Artist's impression showing three planets orbiting across the face of their host star.
Watching as a star's light decreases due to a planet transiting in front of it is one way to discover and investigate planets around other stars outside of our own Solar System.
ESA's Characterising Exoplanet Satellite, CHEOPS, will study known transiting planets around bright stars in order to build up precise measurements of their size. Together with known information about their mass, it will then be possible to determine the density of the planet. 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.