  Ground based radial velocity (RV) searches continue to discover exoplanets
below Neptune mass down to Earth mass. Furthermore, ground based transit
searches now reach milli-mag photometric precision and can discover Neptune
size planets around bright stars. These searches will find exoplanets around
bright stars anywhere on the sky, their discoveries representing prime science
targets for further study due to the proximity and brightness of their host
stars. A mission for transit follow-up measurements of these prime targets is
currently lacking. The first ESA S-class mission CHEOPS (CHaracterizing
ExoPlanet Satellite) will fill this gap. It will perform ultra-high precision
photometric monitoring of selected bright target stars almost anywhere on the
sky with sufficient precision to detect Earth sized transits. It will be able
to detect transits of RV-planets by photometric monitoring if the geometric
configuration results in a transit. For Hot Neptunes discovered from the
ground, CHEOPS will be able to improve the transit light curve so that the
radius can be determined precisely. Because of the host stars' brightness, high
precision RV measurements will be possible for all targets. All planets
observed in transit by CHEOPS will be validated and their masses will be known.
This will provide valuable data for constraining the mass-radius relation of
exoplanets, especially in the Neptune-mass regime. During the planned 3.5 year
mission, about 500 targets will be observed. There will be 20% of open time
available for the community to develop new science programmes.
