mass? That’s an astounding statistic!

The Planets: Our Cosmic Companions

Our solar system is home to eight planets, each with its unique characteristics. The four inner planets—Mercury, Venus, Earth, and Mars—are rocky and terrestrial. The four outer planets—Jupiter, Saturn, Uranus, and Neptune—are giant gas planets. Jupiter, the largest, is even larger than all the other planets combined!

Earth: Our Precious Home

Earth is the third planet from the Sun and the only one known to support life. Its diverse ecosystems, from lush rainforests to vast oceans, provide habitats for countless species, including us humans. It’s crucial that we take good care of our planet to ensure a healthy future for all.

The Moon: Earth’s Faithful Companion

Earth is not alone in its journey around the Sun; it has a loyal companion—the Moon. The Moon’s gravitational pull creates tides, and its surface is marked by craters, mountains, and plains. Human beings have even set foot on the Moon during the Apollo missions!

Asteroids and Comets: Cosmic Wanderers

Beyond the planets, our solar system is teeming with smaller objects like asteroids and comets. Asteroids are rocky remnants from the early solar system, while comets are icy bodies that release beautiful tails when they approach the Sun. Studying these objects helps us understand the solar system’s history.

Space Exploration: Unraveling Mysteries

Humans have always been curious about the solar system, and our desire to explore it has led to amazing discoveries. Space missions like Voyager, Hubble, and Mars rovers have provided us with breathtaking images and invaluable information about distant planets, stars, and galaxies.

The Solar System’s Mysteries

Despite our advances in space exploration, there is still much we don’t know about the solar system. Mysteries abound, from the potential existence of a ninth planet beyond Neptune to the origin of life on Earth. Scientists continue to conduct research and missions to uncover these secrets.

Conclusion of Essay on Solar System

In conclusion, the solar system is a source of wonder and inspiration for people of all ages. It reminds us of our small place in the vast universe and the beauty and complexity of the cosmos. From the blazing Sun to the distant reaches of space, there is always something new and exciting to discover.

As we gaze up at the night sky, let us remember the remarkable journey of exploration and discovery that has brought us closer to understanding the solar system’s wonders. Let us also recognize the importance of protecting our own planet, Earth, and preserving the beauty of the solar system for future generations. In doing so, we honor the legacy of those who have ventured into the cosmos and continue to inspire future generations of explorers. The solar system, our cosmic neighborhood, beckons us to explore, learn, and marvel at the wonders of the universe.

Also Check: List of 500+ Topics for Writing Essay

Exploring the Solar System

The History and Science of Planetary Exploration

• © 2013
• Roger D. Launius

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Three eras of planetary exploration

Probing space to understand Earth

Venus, the Planet: Introduction to the Evolution of Earth’s Sister Planet

• climatology
• development

Table of contents (15 chapters)

Front matter, introduction, managing planetary science, homer newell and the origins of planetary science in the united states.

• John D. Ruley

The Survival Crisis of the US Solar System Exploration Program in the 1980s

• John M. Logsdon

Faster, Better, Cheaper: A Sociotechnical Perspective on Programmatic Choice, Success, and Failure in NASA’s Solar System Exploration Program

• Amy Paige Kaminski

Developing New Approaches to Planetary Exploration

Redefining celestial mechanics in the space age: astrodynamics, deep-space navigation, and the pursuit of accuracy.

• Andrew J. Butrica

Big Science in Space: Viking, Cassini, and the Hubble Space Telescope

• W. Henry Lambright

Visual Imagery in Solar System Exploration

• Peter J. Westwick

Returning Scientific Data to Earth: The Parallel but Unequal Careers of Genesis and Stardust and the Problem of Sample Return to Earth

Exploring the terrestrial planets, planetary science and the “discovery” of global warming.

• Erik M. Conway

Exploring Planet Earth: The Development of Satellite Remote Sensing for Earth Science

• Andrew K. Johnston

Venus-Earth-Mars: Comparative Climatology and the Search for Life in the Solar System

Missions to mars: reimagining the red planet in the age of spaceflight.

• Robert Markley

Unveiling the Outer Solar System

Parachuting onto another world: the european space agency’s huygens mission to titan.

• Arturo Russo

Pluto: The Problem Planet and its Scientists

• David H. DeVorkin

Transcendence and Meaning in Solar System Exploration

• William E. Burrows

"Today, the scientific community is concerned about NASA's declining budget and the corresponding impact on the future of planetary exploration. In this context, Exploring the Solar System, edited by Launius (Smithsonian National Air and Space Museum), is a very timely work. The book's 14 essays focus on the history of NASA and solar system exploration from the earliest days of the space agency. The volume fills a unique niche in the literature since the essays on the scientific motivation behind and accomplishments of missions like Voyager, Galileo, Magellan, and Cassini are accompanied by others on the organization and management of NASA during the development and launch of these and other missions. These latter essays discuss the tensions between scientists, administrators, and the politicians who were responsible for the NASA budget, revealing many of the important issues that surround past, current, and future solar system exploration. This combination of aerospace history and planetary science provides the entire picture of the full life cycle of these missions - not just their most successful moments. Part of the 'Palgrave Studies in the History of Science and Technology' series." - Chris Palma, Senior Lecturer, Pennsylvania State University, USA

"Fifty years after Mariner 2 flew past Venus, Exploring the Solar System, instrumented with some of the best scholars around, gives us a historical flyby of what has evolved since that first encounter. The book instantly claims standing in the canon." - Stephen J. Pyne, Regent's Professor in the School of Life Sciences, Arizona State University, USA, and author of Voyager: Exploration, Space, and the Third Great Age of Discovery

"For anyone who wants to understand how planetary exploration actually occurs, this book is essential reading. It adroitly explains the management, science, imagination, and public policy that have produced spectacular revelations about our planetary neighbors." - Howard McCurdy, Professor of Public Administration and Policy, American University, USA

"In Exploring the Solar System, Roger D. Launius brings together a fascinating collection of essays that explains our attempts to scientifically understand our planetary neighbors. As important, by examining the political, technological, and even cultural history of this quest during the post-World War II era, this book highlights the essential role played by space exploration in shaping our understanding of life back on Earth." - Neil M. Maher, Associate Professor of History, Federated History Department at NJIT - Rutgers University, Newark, USA

About the authors

Bibliographic information.

Book Title : Exploring the Solar System

Book Subtitle : The History and Science of Planetary Exploration

Editors : Roger D. Launius

Series Title : Palgrave Studies in the History of Science and Technology

DOI : https://doi.org/10.1057/9781137273178

Publisher : Palgrave Macmillan New York

eBook Packages : Palgrave History Collection , History (R0)

Copyright Information : Palgrave Macmillan, a division of Nature America Inc. 2013

Hardcover ISBN : 978-1-137-27316-1 Published: 28 December 2012

Softcover ISBN : 978-1-349-44514-1 Published: 28 December 2012

eBook ISBN : 978-1-137-27317-8 Published: 28 December 2012

Series ISSN : 2730-972X

Series E-ISSN : 2730-9738

Edition Number : 1

Number of Pages : IX, 390

Topics : History of Science , History of the Americas , Modern History , US History

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Solar System Facts

Our solar system is made up of a star, eight planets, and thousands of smaller bodies including dwarf planets, moons, asteroids, and comets.

10 ThingS ABOUT OUR SOLAR SYSTEM

1. Our planetary system is called “the solar system” because we use the word “solar” to describe things related to our star, after the Latin word for Sun, "solis."

2. Our solar system orbits the center of the Milky Way galaxy at about 515,000 mph (829,000 kph).

3. It takes our solar system about 230 million years to complete one orbit around the galactic center.

4. The hottest planet in our solar system is Venus, even though Mercury is closer to the Sun.

5. The largest planet is Jupiter. If Jupiter was a hollow shell, 1,000 Earths could fit inside.

6. There are hundreds of moons in our solar system. Most orbit planets, but some asteroids have moons.

7. The four giant planets – and at least one asteroid – have rings. None are as spectacular as Saturn’s gorgeous rings.

8. More than 300 robotic spacecraft from many nations have explored destinations beyond Earth's orbit.

9. Our solar system is the only one known to support life. So far, we only know of life on Earth, but we’re looking for more everywhere we can.

