Solar System History 101 (2024)

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

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

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

The Sun Shines

The Big Bang brought the Universe into existence 13.8 billion yearsago. Our solar system formed much later, about 4.6 billion years ago. Itbegan as a gigantic cloud of dust and gas created by leftover supernovadebris—the death of other stars created our own. The cloud, whichorbited the center of our galaxy, was mostly hydrogen with some heliumand 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 whichpoint temperatures and pressures in the core became so intense thathydrogen began fusing into helium.

And then there was light. Hydrogen fusion released tremendous amountsof energy that countered the Sun’s gravity, stabilizing the young starand keeping it from accumulating more material out of the rotating diskof leftover debris around it. The Sun entered the longest phase of itslife, becoming a main sequence star. It is still in this phase today andwill 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. Thisprocess continued for several thousands of years, formingkilometer-sized objects big enough to gravitationally attract eachother. This led to more collisions and accretions, forming Moon-sizedprotoplanets in less than a million years.

In the inner, hotter part of the solar disk, the planets grewprimarily from rocks and metals because it was too warm for water andother volatiles—substances that evaporate at room temperature—tocondense. Up to hundreds of these worlds collided and combined in theinner solar system for about 100 million years until only four largebodies remained: Mercury, Venus, Earth, and Mars.The inner planets didn’t get as big as the outer planets because thepercentage of rocks and metals available in the Universe—and thus oursolar system’s starting materials—is lower compared to hydrogen, heliumand 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 offMercury’s outer layer, which would explain why the planet’s core makesup so much of its volume. The resulting debris may have spread out intospace instead of forming a moon.

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

Saturn formedshortly after, amassing less material since Jupiter gobbled such a largeportion of the outer disk. With little hydrogen and helium left, thenext planets to form––Uranusand Neptune––accumulatedmore 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 planetsthere, causing them to either collide and shatter with other bodies orleave the region. This process took a few ten million years afterJupiter’s formation, leaving the asteroid belt with only small bodies ofrock, ice and metal that collectively weigh less than 1% of Earth’smass. Ceres, the largest object in the asteroid belt, is considered anoutlier 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 thebelt was much denser than it is today. Just as Earth’s Moon formed aftera collision between Earth and another world, similar smashups in theKuiper 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 sameway. Some outer planet moons like Triton at Neptune may have beenindependent worlds captured by the giant planets’ gravity fields.

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

Giant Planets Wreck Havoc

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

Simulations suggest that the orbits of the giant planets shiftedabout 4.1 billion years ago. Gravity from the numerous Kuiper beltobjects nudged Jupiter and Saturn into a 2:1 resonance, meaning Jupiterorbited the Sun twice for every Saturn orbit. This periodically broughtthe two planets close together, causing wide-ranging gravitationaleffects.

Uranus and Neptune got pushed further away from the Sun, ploughingthrough the Kuiper belt, scattering most of its objects either inward oroutward over the next millions of years. Any additional ice giants thathad formed were kicked out of the solar system entirely. The outwardlyscattered worlds formed today’s sparsely populated Kuiper belt and thefarther-away sphere of icy bodies we call the Oort cloud. This is wheremost comets come from.

The inwardly scattered worlds raced through the inner solar system,smashing into the worlds there and creating basins as large as athousand kilometers or more on Mercury, Venus, Earth, the Moon, andMars. 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 water3 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 Earthis 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 calmerplace. Asteroid impacts still happen but their frequency and the sizesof impacts have reduced drastically. There is still cause for concern assome relatively large impacts have happened in the last 100 millionyears:

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 cityof Chelyabinsk, damaging buildings and sending more than a hundredpeople to area hospitals. Our planet remains at risk for dangerousasteroid impacts, highlighting the need for planetary defense.

Space Exploration Teaches the Timeline

How do we know all this? Space exploration missions, Earth-basedobservations, and other scientific activities help us piece together ourpast. 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 disksthat will evolve into star systems like our own. We know the Sun formedalongside 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 fromthe original disk that formed the solar system have fallen on Earth asmeteorites. Using extensive laboratory analysis, scientists found theoldest 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 theevidence on Venus, Earth, and Mars. Fortunately, the airless Moon stillbears scars from those days that you can see from Earth. Moon samples brought back from Apollo missionsrevealed the ages of some of its largest basins to be between 4.1 to3.8 billion years old, which is how we even inferred the possibility ofsuch 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 orbitaround the Sun. Jupiter’s journey from its original location may beimprinted on these asteroids as they moved along with Jupier. A NASAmission called Lucyis slated for launch in October 2021 to visit 7 of Jupiter’s Trojansfor the first time, helping us figure out what really happened in theearly solar system.

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

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