Artist's impression of a small, rocky planet – perhaps one flooded with lava – transiting across the face of its parent star.
ESA's Characterising Exoplanet Satellite, CHEOPS, will study known transiting planets to build up precise measurements of their size. Together with known information about the mass of the planet, CHEOPS will allow the density of the planet to be estimated. This will constrain the planet's possible composition and structure, indicating for example if it is predominantly rocky or gaseous, or perhaps harbours significant oceans. CHEOPS will focus particularly on bright stars hosting Earth- to Neptune-sized planets. This first-step characterisation of these worlds – many with no Solar System equivalents – is a critical process towards understanding the formation, origin and evolution of exoplanets in this size range.