10. NASA’s Voyager 1 and Voyager 2 are the only spacecraft leaving our solar system. Three other spacecraft – Pioneer 10, Pioneer 11, and New Horizons – will eventually hit interstellar space.

Introduction

The solar system includes the Sun, eight planets, five officially named dwarf planets , and hundreds of moons, and thousands of asteroids and comets. Our solar system is located in the Orion Spur of the Milky Way, a barred spiral galaxy that's about 100,000 light years across.

Our planetary system is called “the solar system” because we use the word “solar” to describe things related to our star, after the Latin word for Sun, "solis."

Potential for Life

So far, we've only know about life on Earth, but NASA is searching for life on other worlds in our solar system and beyond.

Size and Distance

Our solar system extends much farther than the planets that orbit the Sun. The solar system also includes the Kuiper Belt that lies past Neptune's orbit. This is a ring of icy bodies, almost all smaller than the most popular Kuiper Belt Object – dwarf planet Pluto .

Beyond the fringes of the Kuiper Belt is the Oort Cloud . This giant spherical shell surrounds our solar system. It has never been directly observed, but its existence is predicted based on mathematical models and observations of comets that likely originate there.

The Oort Cloud is made of icy pieces of space debris - some bigger than mountains – orbiting our Sun as far as 1.6 light-years away. This shell of material is thick, extending from 5,000 astronomical units to 100,000 astronomical units. One astronomical unit (or AU) is the distance from the Sun to Earth, or about 93 million miles (150 million kilometers).

The Oort Cloud is the boundary of the Sun's gravitational influence, where orbiting objects can turn around and return closer to our Sun.

The Sun's heliosphere doesn't extend quite as far. The heliosphere is the bubble created by the solar wind – a stream of electrically charged gas blowing outward from the Sun in all directions. The boundary where the solar wind is abruptly slowed by pressure from interstellar gases is called the termination shock. This edge occurs between 80-100 astronomical units.

Two NASA spacecraft launched in 1977 have crossed the termination shock: Voyager 1 in 2004 and Voyager 2 in 2007. Voyager 1 went interstellar in 2012 and Voyager 2 joined it in 2018. But it will be many thousands of years before the two Voyagers exit the Oort Cloud.

Our solar system has hundreds of moons orbiting planets, dwarf planets, and asteroids.

Of the eight planets, Mercury and Venus are the only ones with no moons, although Venus does have a quasi-satellite that has  officially been named Zoozve . 

The giant planets Jupiter and Saturn lead our solar system’s moon counts. In some ways, the swarms of moons around these worlds resemble mini versions of our solar system.

Pluto, smaller than our own moon, has five moons in its orbit, including Charon, a moon so large it makes Pluto wobble.

Moons in Our Solar System

Latest Count

For the latest tally of moons, or planetary satellites, in our solar system, visit NASA/JPL's Solar System Dynamics website.

Our solar system formed about 4.6 billion years ago from a dense cloud of interstellar gas and dust. The cloud collapsed, possibly due to the shockwave of a nearby exploding star, called a supernova. When this dust cloud collapsed, it formed a solar nebula – a spinning, swirling disk of material.

At the center, gravity pulled more and more material in. Eventually, the pressure in the core was so great that hydrogen atoms began to combine and form helium, releasing a tremendous amount of energy. With that, our Sun was born, and it eventually amassed more than 99% of the available matter.

Matter farther out in the disk was also clumping together. These clumps smashed into one another, forming larger and larger objects. Some of them grew big enough for their gravity to shape them into spheres, becoming planets, dwarf planets, and large moons. In other cases, planets did not form: the asteroid belt is made of bits and pieces of the early solar system that could never quite come together into a planet. Other smaller leftover pieces became asteroids, comets, meteoroids, and small, irregular moons.

The order and arrangement of the planets and other bodies in our solar system is due to the way the solar system formed. Nearest to the Sun, only rocky material could withstand the heat when the solar system was young. For this reason, the first four planets – Mercury, Venus, Earth, and Mars – are terrestrial planets. They are all small with solid, rocky surfaces.

Meanwhile, materials we are used to seeing as ice, liquid, or gas settled in the outer regions of the young solar system. Gravity pulled these materials together, and that is where we find gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune.

Latest Solar System News

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Turn Supermoon Hype into Lunar Learning

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Asteroids, Comets & Meteors

Kuiper Belt

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solar system

Introduction.

The solar system itself is only a small part of a huge system of stars and other objects called the Milky Way galaxy . The solar system orbits around the center of the galaxy about once every 225 million years. The Milky Way galaxy is just one of billions of galaxies that in turn make up the universe .

The Solar Wind

The gases that surround the Sun shoot out a stream of tiny particles called the solar wind. It flows outward through the whole solar system. The solar wind is what causes auroras, or displays of colored light in the night sky in parts of Earth. In the Northern Hemisphere these auroras are called the northern lights.

The Planets

Scientists used to call Pluto the ninth planet. But in 2006 scientists decided that several objects in the solar system, including Pluto, should be called dwarf planets.

Millions of small chunks of metal and rock called asteroids also orbit the Sun. Most asteroids are found in a ring between Mars and Jupiter. They are believed to be debris, or bits of material, left over from collisions between other bodies in the solar system. The largest asteroids are hundreds of miles in diameter, but most are much smaller. Small asteroids regularly fall to Earth or burn up in the sky as glowing meteors .

Outer Regions

Beyond Neptune lies the Kuiper Belt, a flat ring of millions of small, icy objects. These objects orbit the Sun at a very great distance. They are mostly 30 to 50 times farther from the Sun than Earth is.

At the outer reaches of the solar system is the Oort Cloud. It is a huge cloud of countless small, icy objects. The Oort Cloud surrounds the rest of the solar system.

How the Solar System Was Formed

The solar system was formed about 4.7 billion years ago. It probably started as a loose cloud of gas and dust. Scientists think that a force called gravity pulled parts of the cloud together into clumps. The largest clump was squeezed together so tightly that it got very hot. This clump eventually became the Sun. Over millions of years the other clumps became the planets. The Sun’s strong gravity eventually pulled the planets into their orbits. Over time some of the leftover clumps became asteroids, comets, and other small, icy objects.

Exploring the Solar System

Other Planetary Systems

The solar system is also known as a planetary system. Since the 1990s scientists have found many planetary systems beyond our solar system. In these systems, one or more planets orbit a star—just as the eight planets in our solar system orbit the Sun. These planets are called extrasolar planets. Finding other planetary systems is not easy, however, because extrasolar planets appear much dimmer than the stars they orbit. As space probes travel farther away from Earth, they are likely to discover more extrasolar planets.

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Essay on Solar System for School Students

• Updated on  
• Dec 23, 2023

Essay on Solar System: Our solar system consists of one Sun and eight (formerly nine) planets. These eight planets are gravitationally bound by the Sun on their orbits. Apart from these eight planets, there are more than 210 known planetary satellites, asteroids, comets, and other icy bodies that are assembled in the Solar system. 

The first four planets are called terrestrial planets (Mercury, Venus, Earth, and Mars) the two gas planets (Jupiter and Saturn), and the other remaining ones are ice giants (Neptune and Uranus.)

Table of Contents

• 2 Inner Planets (Terrestrial Planets):
• 3 Outer Planets (Gas Giants)
• 5 FAQs 

Learn about the smallest planet in our solar system

The Sun is the primary source of light and energy and is about 93 million miles from the Earth. It is the only star in our solar system and one of the more than 100 billion stars in the Milky Way. The surface of the Sun is about 5,500 degrees Celsius (10,000 degrees Fahrenheit) hot and the temperature reaches 15 million Celsius (27 million Fahrenheit). 

In terms of age and size, the Sun is 4.5 billion years old, composed of hydrogen and helium with a diameter of about 865,000 miles which is approximately 1.4 million kilometres. 

