How the Solar System formed

About 4,567 million years ago a small part of a cold cloud in our galaxy collapsed and, within a few tens of millions of years, became the Sun and its planets. Nobody watched it happen, but the steps can be pieced together from meteorites, which are leftovers from that time, and from telescopes that see young stars forming the same way today.

1. Step 1 of 7

   A cold cloud of gas and dust

   The story begins in a molecular cloud: a vast, cold, dark region of gas and dust between the stars. By mass it was about 98 per cent hydrogen and helium. The rest was dust grains and ices made of heavier elements, forged in earlier generations of stars that had lived and died.

   Clouds like this can drift for millions of years without change. But where a patch becomes dense enough, its own gravity starts to win against the gas pressure holding it up. Meteorites show that the cloud also held short-lived radioactive atoms such as aluminium-26, freshly made in massive stars. Whether a nearby exploding star also helped to squeeze the cloud is still debated.

2. Step 2 of 7

   Collapse and spin

   Once collapse begins it speeds up. The cloud core was turning very slightly, and as it shrinks it must spin faster to keep its angular momentum, the same reason a skater spins faster when they pull in their arms.

   Spinning matter cannot fall straight to the centre, so it settles into a flat, rotating disc. Most of the mass still reaches the middle and piles up into a hot, growing protostar, the young Sun. The disc around it, called the protoplanetary disc or solar nebula, holds only a small share of the mass, but it is where the planets will be built.

3. Step 3 of 7

   A disc, hot inside and cold outside

   The disc was far from uniform. Close to the young Sun it was hot enough that only rock-forming minerals and metal could exist as solids. Further out it was colder, and beyond a boundary called the snow line water could freeze into ice.

   That boundary mattered enormously. Beyond it there was much more solid material to build with, ice as well as rock, which is one reason the giant planets grew out there. Inside it, the planets that formed were small and rocky. The snow line was not fixed: it moved inwards as the disc cooled over time.

4. Step 4 of 7

   The first solids

   The first solid grains condensed from hot gas close to the Sun. The oldest of all are calcium–aluminium-rich inclusions, or CAIs: pale, irregular clumps of minerals that only form at very high temperatures. Lead–lead dating puts them at about 4,567 million years old, and that moment is used as the start of the Solar System.

   Soon after came chondrules, millimetre-sized droplets of molten rock that cooled quickly into tiny spheres. Most formed within about 1 to 4 million years of the CAIs. Both are still found today, almost unchanged, inside the meteorites called chondrites, packed together with fine dust called matrix.

5. Step 5 of 7

   From dust to planetesimals

   Dust grains in the disc collided gently and stuck together, growing into fluffy clumps and then pebbles. Beyond that, simple sticking struggles: pebbles hitting each other at a few metres per second tend to bounce or shatter, and drag from the gas makes them spiral in towards the Sun.

   The leading explanation is the streaming instability. Pebbles moving through the gas bunch together into dense swarms, rather as cyclists ride in a pack. When a swarm is crowded enough, its own gravity pulls it together in one go into a body tens to hundreds of kilometres across, called a planetesimal. Many asteroids, and the parent bodies of most meteorites, began like this.

6. Step 6 of 7

   Planets grow, and Jupiter takes the gas

   Planetesimals collided and merged. The largest grew fastest, because their gravity pulled in more material, and became planetary embryos somewhere between the size of the Moon and Mars.

   Beyond the snow line, Jupiter's core grew big enough, probably within the first few million years, to pull in a vast envelope of hydrogen and helium. Its gravity opened a gap in the disc. Differences in the isotopes of meteorites from the inner and outer Solar System suggest that young Jupiter helped keep those two regions apart. Within roughly 5 to 10 million years the gas was gone, blown away by the young Sun, so the giant planets had to finish growing before then.

7. Step 7 of 7

   Giant impacts finish the job

   The last stage for the rocky planets was violent. Embryos on crossing orbits smashed together in giant impacts over tens of millions of years. One such collision, between the young Earth and a Mars-sized body, is thought to have thrown out the debris that formed the Moon, about 4.5 billion years ago.

   Not everything ended up in a planet. Between Mars and Jupiter, Jupiter's gravity stirred the planetesimals so much that they never assembled into a planet: they remain as the asteroid belt. Icy leftovers survive far out in the Kuiper belt and the Oort cloud as comets. Pieces of these leftovers still fall to Earth as meteorites, which is why they are our best record of how it all began.

Related pages

See where these events sit in deep time on the 4.6-billion-year timeline.