The goal of the CHEOPS mission will be to characterize the structure of exoplanets with typical sizes ranging from Neptune down to Earth diameters orbiting bright stars. This will be achieved by measuring high precision photometric sequences to detect a variation in the stellar brightness induced by a transiting planet.
CHEOPS is built to achieve a photometric precision similar to Kepler while observing much brighter stars located anywhere on the sky. The CHEOPS target list will consist of stars with:
a) small exoplanets previously detected by accurate Doppler surveys,
b) known transiting Neptune size planets detected by ground-based surveys.
In comparison with random searches such as those carried out by CoRoT and Kepler, CHEOPS will be incredibly more efficient. Knowing where to look and at what time to observe is priceless information particularly to detect (and characterize) long period transiting systems.
The mission goals are:
- To search for shallow transits on stars already known to host planets obtaining a transit signal-to-noise ratio of 5 for an Earth-size planet with a period of 50 days on G5 dwarf stars with V- magnitude brighter than 9th. This signal-to-noise ratio is sufficient to enable identifying the presence or absence of a significant atmosphere for planets with masses ranging from Neptune to Earth. We note that the handful of the smallest planets (super-Earth of even Earth-like) transiting bright stars will be “perfect” targets for further studies with spectroscopic capabilities (e.g. JWST). Such measurements will lift the degeneracy in composition of the envelope enabling a quantitative constraint on the upper limit on the planet’s envelope mass.
- To provide precision radii for a number of hot Neptune planets orbiting stars brighter than 12th V magnitude and to search for co-aligned smaller mass planets. The new generation of ground-based transit searches in the southern hemisphere (e.g. NGTS or HAT-S) will detect a significant number of Neptune-sized planets orbiting late K dwarfs and early M stars. For transits with CHEOPS having signal-to-noise ratios above 30, radii for these objects will be measured with a precision of 10% or better. This precision and the unprecedented sample size (dozens of planets with accurately known radii between 1.5 and 6 REarth, see Table 2) will provide new insights into the physical structure of hot Neptunes. Moreover detecting another transit in these systems from a smaller planet undetectable by NGTS is almost guaranteed. As shown by Kepler, about 1/3 of the transiting hot Neptune size planet candidates are observed to harbour additional smaller transiting companions. CHEOPS will be ideally suited to detect them.
- To measure the phase modulation due to the different contribution of the dayside of hot Jupiter planets and in some cases to measure the secondary eclipse. These measurements provide information about the energy flux in the atmosphere of the planet.
Finally, CHEOPS will have the capability to provide precise photometric measurements (light curves) of a large number of variable light sources in the universe. Time will be made available for this ancillary science.