Formation and evolution of the Solar System

• 1. Introduction

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  The formation and evolution of the Solar System began not with a burst of light, but with a quiet collapse. About 4.6 billion years ago, a fragment of a giant molecular cloud gave way under its own gravity. That collapse started a chain of events that produced the Sun, the planets, our Moon, and every asteroid, comet, and icy body in the outer reaches of space. What set it off? Why does Earth have water at all? And what will become of all of it, in the end? These are the questions at the heart of one of science's longest-running investigations, one that draws on astronomy, chemistry, geology, physics, and planetary science all at once. The oldest solid material yet found, locked inside ancient meteorites, dates to 4,568.2 million years ago. That number is, by one definition, the age of the Solar System itself.

• 2. The Nebular Hypothesis

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  Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace each contributed to the idea now called the nebular hypothesis in the 18th century. The hypothesis holds that the Solar System grew from a collapsing cloud of gas and dust, with the Sun forming at the center and the remaining material flattening into a disc from which everything else took shape. It sounds tidy, but for a long time the idea had a serious problem: critics pointed out that the Sun rotates far more slowly than the planets orbit it, which seemed to contradict the physics the model predicted. The hypothesis fell out of favour until the early 1980s, when studies of young stars revealed them surrounded by cool discs of dust and gas, exactly as the model requires. That evidence brought the hypothesis back into wide acceptance. Since the discovery of exoplanets in the 1990s and the dawn of the Space Age in the 1950s, researchers have continued pushing the model to account for new observations it did not originally anticipate. Arthur Stanley Eddington's work on Albert Einstein's theory of relativity led Eddington to establish that the Sun's energy comes from fusing hydrogen into helium in its core. In 1935, Eddington proposed that other elements might also form inside stars. Fred Hoyle later argued that red giants forge many elements heavier than hydrogen and helium, and that when those stars shed their outer layers, those heavier elements are recycled into new star systems.

• 3. Birth of the Sun

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  The presolar nebula most likely formed at the edge of a Wolf-Rayet bubble and covered a region about 20 parsecs across, with individual fragments roughly 1 parsec wide. The composition of that region was nearly identical to what the Sun contains today: hydrogen, helium, and trace lithium from the Big Bang making up about 98% of the mass, with the remaining 2% consisting of heavier elements forged in earlier stellar generations. Some scientists have given the name Coatlicue to a hypothetical star whose supernova may have triggered the Solar System's birth by compressing the surrounding cloud into collapse. Evidence for a nearby supernova comes from traces of iron-60 found in ancient meteorites. Iron-60 is a short-lived isotope produced only in exploding stars, and its even distribution throughout the Solar System points to a supernova injection occurring well before any of the nebular dust had accreted into planetary bodies. The collapse was not instantaneous. Conservation of angular momentum caused the cloud to spin faster as it shrank, and over roughly 100,000 years gravity, gas pressure, magnetic fields, and rotation together flattened the nebula into a spinning disc about 200 AU across, with a hot, dense protostar at its center. At that stage, the young Sun would have been classified as a T Tauri star, surrounded by a disc that could extend to several hundred AU. The Hubble Space Telescope has photographed protoplanetary discs up to 1,000 AU in diameter in star-forming regions including the Orion Nebula. Within 50 million years, conditions at the Sun's core became extreme enough for hydrogen fusion to begin, marking its entry into the main sequence, the stable phase it still occupies today.

• 4. Building the Planets

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  Accretion, the process by which small dust grains stick together and grow, is how the current scientific consensus explains planet formation. Grains in the solar disc first clumped into bodies up to 200 metres across, then collided to form planetesimals roughly 10 kilometres in size, growing at the rate of centimetres per year over a span of millions of years. Inside a boundary called the frost line, located between the present orbits of Mars and Jupiter, temperatures were too high for water and other volatile compounds to solidify. Only materials with high melting points, such as iron, nickel, aluminium, and rocky silicates, could condense there. Because those compounds make up only about 0.6% of the nebula's total mass, the inner planets could not grow very large. Mercury, Venus, Earth, and Mars all trace their origins to this region. Beyond the frost line, icy compounds were far more abundant, letting the outer planet cores grow massive enough to gravitationally capture hydrogen and helium. Today, Jupiter, Saturn, Uranus, and Neptune together hold just under 99% of all mass orbiting the Sun. Saturn may be less massive than Jupiter simply because it formed a few million years later, when less gas remained available in the disc. Uranus and Neptune are sometimes called failed cores because the strong solar wind from the young T Tauri Sun had already blown away much of the disc material by the time they formed, limiting how much hydrogen and helium each could accumulate. After three to ten million years, that same solar wind cleared the remaining gas and dust from the disc entirely, ending planetary growth.

