How the Solar System formed

The molecular cloud

Before the Sun, this region of the Milky Way held a molecular cloud: cold gas and dust, suspended in one of the Galaxy's arms. Clouds of this kind measure from a few tens to a few hundred light years across, weigh from ten thousand to a million times the Sun and sit at around 10 kelvin, about −263 °C. At that temperature hydrogen exists in molecular form and the density stays extremely low: a hundred to a thousand particles per cubic centimetre, a better vacuum than anything you can produce in a laboratory, but spread over enormous distances.

The composition is almost all hydrogen and helium. The little that remains — carbon, oxygen, silicon, iron — comes from earlier generations of stars, and it is the material the rocky planets, and whoever lives on them, are made of.

One detail of the most primitive meteorites suggests the cloud was not alone. Their grains preserve the traces of aluminium-26, a radioactive isotope with a half-life of about 700,000 years: for it to be present in the starting material, it must have been produced shortly before and not far away. The evidence suggests the involvement of a massive star nearby, which exploded as a supernova or otherwise reached the end of its life, and which would have both enriched the cloud with freshly forged elements and compressed one of its regions until it collapsed. It is a robust hypothesis, not a certainty: that scene cannot be reconstructed in detail.

The collapse

When internal pressure is no longer enough to balance gravity, the densest region of the cloud starts to fall in on itself. The free-fall time depends only on the density of the gas: for a molecular cloud core it runs from a hundred thousand to a million years. It is not an instantaneous crash, but on an astronomical scale it is fast.

The date is known with surprising precision. The calcium-aluminium-rich inclusions contained in chondritic meteorites are the first solids to condense out of the disk, and lead-lead dating puts them at 4,567.30 ± 0.16 million years. More recent analyses have proposed slightly different values, around 4,568.7 million years. In both cases the uncertainty is a few hundred thousand years out of four and a half billion: about 0.01%. Few other quantities in astronomy are known this well.

The collapse is not symmetric. Matter converges on the centre, where density and temperature climb fast, while around it an envelope keeps falling in. By the end of the process 99.8% of the available mass will have ended up in the central object. Everything else — eight planets, hundreds of moons, millions of asteroids and comets — will be built out of the 0.2% left over.

The spinning disk

The original cloud was rotating, if slowly. As it contracts, angular momentum is conserved: the same rotation spread over a much smaller radius becomes a much faster rotation, like a skater pulling in her arms. The infalling matter can no longer reach the centre directly. The matter with more angular momentum settles into orbits, and collisions between those orbits flatten the whole into the plane perpendicular to the rotation axis.

In about a hundred thousand years the three-dimensional, disordered cloud becomes a protoplanetary disk: thin, spanning a hundred or so astronomical units (one astronomical unit is the mean Earth-Sun distance, 149.6 million km), with the protosun at the centre still wrapped in gas. This is not theory alone: radio telescopes see disks of the same kind today around young stars, some already grooved by rings and gaps carved out by bodies in formation.

The disk explains a regularity that would otherwise be strange. All the planets orbit on almost the same plane, in the same direction, and almost all of them spin in the direction they orbit. That geometry is the imprint of the disk they were born from, still readable four and a half billion years later.

The protosun ignites

At the centre of the disk, contraction keeps turning gravitational energy into heat. The protosun burns deuterium first; then, when the core reaches about 15 million degrees, the fusion of hydrogen into helium ignites: from that moment the star runs on its own energy and stops contracting. On exactly when the Sun reached the main sequence, estimates differ, from a few million to a few tens of millions of years after the collapse.

Before getting there, the young star goes through the T Tauri phase: fast rotation, intense magnetic fields, jets along the axis and a stellar wind much stronger than today's. That wind, together with ultraviolet radiation, sweeps the gas out of the disk from the inside outwards. Observations of young star clusters give a median disk lifetime of around 3 million years: past that point, the gas is gone.

