Artist impression of the different types of exoplanets and their host stars that might be studied by ESA’s Characterising Exoplanet Satellite, Cheops.

Cheops will make precise measurements of planets known to be orbiting around nearby bright stars. The mission 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.

It will focus particularly on stars with Earth- to Neptune-sized planets. By determining the size of the planet, and combining this with known measurements of the planets’ masses, Cheops will allow the density of the planet to be estimated, and therefore enable a first step towards characterising the nature of these planets – from lava worlds to rocky Earth-like planets, to gas planets or even ocean worlds.

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