Inner Planets (Terrestrial Planets):

The planets that are made of rocks and metals are known as Inner Planets or Terrestrial Planets. These planets are comparatively small in size compared to the other outer planets. The description of these four planets is as follows:

1. Mercury—The Swift Planet

Mercury is the swiftest planet in our solar system which completes an orbit around the Sun in just 88 Earth days. Its proximity to the Sun contributes to extreme temperature variations, from scorching highs to freezing lows. 

With minimal atmosphere, Mercury lacks the protective blanket found on the Earth, exposing its surface to harsh solar radiation. 

2. Venus—The Evening Star or Morning Star

Venus, which is often referred to as the evening star or morning star, depends on its position relative to the Sun. When Venus is trailing the Sun, it is the evening star, visible after the sunset. Conversely, when ahead of the Sun, it is the morning star, appearing before sunrise. 

This dual identity arises from Venus´s orbit, positioning it closer to the Sun than Earth and causing varied visibility during different parts of the orbital journey. 

3. Earth—Blue Planet

The home planet to all living things is Earth. It is the only planet that is known for the existence of life. 

The surface of the Earth is made up of the crust, the core, and the mantle. It is a giant rocky planet with a circumference of about 40,075 kilometers; 71 percent or ¾ th of the Earth is covered with oceans and seas. A large area covered with water makes this planet a Blue Planet. 

4. Mars—Red Planet

The fourth planet of the solar system, Mars, is the most explored planet by the National Aeronautics and Space Administration (NASA.) The reason behind so many missions or research for Mars is to hope for the existence of extraterrestrial life on the planet. 

Apart from the possibility of life on Mars, the planet is also known for its presence of iron oxide that turns the planet reddish in appearance. 

Want to know more about our Planet Earth? Read Essay on Earth for more information.

Outer Planets (Gas Giants)

5. Jupiter—King of Planets

Jupiter is the first planet of our solar system in the category of outer planets, also known as gas giants. According to NASA, the U.S. government agency, the planet’s size is more than twice that of all other planets combined. 

Except for Jupiter’s size, the solar system’s first outer planet is made up of leftover gases from the formation of the Sun. 

6. Saturn—Ringed Planet

The sixth planet from the Sun is Saturn. It is also known as the ringed planet and the second-largest solar system planet. 

The three distinctive features that make Saturn different from other planets are its huge 145 moons, visibility from the Earth with the naked eye, and the seven main rings named D, C, B, A, F, G, and E from the outward side of the planet. 

7. Uranus—Ice Giant

The seventh planet from the Sun, Uranus, is one of the two ice giants in the list of the outer solar system. The planet is featured with the third largest diameter which makes the planet the third largest in the solar system. 

Other than massive size, Uranus is made up of three dense icy materials, methane, ammonia, and water – above all a small rocky core. 

8. Neptune—Blue Giant

The third largest and eighth planet of the solar system is Neptune. According to NASA, the farthest planet from the Sun is more than 17 times Earth’s size and nearly 58 times the dimensions of Earth’s volume. 

The cool blue planet, due to the absorption of infrared light by the planet’s Methane atmosphere, comprises a core with the capacity to pick up a lot of gas, making Neptune impossible for the existence of life. 

Also Read: Essay on Space Exploration

Our Solar system is incomplete without the Moon, a planetary large natural object that travels around the Earth. However, the Moon does not make its light but it reflects the light of the sunlight. 

The total number of moons in our Solar system is 290, out of which one Moon belongs to Earth, two to Mars, 27 to Uranus, 95 to Jupiter, 146 to Saturn, 5 to dwarf planet Pluto, and 14 to Neptune.

The solar system consists of the Sun, terrestrial planets, gas giants, Earth’s Moon, celestial bodies , and various other objects. The unique formation and dynamics continue to amaze scientists offering a glimpse into the vastness and beauty of our cosmic neighbourhood. 

Also Read: How to Prepare for UPSC in 6 Months?

Ans: The Nebular Theory, which states that the solar system is made up of interstellar clouds of dust and gas, is the best theory for the solar system.

Ans: Arybhatta, the mathematician and astronomer was the first to discover that the Earth revolves around the Sun. 

Ans: There is only one solar system in the universe. 

Ans: Our solar system consists of only stars and we know it as The Sun. 

Ans: The size of the solar system is almost 12 trillion miles, nearly 2 light years. 

Related Articles: 

For more information on such interesting topics, visit our essay writing page and follow Leverage Edu .

Deepika Joshi

Deepika Joshi is an experienced content writer with educational and informative content expertise. She has hands-on experience in Education, Study Abroad and EdTech SaaS. Her strengths lie in conducting thorough research and analysis to provide accurate and up-to-date information to readers. She enjoys staying updated on new skills and knowledge, particularly in the education domain. In her free time, she loves to read articles, and blogs related to her field to expand her expertise further. In her personal life, she loves creative writing and aspires to connect with innovative people who have fresh ideas to offer.

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Science and Creationism: A View from the National Academy of Sciences, Second Edition (1999)

Chapter: the origin of the universe, earth, and life, the origin of the universe, earth, and life.

The term "evolution" usually refers to the biological evolution of living things. But the processes by which planets, stars, galaxies, and the universe form and change over time are also types of "evolution." In all of these cases there is change over time, although the processes involved are quite different.

In the late 1920s the American astronomer Edwin Hubble made a very interesting and important discovery. Hubble made observations that he interpreted as showing that distant stars and galaxies are receding from Earth in every direction. Moreover, the velocities of recession increase in proportion with distance, a discovery that has been confirmed by numerous and repeated measurements since Hubble's time. The implication of these findings is that the universe is expanding.

Hubble's hypothesis of an expanding universe leads to certain deductions. One is that the universe was more condensed at a previous time. From this deduction came the suggestion that all the currently observed matter and energy in the universe were initially condensed in a very small and infinitely hot mass. A huge explosion, known as the Big Bang, then sent matter and energy expanding in all directions.

This Big Bang hypothesis led to more testable deductions. One such deduction was that the temperature in deep space today should be several degrees above absolute zero. Observations showed this deduction to be correct. In fact, the Cosmic Microwave Background Explorer (COBE) satellite launched in 1991 confirmed that the background radiation field has exactly the spectrum predicted by a Big Bang origin for the universe.

As the universe expanded, according to current scientific understanding, matter collected into clouds that began to condense and rotate, forming the forerunners of galaxies. Within galaxies, including our own Milky Way galaxy, changes in pressure caused gas and dust to form distinct clouds. In some of these clouds, where there was sufficient mass and the right forces, gravitational attraction caused the cloud to collapse. If the mass of material in the cloud was sufficiently compressed, nuclear reactions began and a star was born.

Some proportion of stars, including our sun, formed in the middle of a flattened spinning disk of material. In the case of our sun, the gas and dust within this disk collided and aggregated into small grains, and the grains formed into larger bodies called planetesimals ("very small planets"), some of which reached diameters of several hundred kilometers. In successive stages these planetesimals coalesced into the nine planets and their numerous satellites. The rocky planets, including Earth, were near the sun, and the gaseous planets were in more distant orbits.

The ages of the universe, our galaxy, the solar system, and Earth can be estimated using modem scientific methods. The age of the universe can be derived from the observed relationship between the velocities of and the distances separating the galaxies. The velocities of distant galaxies can be measured very accurately, but the measurement of distances is more uncertain. Over the past few decades, measurements of the Hubble expansion have led to estimated ages for the universe of between 7 billion and 20 billion years, with the most recent and best measurements within the range of 10 billion to 15 billion years.

A disk of dust and gas, appearing as a dark band in this Hubble Space Telescope photograph, bisects a glowing nebula around a very young star in the constellation Taurus. Similar disks can be seen around other nearby stars and are thought to provide the raw material for planets.

The age of the Milky Way galaxy has been calculated in two ways. One involves studying the observed stages of evolution of different-sized stars in globular clusters. Globular clusters occur in a faint halo surrounding the center of the Galaxy, with each cluster containing from a hundred thousand to a million stars. The very low amounts of elements heavier than hydrogen and helium in these stars indicate that they must have formed early in the history of the Galaxy, before large amounts of heavy elements were created inside the initial generations of stars and later distributed into the interstellar medium through supernova explosions (the Big Bang itself created primarily hydrogen and helium atoms). Estimates of the ages of the stars in globular clusters fall within the range of 11 billion to 16 billion years.

