The exoplanets discovered by using different methods as of 6 December 2019 (source: exoplanet.eu).
ESA’s Characterising Exoplanet Satellite, Cheops, will characterise several hundreds of these known planets by 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.