Artist’s impression of 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.

The exquisite precision of Cheops, together with the stability with which the telescope will be able to measure the transit depths, will enable astronomers to determine the planet sizes both accurately and precisely. 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.

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