A quick look at the science behind Cheops, ESA's first mission to focus on exoplanets – planets orbiting stars other than the Sun. By observing stars that are already known to host exoplanets, Cheops will make measure very precisely the sizes of planets in the super-Earth to Neptune size range. Combined with existing measurements of planet masses, the Cheops data will make it possible to determine the density of these planets, giving us vital clues about its composition and structure, indicating for example if it is predominantly rocky or gassy, or perhaps harbours significant oceans.