Executive Summary

CHEOPS – CHaracterizing ExOPlanet Satellite

The CHaracterizing ExOPlanet Satellite (CHEOPS) will be the first mission dedicated to search for transits by means of ultrahigh precision photometry on bright stars already known to host planets. By being able to point at nearly any location on the sky, it will provide the unique capability of determining accurate radii for a subset of those planets for which the mass has already been estimated from ground-based spectroscopic surveys. It will also provide precision radii for new planets discovered by the next generation ground-based transits surveys (Neptune-size and smaller).

Large ground-based high-precision Doppler spectroscopic surveys carried out during the last years have identified hundreds of stars hosting planets in the super-Earth to Neptune mass range (1<Mplanet/MEarth<20) and will continue to do so into the foreseeable future. The characteristics of these stars (brightness, low activity levels, etc.) and the knowledge of the planet ephemerids make them ideal targets for precision photometric measurements from space. CHEOPS will be the only facility able to follow-up all these targets for precise radius measurements.

The new generation of ground-based transit surveys (e.g. NGTS), capable of reaching 1 mmag precision on V < 13 magnitude stars, provide yet another source of targets. By the end of 2017, NGTS will provide of order 50 targets with R < 6 REarth for which CHEOPS will be able to measure radii to a precision of 10%. These stars are also bright enough for precise radial velocity follow-up measurements to be practical. While unbiased ground-based searches are well-suited to detect the transits and fix the ephemerids, CHEOPS is crucial to obtain precise measurements of planet radii.

Knowing where to look and at what time to observe makes CHEOPS the most efficient instrument to search for shallow transits and to determine accurate radii for planets in the super-Earth to Neptune mass range.

The main science goals of the CHEOPS mission will be to study the structure of exoplanets with radii typically ranging from 1-6 REarth orbiting bright stars. With an accurate knowledge of masses and radii for an unprecedented sample of planets, CHEOPS will set new constraints on the structure and hence on the formation and evolution of planets in this mass range. In particular, CHEOPS will:

To reach its goals, CHEOPS will measure photometric signals with a precision limited by stellar photon noise of 150 ppm/min for a 9th magnitude star. This corresponds to the transit of an Earth-sized planet orbiting a star of 0.9 Rsun in 60 days detected with a S/Ntransit >10 (100 ppm transit depth). This precision will be achieved by using a single frame-transfer back-side illuminated CCD detector located in the focal plane assembly (FPA) of an F/8 ~32 cm diameter on-axis telescope. The optical design is based on a Ritchey-Chretien style telescope to provide a de-focussed image of the target star. An industrial study has led to a suitable optical design, which also minimizes stray light onto the detector utilizing a dedicated field stop and a baffling system. This design meets the requirement of < 10 ppm stray light onto the detector even in the worst case observing geometry on the baseline orbit. Thermal control of the detector (stable within ~10 mK to minimize noise) will be obtained by coupling the detector to a radiator always exposed to deep space.

The telescope will reside on a spacecraft (S/C) platform providing pointing stability of < 4 arcsec rms over a typical 48 hour observing period. The S/C will be 3-axis stabilized but nadir locked with the thermal interface between the spacecraft bus and instrument payload remaining stable to within one degree. The S/C will provide 64W continuous power for instrument operations and allow for at least 1.2 GBit/day downlink. The S/C will be provided by EADS CASA Espacio based on the SEOSAT platform.

The baseline orbit satisfying the science requirements is a sun-synchronous 650 to 800 km altitude orbit (SSO) with a mean local time of the ascending node of 6 a.m.. This choice optimizes uninterrupted observations and keeps thermal variations of the S/C and stray light on the satellite to a minimum as the orbital plane follows as close as possible the day/night terminator. A shared launch is envisioned which, given the mass of the S/C (< 300 kg), will be possible using a number of existing launchers (VEGA, Dnepr, Rockot, Soyuz).

The CHEOPS mission baseline relies completely on components with flight heritage. This is valid for the platform as well as for the payload components. For the latter, the team can exploit significant heritage from the CoRoT mission minimizing both cost and risk.

The baseline CHEOPS mission fits within both the technical readiness requirements and the cost envelope defined by the ESA call for S-missions, yet represents a break-through opportunity in furthering our understanding of the formation and evolution of planetary systems. A number of options have originally been identified that could potentially significantly enhance the scientific return. These old options are presented in the table at the end of this page. However, in the meantime the mission baseline is frozen:

The baseline mission profile is:

What Details
Orbit: Low Earth, Sun Synchronous Orbit (LEO, SSO) 6am/pm at 700 km altitude
Detector: CD detector, one wide wavelength band from 0.4 to 1.1 micron