Formation of the Solar System

The story

A cloud that began to spin

About 4.6 billion years ago, somewhere in the disc of the Milky Way, there was a big, cold cloud of gas and dust. It was made mostly of hydrogen and helium, but it also held traces of the heavy elements made by stars that had died earlier. For some reason, perhaps because a nearby star had exploded and given it a push, or perhaps because the cloud had simply become denser, part of it began to fall in on itself. Once again, gravity was the star of the show.

As the cloud shrank, it spun faster and faster, like a skater pulling in her arms. A spinning cloud does not gather into a ball: it flattens into a disc, with a bulge in the middle. Scientists call it the “solar nebula”. The idea that the Solar System was born like this is an old one: the philosopher Immanuel Kant suggested it in 1755 and the French mathematician Pierre-Simon Laplace took it up in 1796.

The Sun at the centre

Almost all the mass of the cloud, more than 99 per cent, slid towards the middle and formed the Sun. With the weight, the centre grew hotter and hotter, until the temperature was so high that the fusion of hydrogen began. From that moment the Sun was a real star. Today it is already halfway through its life: in about five billion years it will use up the hydrogen in its core and become a red giant.

A disc of gas and dust was left around the Sun. In the disc the grains bumped into one another and stuck together, becoming pebbles, then rocks, then bodies as big as mountains, called planetesimals. The biggest planetesimals pulled in the others with their gravity and kept growing. The giant planets formed in a few million years, which is very little for astronomy; the rocky ones took a few tens of millions of years.

Rocky planets and giant planets

Close to the Sun it was too hot for ice to stay solid. So small, rocky planets formed there: Mercury, Venus, the Earth and Mars. Further out, beyond a distance called the “frost line”, water was frozen and there was far more solid material. There the planets grew big enough to capture huge amounts of hydrogen and helium: Jupiter and Saturn were born, gas giants, and further out Uranus and Neptune, ice giants.

Not all the material became a planet. Between Mars and Jupiter a great many fragments were left, forming the asteroid belt: Jupiter, with its gravity, stopped them from joining into a planet. Beyond Neptune lies the Kuiper belt, with Pluto and many icy bodies, and much further out there is thought to be a cloud of comets, the Oort cloud. Comets are the frozen leftovers of that material.

How do we know?

Nobody was there, of course, but there is plenty of evidence. The oldest meteorites, pieces of rock that have fallen to Earth from the asteroid belt, have an age that can be measured with radioactive atoms: the oldest grains in some of them go back 4.567 billion years and are thought to be the first solids in the Solar System. Also, all the planets go around the Sun in the same direction and almost in the same plane, as if they had been born from a single disc.

Today telescopes, such as ALMA in Chile, can see discs of dust around young stars that look like solar systems under construction. The ring shapes in some discs show that planets are already forming inside them. Meanwhile satellites have discovered thousands of planets around other stars, the exoplanets, and many of those systems are very different from ours. This has shown that the story of the Solar System is not the only one possible, and has forced scientists to revise the details of the model.