It is a deadline that matters more than it looks. The giant planets have to capture their hydrogen and helium atmospheres while the gas is still there; whatever arrives late is left with a core of rock and ice and nothing else. When the disk empties, what remains is the dust and the solid bodies that have already grown: from then on planets are built by collision, not by gas capture.

The snow line

The disk is not the same temperature everywhere: it is hot near the star and cold far away. So there is a distance beyond which water stops being vapour and condenses into ice. Estimates place it around 2.7 astronomical units from the Sun, where the temperature drops below about 170 K. It is not a fixed boundary: it moved inwards as the disk cooled.

The jump is abrupt because water is abundant. Beyond the snow line the amount of solid material available increases several times over, and bodies grow much faster. It is the most widely accepted explanation for the division we still see today: inside the line formed Mercury, Venus, Earth and Mars, small, dense and poor in volatiles; outside it Jupiter grew first, its core becoming massive enough to hold on to hydrogen and helium while the gas was still there.

One question stays open. If the Earth formed inside the snow line, where does its water come from? There are two main hypotheses: water-rich planetesimals arriving from outside, or water already bound to the dust grains from the start. The isotopic composition of the oceans constrains the proportions, but it has not yet closed the question.

The planetesimals

The step from dust grain to planet is the worst understood part of the whole story. Micrometre-sized grains stick to each other through electrostatic forces until they form millimetre-sized aggregates, but there the mechanism jams: centimetre-sized bodies tend to bounce instead of sticking, and metre-sized ones spiral into the star within a few hundred years through friction with the gas. One metre is the size at which, on paper, nothing should survive.

The most widely accepted way out is the streaming instability. Pebbles dragged along by the gas concentrate into filaments dense enough to collapse under their own gravity, skipping the problematic sizes in one go: the simulations produce planetesimals of around 100 km directly. From then on gravity does the rest, the larger bodies grow faster than the small ones, and within a few million years embryos the size of the Moon or of Mars take shape.

The asteroid belt preserves the remains of that process, and it also shows how much material was lost: it holds between 1.1 and 1.9 million bodies larger than a kilometre, but their combined mass is less than the Moon's, and a quarter of it sits in Ceres. For the Earth, hafnium-tungsten dating indicates accretion completed between 30 and 100 million years after the first solids: not an event, a long process.

The Theia impact

Towards the end of Earth's accretion a protoplanet roughly the size of Mars, which astronomers call Theia, struck the Earth with a glancing blow. Part of the mantle of both bodies was thrown into orbit, and out of that debris disk the Moon came together in a very short time on a geological scale. Theia's metallic core, instead, sank and merged with the Earth's.

The giant impact model explains several things that otherwise go unanswered: why the Moon is so large compared with its planet — 3,474 km across, more than a quarter of the Earth's diameter — why it is poor in iron and volatiles, and why the Earth-Moon system carries so much angular momentum. It explains less well the near-perfect isotopic similarity between terrestrial and lunar rocks, which keeps putting the simulations to the test.

The date is debated too. Estimates differ between about 4.51 billion years ago, derived from the dating of zircon crystals, and about 4.35 billion, the age of most of the lunar samples brought back to Earth. A recent paper tries to reconcile them by positing an episode of tidal heating that would have remelted the Moon, resetting its isotopic clocks long after it formed. For the Earth the consequences are still with us: the Moon stabilises the tilt of the axis at around 23.4° and is receding by 3.8 cm a year.

The migration of the giants

The giant planets were not born where we see them. According to the Nice model, proposed in 2005 by a group at the Côte d'Azur Observatory, at the end of the gas phase Jupiter, Saturn, Uranus and Neptune occupied far more compact orbits, all within about twenty astronomical units, surrounded by a disk of planetesimals of a few tens of Earth masses.

The exchange of angular momentum with that disk slowly shifted the orbits, until Jupiter and Saturn crossed a resonance and the system became unstable. Neptune and Uranus were pushed outwards — possibly swapping places — while Jupiter moved slightly closer to the Sun. Neptune travelled out to 30 astronomical units, ploughing through the primordial disk and scattering most of it.