A second method for estimating the age of our galaxy is based on the present abundances of several long-lived radioactive elements in the solar system. Their abundances are set by their rates of production and distribution through exploding

supernovas. According to these calculations, the age of our galaxy is between 9 billion and 16 billion years. Thus, both ways of estimating the age of the Milky Way galaxy agree with each other, and they also are consistent with the independently derived estimate for the age of the universe.

Radioactive elements occurring naturally in rocks and minerals also provide a means of estimating the age of the solar system and Earth. Several of these elements decay with half lives between 700 million and more than 100 billion years (the half life of an element is the time it takes for half of the element to decay radioactively into another element). Using these time-keepers, it is calculated that meteorites, which are fragments of asteroids, formed between 4.53 billion and 4.58 billion years ago (asteroids are small "planetoids" that revolve around the sun and are remnants of the solar nebula that gave rise to the sun and planets). The same radioactive time-keepers applied to the three oldest lunar samples returned to Earth by the Apollo astronauts yield ages between 4.4 billion and 4.5 billion years, providing minimum estimates for the time since the formation of the moon.

The oldest known rocks on Earth occur in northwestern Canada (3.96 billion years), but well-studied rocks nearly as old are also found in other parts of the world. In Western Australia, zircon crystals encased within younger rocks have ages as old as 4.3 billion years, making these tiny crystals the oldest materials so far found on Earth.

The best estimates of Earth's age are obtained by calculating the time required for development of the observed lead isotopes in Earth's oldest lead ores. These estimates yield 4.54 billion years as the age of Earth and of meteorites, and hence of the solar system.

The origins of life cannot be dated as precisely, but there is evidence that bacteria-like organisms lived on Earth 3.5 billion years ago, and they may have existed even earlier, when the first solid crust formed, almost 4 billion years ago. These early organisms must have been simpler than the organisms living today. Furthermore, before the earliest organisms there must have been structures that one would not call "alive" but that are now components of living things. Today, all living organisms store and transmit hereditary information using two kinds of molecules: DNA and RNA. Each of these molecules is in turn composed of four kinds of subunits known as nucleotides. The sequences of nucleotides in particular lengths of DNA or RNA, known as genes, direct the construction of molecules known as proteins, which in turn catalyze biochemical reactions, provide structural components for organisms, and perform many of the other functions on which life depends. Proteins consist of chains of subunits known as amino acids. The sequence of nucleotides in DNA and RNA therefore determines the sequence of amino acids in proteins; this is a central mechanism in all of biology.

Experiments conducted under conditions intended to resemble those present on primitive Earth have resulted in the production of some of the chemical components of proteins, DNA, and RNA. Some of these molecules also have been detected in meteorites from outer space and in interstellar space by astronomers using radio-telescopes. Scientists have concluded that the "building blocks of life" could have been available early in Earth's history.

An important new research avenue has opened with the discovery that certain molecules made of RNA, called ribozymes, can act as catalysts in modem cells. It previously had been thought that only proteins could serve as the catalysts required to carry out specific biochemical functions. Thus, in the early prebiotic world, RNA molecules could have been "autocatalytic"—that is, they could have replicated themselves well before there were any protein catalysts (called enzymes).

Laboratory experiments demonstrate that replicating autocatalytic RNA molecules undergo spontaneous changes and that the variants of RNA molecules with the greatest autocatalytic activity come to prevail in their environments. Some scientists favor the hypothesis that there was an early "RNA world," and they are testing models that lead from RNA to the synthesis of simple DNA and protein molecules. These assemblages of molecules eventually could have become packaged within membranes, thus making up "protocells"—early versions of very simple cells.

For those who are studying the origin of life, the question is no longer whether life could have originated by chemical processes involving nonbiological components. The question instead has become which of many pathways might have been followed to produce the first cells.

Will we ever be able to identify the path of chemical evolution that succeeded in initiating life on Earth? Scientists are designing experiments and speculating about how early Earth could have provided a hospitable site for the segregation of

molecules in units that might have been the first living systems. The recent speculation includes the possibility that the first living cells might have arisen on Mars, seeding Earth via the many meteorites that are known to travel from Mars to our planet.

Of course, even if a living cell were to be made in the laboratory, it would not prove that nature followed the same pathway billions of years ago. But it is the job of science to provide plausible natural explanations for natural phenomena. The study of the origin of life is a very active research area in which important progress is being made, although the consensus among scientists is that none of the current hypotheses has thus far been confirmed. The history of science shows that seemingly intractable problems like this one may become amenable to solution later, as a result of advances in theory, instrumentation, or the discovery of new facts.

Creationist Views of the Origin of the Universe, Earth, and Life

Many religious persons, including many scientists, hold that God created the universe and the various processes driving physical and biological evolution and that these processes then resulted in the creation of galaxies, our solar system, and life on Earth. This belief, which sometimes is termed "theistic evolution," is not in disagreement with scientific explanations of evolution. Indeed, it reflects the remarkable and inspiring character of the physical universe revealed by cosmology, paleontology, molecular biology, and many other scientific disciplines.

The advocates of "creation science" hold a variety of viewpoints. Some claim that Earth and the universe are relatively young, perhaps only 6,000 to 10,000 years old. These individuals often believe that the present physical form of Earth can be explained by "catastrophism," including a worldwide flood, and that all living things (including humans) were created miraculously, essentially in the forms we now find them.

Other advocates of creation science are willing to accept that Earth, the planets, and the stars may have existed for millions of years. But they argue that the various types of organisms, and especially humans, could only have come about with supernatural intervention, because they show "intelligent design."

In this booklet, both these "Young Earth" and "Old Earth" views are referred to as "creationism" or "special creation."

There are no valid scientific data or calculations to substantiate the belief that Earth was created just a few thousand years ago. This document has summarized the vast amount of evidence for the great age of the universe, our galaxy, the solar system, and Earth from astronomy, astrophysics, nuclear physics, geology, geochemistry, and geophysics. Independent scientific methods consistently give an age for Earth and the solar system of about 5 billion years, and an age for our galaxy and the universe that is two to three times greater. These conclusions make the origin of the universe as a whole intelligible, lend coherence to many different branches of science, and form the core conclusions of a remarkable body of knowledge about the origins and behavior of the physical world.

Nor is there any evidence that the entire geological record, with its orderly succession of fossils, is the product of a single universal flood that occurred a few thousand years ago, lasted a little longer than a year, and covered the highest mountains to a depth of several meters. On the contrary, intertidal and terrestrial deposits demonstrate that at no recorded time in the past has the entire planet been under water. Moreover, a universal flood of sufficient magnitude to form the sedimentary rocks seen today, which together are many kilometers thick, would require a volume of water far greater than has ever existed on and in Earth, at least since the formation of the first known solid crust about 4 billion years ago. The belief that Earth's sediments, with their fossils, were deposited in an orderly sequence in a year's time defies all geological observations and physical principles concerning sedimentation rates and possible quantities of suspended solid matter.

Geologists have constructed a detailed history of sediment deposition that links particular bodies of rock in the crust of Earth to particular environments and processes. If petroleum geologists could find more oil and gas by interpreting the record of sedimentary rocks as having resulted from a single flood, they would certainly favor the idea of such a flood, but they do not. Instead, these practical workers agree with academic geologists about the nature of depositional environments and geological time. Petroleum geologists have been pioneers in the recognition of fossil deposits that were formed over millions of years in such environments as meandering rivers, deltas, sandy barrier beaches, and coral reefs.

The example of petroleum geology demonstrates one of the great strengths of science. By using knowledge of the natural world to predict the consequences of our actions, science makes it possible to solve problems and create opportunities using technology. The detailed knowledge required to sustain our civilization could only have been derived through scientific investigation.

