How to make a planet

From stardust to rocky realms and gas giants, here's what it takes to build a world

- Jamie Middleton

To make a planet, first you'll need a massive, reasonably dense cloud of cold gas and dust.

Luckily, later generations of the cosmos have provided just that. These clouds, rich in hydrogen, helium and elements forged in earlier stars, are the raw ingredients for planets. In the case of Earth, this happened about 4.5 billion years ago. So if you're starting from scratch, you may want to clear your schedule for the next few aeons...

Next, you'll need something to give that cloud a cosmic nudge. A nearby supernova blast, a passing star or some other gravitational disturbance will do the trick. This triggers the cloud to begin collapsing under its own gravity. As it contracts, most of the mass falls towards the centre, forming a protostar. The rest flattens into a spinning protoplanetary disc. This is where the magic of planet building happens.

As the protostar continues to feed on surrounding material, it grows hotter and denser. Eventually, its core becomes hot enough to ignite nuclear fusion, and - voilà! - a new star is born. But we're here for planets, not stars, so let's turn our attention to what's still swirling in the disc. This material isn't all the same temperature. The farther you go from the star, the cooler it gets. At certain distances, you'll hit what's called a snow line (or frost line), the point where gases like water, methane and ammonia are cold enough to condense into ice. As these ices start sticking to dust grains, they form larger and larger clumps. These sticky seeds are called planetesimals.