That passage left signatures that are still readable. Pluto and dozens of other bodies sit in a 3:2 resonance with Neptune — two Pluto orbits for every three of the planet's — a configuration well explained by capture during a slow migration. Eris and the other scattered disk objects ended up on eccentric, steeply inclined orbits.

The model exists in several variants, and the timing of the instability is debated. Recent reconstructions move it much earlier: within the first 100 million years of the Solar System, and according to some authors a few million years after the gas dispersed.

The Late Heavy Bombardment

The rocks brought back by the Apollo missions show a clustering of ages around 3.9 billion years. In the 1970s the interpretation was that the inner Solar System had gone through a sudden spike in impacts at that moment: the Late Heavy Bombardment, or lunar cataclysm. The surface of Mercury, saturated with craters and with no erosion to erase them, seemed to confirm that the whole inner region had been hit in the same period.

Today that reading is contested. The main objection is a sampling bias: the Apollo landing sites all lie on the near side, within reach of the debris thrown out by the Imbrium basin, and a single large collision may have reset the isotopic clocks of many different samples. A 2016 study showed that the same distribution of ages clustered around 3.9 billion years also comes out of a steadily declining impact flux, with no spike at all. The dating of the far-side samples collected by the Chang'e-6 mission, published in 2026, covers a range from 4.33 to 1.13 billion years and shows no concentration at 3.9: it points to a gradual decline.

This has to be said explicitly: the causal link between the migration described in the previous stage and this bombardment is today the most disputed part of the whole reconstruction. The Nice model was born precisely because the migration of the giants supplied an explanation for a spike in impacts 3.9 billion years ago; if the instability happened within the first 100 million years, as recent reconstructions indicate, that link falls away. The two stages remain plausible taken on their own, but they should not be read as cause and effect.

The Solar System today

The result of four and a half billion years is a system that is ordered but not finished. The Sun holds 99.8% of the total mass, and almost everything left over is in Jupiter. Around it orbit eight planets, the five dwarf planets recognised by the IAU, the asteroid belt between Mars and Jupiter and the icy populations of the Kuiper Belt and the Oort Cloud.

The traces of the earlier stages are still readable, one by one. The division between inner rocky planets and outer giants comes from the snow line; the nearly coplanar orbits come from the disk; the resonances of the Kuiper Belt come from the migration; the tilt of the Earth's axis and the Moon come from an impact. A planetary system is, in essence, its own history frozen into a configuration.

The Sun has covered a little less than half of its stay on the main sequence: it has been fusing hydrogen in its core for about 4.6 billion years. The planets' orbits are stable on human timescales, but they are not eternal: long-term numerical integrations show that the inner Solar System is weakly chaotic, and on scales of billions of years the positions are not predictable in detail.

The profiles of the individual bodies collect the up-to-date data for each of them; the 3D simulation shows them in motion, with orbital periods proportional to the real ones.

The future

The Sun is gradually growing brighter: it already radiates more energy than when it first ignited, and in about a billion years the increase will be enough to make the Earth uninhabitable, long before the star changes shape.

In about 5 billion years the hydrogen in the core will run out. The core will contract, fusion will move to a shell around it and the outer layers will expand: the Sun will become a red giant. According to a 2008 model, at the peak of this phase — in about 7.6 billion years — the solar radius will reach 256 times its present value, roughly 1.2 astronomical units, and the star will already have lost a third of its mass.

The fate of the Earth is uncertain, and it depends precisely on that mass loss: the less mass the Sun has, the wider the Earth's orbit becomes. With the values from that model the orbit would move out to around 1.5 astronomical units, but tidal friction with the giant's envelope would make it decay until the Earth was engulfed. A 2026 study, with a different treatment of the tides and mass-loss rates measured on real stars, concludes instead that the Earth survives. Estimates differ, and almost all of the difference lies in how much material the Sun manages to expel first. Mercury and Venus, in any case, do not make it.

In the end the Sun will expel its outer layers and the bare core will remain: a white dwarf of about 0.54 solar masses, roughly the size of the Earth, which will cool slowly for billions of years.