The arguments of creationists are not driven by evidence that can be observed in the natural world. Special creation or supernatural intervention is not subjectable to meaningful tests, which require predicting plausible results and then checking these results through observation and experimentation. Indeed, claims of "special creation" reverse the scientific process. The explanation is seen as unalterable, and evidence is sought only to support a particular conclusion by whatever means possible.

While the mechanisms of evolution are still under investigation, scientists universally accept that the cosmos, our planet, and life evolved and continue to evolve. Yet the teaching of evolution to schoolchildren is still contentious.

In Science and Creationism , The National Academy of Sciences states unequivocally that creationism has no place in any science curriculum at any level.

Briefly and clearly, this booklet explores the nature of science, reviews the evidence for the origin of the universe and earth, and explains the current scientific understanding of biological evolution. This edition includes new insights from astronomy and molecular biology.

Attractive in presentation and authoritative in content, Science and Creationism will be useful to anyone concerned about America's scientific literacy: education policymakers, school boards and administrators, curriculum designers, librarians, teachers, parents, and students.

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Formation of Earth

Our planet began as part of a cloud of dust and gas. It has evolved into our home, which has an abundance of rocky landscapes, an atmosphere that supports life, and oceans filled with mysteries.

Chemistry, Earth Science, Astronomy, Geology

Manicouagan Crater

Asteroids were not only important in Earth's early formation, but have continued to shape our planet. A five-kilometer (three-mile) diameter asteroid is theorized to have formed the Manicouagan Crater about 215.5 million years ago.

We live on Earth’s hard, rocky surface, breathe the air that surrounds the planet , drink the water that falls from the sky, and eat the food that grows in the soil. But Earth did not always exist within this expansive universe, and it was not always a hospitable haven for life. Billions of years ago, Earth, along with the rest of our solar system, was entirely unrecognizable, existing only as an enormous cloud of dust and gas. Eventually, a mysterious occurrence—one that even the world’s foremost scientists have yet been unable to determine—created a disturbance in that dust cloud, setting forth a string of events that would lead to the formation of life as we know it. One common belief among scientists is that a distant star collapsed, creating a supernova explosion, which disrupted the dust cloud and caused it to pull together. This formed a spinning disc of gas and dust, known as a solar nebula . The faster the cloud spun, the more the dust and gas became concentrated at the center, further fueling the speed of the nebula . Over time, the gravity at the center of the cloud became so intense that hydrogen atoms began to move more rapidly and violently. The hydrogen protons began fusing, forming helium and releasing massive amounts of energy. This led to the formation of the star that is the center point of our solar system—the sun—roughly 4.6 billion years ago. Planet Formation The formation of the sun consumed more than 99 percent of the matter in the nebula . The remaining material began to coalesce into various masses. The cloud was still spinning, and clumps of matter continued to collide with others. Eventually, some of those clusters of matter grew large enough to maintain their own gravitational pull, which shaped them into the planets and dwarf planets that make up our solar system today. Earth is one of the four inner, terrestrial planets in our solar system. Just like the other inner planets —Mercury, Venus, and Mars—it is relatively small and rocky. Early in the history of the solar system, rocky material was the only substance that could exist so close to the Sun and withstand its heat. In Earth's Beginning At its beginning, Earth was unrecognizable from its modern form. At first, it was extremely hot, to the point that the planet likely consisted almost entirely of molten magma . Over the course of a few hundred million years, the planet began to cool and oceans of liquid water formed. Heavy elements began sinking past the oceans and magma toward the center of the planet . As this occurred, Earth became differentiated into layers, with the outermost layer being a solid covering of relatively lighter material while the denser, molten material sunk to the center. Scientists believe that Earth, like the other inner planets , came to its current state in three different stages. The first stage, described above, is known as accretion, or the formation of a planet from the existing particles within the solar system as they collided with each other to form larger and larger bodies. Scientists believe the next stage involved the collision of a proto planet with a very young planet Earth. This is thought to have occurred more than 4.5 billion years ago and may have resulted in the formation of Earth’s moon. The final stage of development saw the bombardment of the planet with asteroids . Earth’s early atmosphere was most likely composed of hydrogen and helium . As the planet changed, and the crust began to form, volcanic eruptions occurred frequently. These volcanoes pumped water vapor, ammonia, and carbon dioxide into the atmosphere around Earth. Slowly, the oceans began to take shape, and eventually, primitive life evolved in those oceans. Contributions from Asteroids Other events were occurring on our young planet at this time as well. It is believed that during the early formation of Earth, asteroids were continuously bombarding the planet , and could have been carrying with them an important source of water. Scientists believe the asteroids that slammed into Earth, the moon, and other inner planets contained a significant amount of water in their minerals, needed for the creation of life. It seems the asteroids , when they hit the surface of Earth at a great speed, shattered, leaving behind fragments of rock. Some suggest that nearly 30 percent of the water contained initially in the asteroids would have remained in the fragmented sections of rock on Earth, even after impact. A few hundred million years after this process—around 2.2 billion to 2.7 billion years ago—photosynthesizing bacteria evolved . They released oxygen into the atmosphere via photosynthesis and, in a few hundred million years, were able to change the composition of the atmosphere into what we have today. Our modern atmosphere is comprised of 78 percent nitrogen and 21 percent oxygen, among other gases, which enables it to support the many lives residing within it.

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A Supermoon and Lunar Eclipse Will Light Up and Darken Tuesday’s Night Sky

Earth’s shadow will partially cover one of the biggest and brightest full moons this year.

By Katrina Miller

The night sky on Tuesday will treat us to a partial lunar eclipse during a supermoon. It’s an astronomical wonder that almost anyone in the Western Hemisphere under clear skies can experience, and it’s the latest reminder that our home planet is part of a bigger cosmic system.

When is the lunar eclipse, and where can I see it?

This lunar eclipse will happen between 8:41 p.m. Eastern time on Tuesday and 12:47 a.m. on Wednesday, with the maximum partial phase occurring at 10:44 p.m. It will be visible across most of the United States, Canada, Latin America and the Caribbean, Africa and Europe.

Bruce Betts, the chief scientist at the Planetary Society, will be watching the show from his front yard in Pasadena, Calif.

Eclipses offer a chance to feel “the three-dimensional nature of everything,” Dr. Betts said. “It’s something that just in a visceral way is like, wow.”

What’s the U.S. weather forecast for the eclipse?

People in states bordering the Mississippi River may have the best shot at an unobstructed view of the eclipse, with the skies forecast to become more clear as the event goes on. In addition, the Southern Rockies and the Southwest will have a beautiful evening for enjoying the event.

Up and down the East Coast, the likelihood of getting a glimpse of the moon through the cloud cover will be slim this evening, especially in the mid-Atlantic region. A few places, like portions of northern Maine, southern Georgia and north Florida, may have a chance.

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The Transbay Tube turns 50: Inside the groundbreaking history and future of the Bay's underwater crossing

An Engineering Marvel: "Nobody has a tube like ours"  

September 16, 1974, was a banner day in the history of the Bay Area. On this day, 50 years ago, BART’s Transbay Tube opened for passenger service.  

The 3.6-mile-long Tube was unlike anything the world had ever seen. At the time, it was the longest immersed tube in the world, a title it held until 2010, and it mystified the public, who were mesmerized by the idea of a miles-long structure sitting 135 feet below the water’s surface. To some, it was a preposterous notion – train rides under the ocean?!? -- but the nonbelievers quickly hushed on that fateful day in 1974. They, too, wanted to take an underwater voyage from Oakland to San Francisco.  

This is the story of a modern marvel – its past, present, and future. It is also a story about people – those who ushered the Tube into existence and those who are still working to ensure it stands for generations to come. Long after all of us have departed this earth, the Tube will remain.  

On the Transbay Tube’s 50th anniversary, BART honors this one-of-a-kind structure that has ferried millions of passengers across the Bay for five decades and will continue to do so for decades to come. In the above video, we invite you to look inside the engineering opus for yourself. 

1961: The first concept drawing of the Transbay Tube.  

It was 1973 when budding actor Kenny Meyers got a job power washing the Tube to prepare it for its public debut. After that first gig with BART, Meyers never left. He retired from BART after 48 years in 2021. 

