Artist’s impression of the Cheops satellite observing an exoplanet transiting its parent star.
ESA’s Characterising Exoplanet Satellite, Cheops, 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.
Cheops makes use of the technique of ‘ultra-high-precision transit photometry’ to measure very precisely the sizes of exoplanets. The size of the dip in the light due to the exoplanet transit is known as the ‘depth’ of the transit, and relates directly to the size of the planet relative to the star: a large planet will block a larger fraction of the light from the star than would a small one.
By knowing when and where to point in order to catch planetary transits, Cheops will maximise the time it spends monitoring actual transit events. It will point at stars over most of the sky, returning to the same stars to observe multiple transits over the course of the mission, thus building up the accuracy of measurement of planet sizes.
Relative sizes and distances are not to scale.