WORLDS WITHOUT SUNS

Astronomers are still on the hunt for rogue planets, but what can they tell us about the layout of our cosmos?

- Colin Stuart

When we think about planets, a star usually comes as part of the package. After all, where would Earth be without the Sun? Imagine Luke Skywalker’s home planet of Tatooine without its twin suns, or Superman without Rao, the red supergiant around which Krypton spun. Yet you’d be wrong to imagine that planets are never starless. Astronomers have already found swarms of rogue planets – worlds wandering the open chasm of space alone.

As far as we know, planets always start off with stars. They are the leftover fragments from star formation, the offcuts that you’d throw away in a skip if you were building a house. A cloud of gas and dust that was otherwise minding its own business may begin to contract if it’s hit by a shock wave from an exploding star. As the cloud compresses, gravity takes over and bundles it into an ever-smaller space until fledgling stars ignite inside.

The shrinking cloud begins to spin faster and faster, flinging leftover gas and dust into discs around new stars. Gravity continues its work here, too. Dust grains merge into lumps the size of golf balls, which continue to snowball until they reach a kilometre or so across.

Astronomers call these planetary building blocks planetesimals. Close to the star they are made of metals – the only materials with high enough melting points to stay solid in the face of the inferno. Further out – beyond an imaginary boundary called the ice line – temperatures plummet below freezing and the planetesimals take the form of frigid lumps of water, ammonia and methane ices. That’s why we have rocky planets close to the Sun and ice giants the furthest out.