Meyers still remembers showing up to that first interview in a suit and tie, not totally sure what position he had applied for. He needed a job and didn’t care much what it entailed.  

“During the interview, the supervisor asked one question: ‘Do you have jeans?” Meyers remembered. “When I nodded, he said, ‘Great. Wear ‘em tomorrow and show up here at this time.’” 

Meyers did as he was told, and after a month as a utility worker, he was sent to the Tube with a fellow worker and a 300-foot hose. Their task: wash the dusty walls before the structure’s premiere.  

"At the time, I didn’t know anything about the project,” said Meyers. “We were looking at each other going, what is this thing? At 20 years old, it was an eye-opening experience. That’s when I realized how crazy engineering is.” 

Not many people have had the chance to see the Tube where it lay – only specially trained divers who can navigate the zero-visibility waters and perhaps the ghosts of long-ago shipwrecks. It’s not just mud and muck down there, though there is plenty of that. There are waves of sand – not unlike the dunes on land – deep holes, and huge mounds of rock. Sharks, whales, porpoises, fish – all the creatures that call the Bay home – float above this feat of engineering that sits 135 feet below the surface at its maximum depth.  

It’s another world down there, and it’s in this world one is zooming through at 70 miles per hour when riding a BART train from Oakland to San Francisco.  

Most of us aren't paying a lot of attention when we make this familiar voyage. We’ve got things to do, phones to scroll, books to read. The exception may be the many generations of children mesmerized by the quick trip through the ocean’s depths, their eyes glued to the windows in the hopes of seeing a leopard shark or a playful seal. 

The Tube isn’t translucent, and if it were, you wouldn’t see anything that deep in the water anyways. But in 1974, not long after the Tube opened, Meyers wanted to make a couple of kids’ day.  

Various Transbay Tube mementos are pictured.  

He got his chance one afternoon when a train came to a halt right in front of a vent in the upper gallery of the Tube, where Meyers happened to be standing.  

“I pushed my face right up to the vent and made a face like I was swimming,” he said. “The kids screamed! They really thought I was swimming down there.” 

Back when the Tube first opened, Meyers said people thought of it like Disneyland’s Monorail or its submarine ride, where you look out the windows and see all the animatronic mermaids and fish flapping around.  

“People would ride it like they were at an amusement park, just for fun,” Meyers said. “It was one of a kind! It was over the top!” 

“There's nothing really down there, though. No Starbucks or anything,” Meyers clarified. “Maybe I should push for that...” 

A drawing of the cross-section of the Tube in the San Francisco Bay.

The diagram of the Tube is pretty simple when it comes down to it (at least it looks that way). There are two bores, each with one trackway. Trains move west from West Oakland Station through the right bore (when looking from Oakland) and east from Embarcadero Station through the left bore. It’s not visually dissimilar to a giant pair of super long-lensed binoculars.  

Between the bores there is an upper gallery and a lower gallery for pedestrian access, ventilation, and utilities. Workers use the galleries to get around the Tube, as evidenced by Meyer’s prank, sometimes on specialized vehicles that are akin to narrow golf carts. There are also bicycles on hand in case a cart loses its charge. Or you can make the trek back to land by foot.  

BART staff enter the tube via five-story vent structures located on either end. Once you descend, you either start walking, cycling, or hitch a ride on a cart. There’s nowhere to go but through or back where you came from.  

What’s it like down there? Pretty dark, for one, and despite cleanings, it’s a bit grimy thanks to all the brake dust from the trains. It’s also blustery due to fans that are constantly circulating air – you better be hanging onto your baseball cap if you get too close.  

Then there’s the motion of the trains coming in and out. “When the trains come through, there’s wind blasting through the cracks of the gallery doors,” Meyers said. “Once it leaves, there’s a suction effect. A train in a Tube is not unlike a piston.” 

Meyers added that the Tube has a very specific smell that he can’t quite place his finger on. “But after I get out of there, I’d smell like it for a week.” 

The Tube isn’t a Disney creation, but there's magic inside, especially when you’re zooming through in the front cab or the very end of a train. In the latter scenario, thanks to the glow of the brake lights, you feel like you’re passing through giant scarlet smoke rings, bigger than any talking caterpillar could blow, as you move through each of the structure’s 57 segments.  

By the way, Kenny Meyers isn't the only actor whose hung around the Tube. In 1971 while the structure was still being built, George Lucas filmed a scene for his first feature film, THX 1138. In the film, Robert Duvall is shown climbing "up" from the subterranean Orwellian world, but he's actually crawling horizontally through the Tube. The structure has also appeared in Predator 2, Eve of Destruction, Eye for an Eye, The Shannara Chronicles , and an episode of Nash Bridges, among others . Its sounds were even in the videogame Dead Space.   

I. An Underwater Bay Crossing is Born

With zero fictional embellishments, the Tube is as fantastic as something Ray Bradbury or Philip K. Dick might envision. So, we’re left with the question: How did BART bring this crazy thing from the realm of imagination into the realm of reality? 

You could say it started with San Francisco’s beloved eccentric Emperor Norton. In 1872, the man who proclaimed himself “Emperor of the United States” issued a proclamation proposing the idea of an underwater rail tunnel that would cross the San Francisco Bay. A few months later, he made another proclamation threatening to arrest Oakland and San Francisco leaders who rudely ignored his initial request.  

“Believing Oakland Point to be the proper and only point of communication from this side of the Bay to San Francisco, we, Norton I, Dei gratia Emperor of the United States and Protector of Mexico, do hereby command the cities of Oakland and San Francisco to make an appropriation for paying the expense of a survey to determine the practicability of a tunnel under water; and if found practicable, that said tunnel be forthwith built for a railroad communication.”   - Emperor Norton’s Tunnel Proclamation of May 12, 1872 

Fast forward to 1947, and Emperor Norton’s grand designs were finally being put into play. That was the year a joint Army-Navy Commission issued their recommendation for an underwater Tube to relieve traffic congestion on the ten-year-old Bay Bridge. Even then traffic was terrible, despite the relief provided by Key System streetcars that ran on the lower deck of the bridge until 1958.  

A copy of the front page of a 1920 San Francisco Chronicle.   

After multiple seismic studies, a soil investigation program, and even the installation of an earthquake recording system on the floor of the Bay, a preliminary route was selected that would avoid as much bay bedrock as possible. Actually, the route wasn’t that far off from the one Emperor Norton initially proposed. The cost of the project was estimated at $132,720,000 (or about $1.4 billion in 2023 dollars) and was to be financed by toll revenues. 

Top image: The route of the Transbay Tube depicted by the square dotted line in a photograph taken during the final stages of construction.   

Bottom image: 1967: A concept drawing demonstrating how the Transbay Tube would be deposited on the floor of the Bay.   

Construction began in 1965 as a joint venture of Peter Kiewit Sons’ Co.; Raymond International, Inc.; Tidewater Construction Corp.; and Healy-Tibbitts Construction Co. Together, they were known as the Trans-Bay Constructors. They kicked things off with a Tube groundbreaking ceremony in 1966 that featured a literal boatload of dignitaries watching a dredge bring up the first of more than 5.7 million cubic yards of Bay mud. It was groundbreaking in every sense of the word.  

The contract called for a demanding two-and-a-half-year deadline for completion of the basic structure, so the Trans-Bay Constructors developed a fierce schedule at a pace of building and placing two Tube sections a month. There simply wasn’t time to make mistakes. 

A photo of a section of the Tube under fabrication at Bethlehem Shipyards in South San Francisco.   

The Tube is made up of 57 sections, each averaging 330 feet in length. The segments were fabricated at Bethlehem Shipyards in South San Francisco, from which they were launched, towed into the bay, and then sunk into their proper positions.  

To get the just barely buoyant segments to the bottom of the bay, each was weighted with 500 tons of gravel ballast. One section weighed approximately 10,000 pounds, about 1/6 of the weight of a BART car. Lowering one segment required a complex system of hydraulic controls and strain gauges that allowed operators to monitor the weight distribution and keep each section level during descent.  

Circa 1969: A section of the Transbay Tube being floated into the Bay.  

