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When a star like the Sun reaches the end of its supply of hydrogen fuel, it swells up into a red giant, then shrinks down into a tiny, super-hot white dwarf. But that’s not the end for its planets- except for the close in ones, which are destroyed by the red giant. The outer ones are still there, and it’s what happens next that I study.

Sometimes the asteroids and planets left over get scattered in to the white dwarf, breaking apart to form massive rings of gas and dust similar to the rings around Saturn. I’ve used giant telescopes to observe some of these discs, and found evidence for asteroids flying in and hitting them in huge explosions.

The materiel in these rings can fall onto the white dwarf, and it’s really easy to spot. This mean that we can look at the stuff going into the white dwarf and work out what the planet that broke apart to form the disc was made of! In fact, this is the only was we have to discover what mix of chemicals the planets around other stars are made of- are they like the Earth, or is there more than one way to build a planet?