How the planets formed

The Sun and its planets were all made together, about 4.6 billion years ago, from one cloud of gas and dust.

The cloud fell in on itself. Most of it went to the middle and became the Sun. The rest could not fall straight in, because the cloud was slowly turning. It spread out into a wide, flat disc going round the new star.

In the disc, grains of dust bumped into each other and stuck. Specks became pebbles. Pebbles became boulders, and boulders became lumps the size of mountains. The biggest lumps pulled in the others with their gravity, and grew into planets.

That is why the planets all go round the Sun the same way, and nearly in the same flat plane. They are what is left of the disc.

Nobody saw it happen. The account below is pieced together from three kinds of evidence: the pattern of the Solar System today, the ages and make-up of meteorites, and discs that telescopes can see around young stars now. The outline is well supported. Several of the steps are still being worked out, and the lesson says where.

The clues in the pattern.

• The planets go round the Sun in the same direction, in nearly the same plane.
• The Sun spins that way too.
• Small rocky planets are close in, and giants are further out.
• Leftover rubble sits in belts between and beyond them.

A flat, spinning disc explains all four.

From dust to planets.

Stage	What happens	How long
A disc forms	The cloud collapses; what cannot fall in settles into a disc of gas with about one part in a hundred of dust	About 100,000 years
Dust sticks	Grains collide gently and cling, growing to pebbles	Thousands to tens of thousands of years
Planetesimals	Swarms of pebbles clump under their own gravity into bodies kilometres across	Not well understood
Runaway	The biggest bodies pull in the most, and grow fastest	Up to a few million years
The gas goes	The young Sun’s light and wind clear the gas away. Giant planets must be finished by now	Within about 10 million years
Final collisions	In the inner system, dozens of bodies the size of the Moon or Mars collide and merge into four planets	Tens of millions of years

The step from pebbles to planetesimals is the hard one. Pebbles tend to bounce or shatter, not stick, and they should spiral into the Sun before they grow. Current thinking is that they gather in dense swarms that collapse all at once.

Why two kinds. Near the Sun the disc was hot, and only rock and metal were solid. Those make up a small share of the material, so the inner planets stayed small. Beyond the frost line, between where Mars and Jupiter are now, water froze as well, and there was several times more solid stuff to build with. Out there, cores grew to about ten times the mass of the Earth while the gas was still around. A core that heavy pulls in hydrogen and helium without limit. Jupiter and Saturn are the result. Uranus and Neptune grew more slowly and caught less gas before it was gone.

The last big collision. The Earth’s final major impact, with a body about the size of Mars, is thought to have thrown off the material that became the Moon, about 4.5 billion years ago.

Planets move. The planets probably did not form exactly where they are. Computer models in which the giants shifted their orbits early on do a better job of explaining the thin asteroid belt, the tilted orbits in the Kuiper Belt and the small size of Mars. This is the least settled part of the story, and there are several competing versions.

Watching it elsewhere. In 2014 the ALMA telescope in Chile made a sharp picture of the disc round HL Tauri, a star about 450 light years away and less than a million years old. It shows bright rings separated by dark gaps, of the kind forming planets would sweep clear. Many such discs have been pictured since. And more than 6,000 planets are now known round other stars. Many of those systems look nothing like ours, with giant planets closer to their stars than Mercury is to the Sun, which is good evidence that planets migrate.