Divers were waiting at the bottom of the bay to direct the segments into exact position. When they surfaced, the divers climbed into two decompression chambers on the barge. It wasn’t 20,000 leagues under the sea, but it was still pretty darn deep.  

Concurrently, the trench for the Tube continued to be excavated. About 5.7 million cubic yards of material were removed from the bay. To keep the trench precisely aligned, engineers installed lasers from different shore positions that pinpointed the exact position required for the dredging barges.  

In the end, the contractors met their schedule. The last section of the Tube was placed just east of Yerba Buena Island in 1969. Later that year, BART held an open house on both sides of the Bay, inviting members of the public to walk through two sections of the Tube. It was reported that the line to get inside stretched more than a mile.

With the segments in place and any stragglers cleared, the next tasks were laying the track, electrification, and installing the train control equipment and ventilation. The process took about four years, and in August 1973, the first train traveled through the Tube. About a year later, trains were carrying their first passengers across the bay.

Come 1974, the original BART system was officially complete.  

Tube sections were built from both the San Francisco and Oakland sides. When the last bulkhead was removed to finally connect the Tube, workers from the San Francisco side found Oakland-side workers Don Hughes and Shad Wilson enjoying champagne together.

II. Loma Prieta: "BART's Finest Hour"

The Tube was just a teenager, barely old enough for a driver’s license, when it was put to a terrifying test. On Oct. 17, 1989, the devastating Loma Prieta earthquake shook the San Francisco Bay Area and Central Coast for eight to 15 seconds, depending on where you were located. Sixty-three people died and 3,757 people were injured. In the aftermath, among other impacts, 12,000 homes and 2,600 business were damaged; the Cypress Street Viaduct on Interstate 880 collapsed; and about 1.4 million people lost power, temporarily knocking radio and tv stations off the air.  

Then there was the San Francisco-Oakland Bay Bridge. A 76-foot-by-50-foot section of the upper deck fell onto the lower deck, causing severe damage and one fatality. The bridge is pictured after the earthquake (image courtesy of Flickr ). 

"There were reports of the bridge being down, flooding and fires in the city, we knew it was serious,” said Frank Wilson, BART General Manager at the time of Loma Prieta, in a 2014 interview with BART . “We thought, 'If the damage was that severe around us, what has BART sustained?' It was all hands-on deck to assess safety and find out how we were going to get back into service." 

At the same moment Loma Prieta started shaking the Bay, Donna “Lulu” Wilkinson was operating a train at 80 miles per hour through the Transbay Tube.  

“I didn’t even feel it,” she said in a 2014 interview. She was halfway through the Tube when the Operations Control Center gave the order to stop and hold position. Passengers didn’t panic because they were accustomed to short holds in the Tube – standard operating procedure even for minor earthquakes. Most of them didn't notice a thing.  

“The Tube absorbed the shock,” Wilson said. “When the engineers and planners first built the system, they were criticized by some from the outside that it was too expensive and unnecessary. But how much is a life worth? How much was it worth to keep people moving between Oakland and San Francisco?” 

Calls began flooding into the Operations Control Center from employees asking for earthquake assignments, while those on the job gathered their wits and began inspecting the system from top to bottom for damage. Incredibly, damage was minimal, and less than 12 hours after the earthquake hit,  BART service resumed. 

Loma Prieta and the weeks that followed are often referred to as “BART’s finest hour,” and with good reason. Staff quickly jumped to take on extra shifts so BART could run 24-hour service in the Transbay Tube, which started the day of the earthquake and ended December 3. The Bay Bridge did not reopen until November 18. 

Said Wilson: “Not only did they get BART back on track, they kept it there for more than a month while the Bay Bridge was out of service, carrying more than 10 million passengers to their jobs and homes around the clock.”  

III. The Earthquake Retrofit: “Like doing open heart surgery every night then bringing your patient back to life, over and over and over again” 

A diagram of the Transbay Tube that shows the outer steel shell, concrete liner, and steel plates that line the Tube’s interior. 

Loma Prieta had a magnitude of 6.9. What would happen to the Tube in the event of a once-in-a-thousand-years earthquake? 

This was the thought rumbling around the mind of Jim Dunn, BART’s then-Chief Engineer, when he instigated what became known as BART’s Earthquake Safety Program (ESP) in the early 2000s. The huge project was officially completed this year.  

“Dunn knew Loma Prieta was not the worst-case scenario. He wanted to know if a real big one would give us a problem,” said Tom Horton, who was BART Group Manager of ESP before retiring in 2017.  

With ESP, BART was not just preparing for a major earthquake, but one that might be centered closer to the core system, which runs directly adjacent to the San Andreas and Hayward faults. 

On November 2, 2004, voters in Contra Costa, San Francisco, and Alameda counties approved Measure AA, which allowed BART to issue general obligation bonds to fund up to $980 million of the $1.4 billion total cost of earthquake safety improvements. The team hit the ground running. The Tube retrofit began years later, in 2017.

“We started with a major evaluation of the system and how vulnerable it was to earthquakes,” Horton said. When Horton started, he had three employees. At the height of the project, more than 100 people were working on the program, 30 of whom were devoted to ESP full-time.  

“I have immense respect and appreciation for the personnel who worked on this,” said Zach Amare, BART Assistant Chief, Infrastructure Delivery. Nearly every department at BART had a role to play, including but not limited to the Operations Control Center, Traction Power, Track Maintenance, System Safety, Power/Mechanical Engineering and Maintenance, Operations Planning, Locomotive Operators, Operations Liaisons, Government and Community Relations, Communications, and many, many more.  

He credits the project’s success to four things: dedicated BART personnel; executive management who offered full support from the get-go; skilled contract personnel “who were up for the challenge to solve problems”; and a cohesive project management team. 

“Without these elements, this complicated, challenging project would not have been successful,” Amare said.  

Chuck Bernardo, BART Group Manager of Capital Projects, said he “didn’t realize how one-of-a-kind the Tube was until I started working on it.” 

“The challenge of the retrofit appealed to me,” he said. “You’re working at night and running the system the rest of the time. It was like doing open heart surgery every night then bringing your patient back to life, over and over and over again. You had to just dive right in. No one could tell you how to do it." 

A key piece of the Earthquake Safety Project was the seismic retrofit of the Transbay Tube, deemed BART’s most valuable asset.  

“It was the most difficult engineering project I have worked on with many unexpected challenges along the way, both in design and construction,” said Mark Salmon, who helped coordinate the retrofit design work for BART. "It really showed me the power of teamwork and perseverance. In many respects the Tube is unique in the world; there is no other similar example we could use to draw experience from and guide us. We had to solve the design and construction problems ourselves.” 

We want to be very clear: The Transbay Tube is structurally sound, but we are preparing for a rare and devastating earthquake, defined as something that happens once every thousand years. In an event this large, the Tube won’t fail, but it could crack and leak.  

“Underground structures in general are less of an issue because they don't see the same level of ground motion as the surface,” Horton said. “The wave is confined by the soil if it’s underground, but when it hits the surface, you get big movements.” As evidenced by the Tube travelers during Loma Prieta, the Tube may be one of the safer places to be during a big quake.  

BART worked with earthquake retrofit specialists using a combination of geotechnical and structural site investigations, computer simulations, and testing of materials and models to develop the retrofit design. 

Part of the modeling process including traveling to UC Davis, which has an old NASA centrifuge once used to train astronauts. Now, researchers use the centrifuge to simulate the pressures earthquakes could apply to buildings, and in our case, the pressures the Tube might face in a major shaker.  

Back in the 1960s, engineers designed the Tube to flex with earthquake ground motions to dissipate earthquake forces and absorb shock. A stiff Tube would become brittle and break in an earthquake, so the “Tube wiggle” is crucial. 

An image of one of three locomotives on the work train used during the Transbay Tube retrofit.  

At both ends of the Tube, there are giant seismic joints that allow the two ends of the Tube to move independently without damaging the structure. During the retrofit, the original joints were modified to increase their movement capacity. Now, the two sides can “separate” further without major damage to the Tube. 