• 5. Migration and Bombardment

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  After the gas cleared, the Solar System did not settle quietly into its present arrangement. The outer giant planets continued changing orbits for hundreds of millions of years, driven by gravitational interactions with the vast swarm of planetesimals still surrounding them. According to the Nice model, about 500-600 million years after the Solar System formed, Jupiter and Saturn fell into a 2:1 resonance, meaning Saturn completed one orbit for every two of Jupiter's. That resonance delivered a gravitational push to the outer planets, and Neptune may have surged past Uranus and ploughed into what had been a much denser, more compact Kuiper belt than the one we see today. The scattered icy bodies from that disruption were flung inward by Neptune, then handed off to Jupiter, whose gravity either ejected them from the Solar System entirely or hurled them into the distant Oort cloud. Some of those same scattered objects, including Pluto, were caught in gravitational resonances with Neptune and remain locked there. A June 2011 study from Southwest Research Institute, known as the Grand Tack hypothesis, proposed that Jupiter migrated inward as far as 1.5 AU before Saturn formed, then both planets reversed course and moved back outward. That migration may explain why Mars ended up so small: Jupiter consumed much of the material that would otherwise have built a larger planet. The same simulations reproduce features of the modern asteroid belt, including both dry asteroids and water-rich objects resembling comets. The disruption caused by the outer planets' migration is also thought to have triggered the Late Heavy Bombardment, a period hypothesised to have occurred around 4 billion years ago. Evidence of that bombardment is still visible in the cratering on the Moon and Mercury. The oldest known signs of life on Earth date to 3.8 billion years ago, almost immediately after that bombardment is thought to have ended.

• 6. Moons and Tidal Forces

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  Earth's Moon is thought to have formed from a single large collision near the end of the period of giant impacts. The impacting object probably had a mass comparable to Mars. The blow kicked mantle material into orbit, and that debris coalesced into the Moon. Mars's two small moons, Deimos and Phobos, are thought to be captured asteroids rather than collision products. The large inner moons of Jupiter and Saturn, including Io, Europa, Ganymede, and Titan, likely formed from discs of gas and dust circling those planets in much the same way the planets themselves formed around the Sun. The outer irregular moons of the giant planets, by contrast, tend to be small, follow eccentric and inclined orbits, and mostly travel in the direction opposite their planet's rotation. All of those are signatures of captured bodies. The largest irregular moon in the Solar System is Neptune's Triton, thought to be a captured Kuiper belt object. Tidal forces govern how moon systems evolve over time. When the Moon orbits in the same direction as Earth's rotation and Earth spins faster than the Moon orbits, angular momentum transfers from Earth's spin to the Moon's orbit. The Moon gradually spirals outward while Earth's rotation slowly slows. Phobos, by contrast, orbits Mars faster than Mars rotates, so angular momentum flows the other way: Phobos spirals inward and is expected to be torn apart by tidal stresses or crash into Mars within 30 to 50 million years. Triton faces a similar fate in about 28 billion years. Pluto and its moon Charon, by contrast, are tidally locked to each other, so no angular momentum is being exchanged and their orbits remain stable.

• 7. The Sun's End and After

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  In roughly 600 million years, the Sun's brightening will disrupt Earth's carbon cycle to the point where forests can no longer survive. Around 800 million years from now, it will have killed all complex surface and ocean life. By 1.1 billion years from now, the habitable zone will have moved outward past Earth's orbit, leaving only single-celled organisms possible. Around 5.4 billion years from now, hydrogen fusion will begin in the shell surrounding the solar core, triggering the red-giant expansion. Within 7.5 billion years, the Sun will have grown to a radius of 1.2 AU, about 256 times its current size. Its surface temperature will drop to roughly 2,600 K while its luminosity reaches up to 2,700 times present levels. Mercury and Venus will be swallowed. Earth's fate is less certain: a study from 2008 concluded that Earth will possibly be slowly vaporized through tidal interactions with the Sun's outer envelope, though more recent research adds nuance to that picture. After the red-giant phase, the Sun will pass through a helium-burning horizontal-branch phase and then an asymptotic-giant phase lasting about 30 million years, before ejecting its outer layers as a planetary nebula over roughly 100,000 years. What remains will be a white dwarf containing 54% of the Sun's original mass but no larger than Earth. That white dwarf will gradually cool over billions of years. About 4 billion years from now, the Andromeda Galaxy, currently approaching the Milky Way at about 120 km/s, will begin to collide with it. Astronomers calculate a 12% chance that the Solar System will be pulled into the Milky Way's tidal tail during that event, and a 3% chance it will become gravitationally bound to Andromeda. Over the course of roughly 6 billion years the two galaxies will merge into a giant elliptical galaxy, and the Solar System will most likely be pushed into that new galaxy's outer halo. Eventually, after about 1 quadrillion years, the gravity of passing stars will have stripped the cold, dim Sun of every remaining planet, and the Solar System, in any meaningful sense, will cease to exist.