Cheops is ESA’s first mission dedicated to the study of extrasolar planets, or exoplanets. It will observe bright stars that are already known to host planets, measuring minuscule brightness changes due to the planet’s transit across the star’s disc.
The mission will target stars hosting planets in the Earth- to Neptune-size range, yielding precise measurements of the planet sizes. This, together with independent information about the planet masses, will allow scientists to determine their density, enabling a first-step characterisation of these extrasolar worlds. A planet’s density provides vital clues about its composition and structure, indicating for example if it is predominantly rocky or gassy, or perhaps harbours significant oceans.