Habitable and not-so-habitable exoplanets

On 29th September this year, astronomers announced the discovery of an exoplanet called Gliese 581 g. This planet, they said, was exactly the right distance from its star for water to exist on its surface, with a good chance that it could hold an atmosphere. These two properties are very important when judging whether a planet can support life so, inevitably, some people got excited. But an exoplanet doesn’t have to be capable of supporting life in order to tell us about the universe we live in. In fact, some planets that are very different to our own may be about to turn our theories about planet and solar system formation upside down.