On the Oakland side of the Tube, a steel liner was added to withstand the expected large forces this area could experience in an earthquake. The steel liner is both a structural fix and a means of preventing leakage. 

"There could be flooding in the Tube due to a 1,000-year earthquake, but it would be slow enough that people could get out safely,” Horton said. As it turns out, the Tube is predicted to sustain very little damage in a 500-year earthquake event. 

BART engineers addressed the potential for leakage after a severe earthquake with two elements. The first was to install the steel inner liner in the sections most likely to experience the largest movements. The second was an upgrade to the Tube’s pumping system to enable it to handle a higher volume of water and pump at a greater rate.  

The steel plate handling machine deserves some special attention as it was crucial to the project. 

The machine, transported into the Tube by a specialized work train (more on that below), placed steel plates fabricated to match the Tubes arched geometry. Once secured by concrete anchors, they were welded together. Grout was then used to fill the void between the existing concrete liner and new steel liner. A different method utilizing a plastic sheet membrane was employed on the Tube section supporting the trackway.

During the retrofit, every minute was precious. When Horton visited the steel liner operation during a work night, he said it ran “like a military exercise.”  

"As soon as the work train arrived, workers got off and went right to work, each person knowing exactly what their job was and going right at it.” 

Some workers put in 12-hour shifts, six days a week, to get the work done. This dedicated cohort includes Frank Greco, BART Structures Equipment Operator, who was one of the operators of the 800-foot, custom-built locomotive train. 

“Working those long hours was a sacrifice, for sure,” Greco said. “But I felt like I was a part of something bigger than myself. The Tube is such a big part of BART. It’s something thousands of people travel through on a weekly basis. Just to be a part of that, I felt very fortunate.”  

Greco said community sprung up around the painstaking work and the hundreds of hours they spent underwater. Often, the community was motivated by food. BART Operations Supervisor Harold Stuart would make regular Costco runs and bring giant boxes of snacks into the Tube for the workers. As Kenny Meyers said in the first half of this story, there’s no Starbucks down there.  

Greco said his first trip into the Tube made him "trip out.” 

“I loved it,” he said. “You’re in the middle of the water! It’s crazy! It’s unreal!”  

A photo of the completed retrofit inside the Tube.

Most of the time, Greco tried not to think about how deep underwater he was on those long graveyard shifts. It’s easy to psych yourself out down there, he said.  

Will Sowell, Jr., BART Structures Equipment Operator, also served as a work train operator, and initially, he didn’t want to do it.  

Sowell was used to big equipment, but not that big. The closest thing to the work train he’d operated was a power flat machine.  

He and other work train operators received specialized training to operate the work train, which can hold up to three 50-foot-long flatcars, seven 40-foot-long flatcars, two 41-foot-long concrete cars, one 55-foot-long steel plate handling device, and one 40-foot-long plate handling device flatcar. Say that five times fast.  

"The first time being on that machine was scary,” Greco said of the work train. “You know what you’re doing, but when you throw in the train with a lot of power and a braking system that’s intricate, it is a lot to handle.” 

Greco said he got used to it quickly. “And at the end of the day, not a lot of people can say they’ve operated a huge train with 17 cars. That’s something to put on your resume for sure.” 

Getting the job done in such a tight timeline took serious coordination and logistics.  

Bernardo said the Contractor would work backwards to build each night’s work schedule. They had to ask questions like: How much concrete do I need? How many steel plates have to go up? How many anchors and ties do we bring?

“The project team and contractor were constantly thinking about how to work under the constraints of the Tube, Bernardo said. “As we worked backwards, we’d realize, oh, we can’t do this task because this is in the way. Then, if we remove that first task, we have to find a place to put it back in.”  

Sowell witnessed these logistics in real-time. Every night before leaving the Oakland Yard, Sowell and his team would go through an extensive safety and supply check for the work train to make sure they had everything they needed, the cars were in the right order, all the equipment was secure, and the brakes, horns, bells, whistles, etc., were working properly. Then Operations Control Center would give the cue for the train to head out. Since work began at 9pm, passenger trains would share the tracks with the work train as it made its way underwater.  

"Every aspect of this project was unique because the Tube is unique,” Horton said. There's just nothing quite like our Tube.  

IV. BART’s Cathodic Protection System: “Without this system, the Tube would be gone by now” 

The Transbay Tube is an underwater steel structure, meaning it is prone to rust. To extend the life of the tube, BART employs a cathodic protection system, which prevents this from happening using electrochemistry.  

For BART's cathodic protection system, an external power source is used to push current from the anode sleds (an old, rusted anode is pictured in the first photo in the above slideshow) to the steel structure of the Tube, which acts as a cathode. The anode sleds sacrifice themselves (i.e. rust) to protect the cathode -- in our case the steel structure of the Tube.

There are a total of 30 400-pound anode sleds on the Tube, each containing16 anodes apiece. A huge wire connects the anodes to a power box contained within the Tube, which pumps 200 amps of current -- that's equivalent to the energy needed to power an entire modern home! 

“Without this, the Tube would be rusted by now and require billions in replacement costs,” said Senior Manager of Engineering Programs Maansii Chirag Sheth, who oversees the cathodic protection system. And don’t worry about the marine life, she stressed. “If you’re a little fishy and you touch the Tube, you won’t feel the current. It’s not an electric current that’ll shock you.” 

Maansii is one of just a handful of certified cathodic protection engineers in the U.S. The work is extremely specialized, and corrosion engineers must take a difficult certification test. Maansii passed the test in 2013. She was just the 39th person in California to receive the specialized certification. 

Large clamshell dredging bucket being lowered into the bay as part of soil handling operations.  

Every few years, the rusted anodes must be replaced, and only a handful of contractors in the U.S. are equipped for this sort of specialized, largescale work, Maansii said.  

Replacing a 400-pound anode attached to a Tube more than 100 feet deep in zero-visibility waters is a tall task, as you can imagine. Best-case scenario, the replacement of a single anode takes about a half a month. With shifting currents and wind, the anodes can move slightly, and down there, you can’t see a thing. The work is faster now that GPS systems have improved, which make it easier to locate the anode in the water. 

“It’s a complex operation with a large amount of variables at play. You have to account for weather and seasons, contracting and securing a barge, hiring trained divers, leveraging specialized equipment, and obtaining permissions from the Coast Guard, and so on," Maansi said.

Diver stepping off barge into the Bay as part of cable installation operations near the Transbay Tube.

You also can’t work when it’s stormy or during certain months, including the entirety of Dungeness crab season. One must manage tide cycles, passing vessels, soil handling restrictions – the list goes on.  

Once all that’s taken care of, it’s time to get down to work. First, a large clamshell bucket on the barge digs out a bunch of mud to expose the anode. Deep-sea divers specially trained for industrial projects pinpoint the exact location of the anode, which is detached then lifted from the water onto the barge. Then, the new anode is lowered down and attached to the cable system. After testing, the anode is energized, and its sacrificial work begins.  

Let us all take a moment to thank the sacrificial anodes for their service. Without them, no Tube would be there and no connectivity in the Bay.  

V. The Transbay Tube’s Next Chapter: “It could be under the Bay forever” 

Though this story is coming to an end, the story of the Tube is far from over. The Tube is fifty, true, but we can’t say it’s hit middle age just yet. 

Thanks to the Earthquake Safety Program, those sacrificial anodes, and the many BART staff who work to keep it maintained, the Tube’s got quite a life ahead of it. 

What might the future hold? Currently, BART is working on a project that will allow us to operate up to 30 ten-car trains per hour in each direction through the Tube, increasing capacity and shortening wait times at stations. Currently, a maximum of 24 trains per hour can travel through the Tube in each direction.  

The program includes four elements: the purchase of additional railcars, a new railcar storage yard at the Hayward Maintenance Complex, a new communications-based train control system that will allow shorter waits between trains, and five additional traction power substations to provide the additional power needed for more frequent service.  

It’s a bright future for a structure with a history long enough to fill the pages of a whole book. We’ll save that project for another day.  

“Nobody has a Tube like ours,” said Horton in conclusion. “It could be under the Bay forever. That gives you something to think about.”