CHEOPS - CHaracterizing ExOPlanet Satellite

Executive Summary

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 $(1 < \mathrm{M_{planet}/M_{Earth}} < 20)$ 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:

  • Determine the mass-radius relation in a planetary mass range for which only a handful of data exist and to a precision never before achieved.
  • Probe the atmosphere of known Hot Jupiter in order to study the physical mechanisms and efficiency of the energy transport from the dayside to the night side of the planet.
  • Provide unique targets for future ground- (e.g. E-ELT) and space-based (e.g. JWST, EChO) facilities with spectroscopic capabilities. With well-determined radii and masses, the CHEOPS planets will constitute the best target sample within the solar neighbourhood for such future studies. 
  • Offer up to 10% of open time to the community to be allocated through competitive scientific review.
  •  Identify planets with significant atmospheres as a function of their mass, distance to the star, and stellar parameters. The presence (or absence) of large gaseous envelopes bears directly on fundamental issues such as runaway gas accretion in the core accretion scenario or the loss of primordial H-He atmospheres.
  • Place constraints on possible planet migration paths followed during formation and evolution for planets where the clear presence of a massive gaseous envelope cannot be discerned.

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 ~33.5 cm diameter on-axis telescope. The optical design is based on a Ritchey-Chretien style telescope and a “beam shaper” 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 < 1 photon/pixel/second 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 < 8 arcsec rms over a 10 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 54W continuous power for instrument operations and allow for at least 1GBit/day downlink. Several platform providers have been contacted and several options fitting the technical and cost requirements exist.

The baseline orbit satisfying the science requirements is a sun-synchronous 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 (< 200 kg), will be possible using a number of existing launchers (VEGA, Dnepr, Rockot).

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.  However, a number of options were identified that would significantly enhance the scientific return but which would need further studies during the phase A to assess costs and mitigate risk. All these options (see Table 1) are related to either the S/C orbit and/or the detector FPA.

The baseline mission profile is:

Orbit: Low Earth, Sun Synchronous Orbit (LEO, SSO) 6am/pm at 800 km altitude
Detector: CCD detector, one wide wavelength band from 0.4 to 1.1 micron

CHEOPS mission options and their implications

Table 1: CHEOPS mission options
Name Changes Science Gain Comments, Implications for cost / etc.
SSO 1200 Increased altitude of SSO orbit to 1200 km. Increased visible sky. No additional S/C costs. Shared launch might be more difficult to find. Alternative: add propulsion to raise orbit.
GTO extended Go into Geosynchronous Transfer Orbit (GTO) and raise perigee to 10’000 km. Significant increase in observable sky resulting in 2-5 times more targets available and a 50% increased efficiency (always 6h+ uninterrupted). Stray light conditions improved. Significant cost increase for S/C (see budget) and small cost increase for Mission Operations. Lower TRL level for S/C as a propulsion system with delta v of 750 m/s is required.
NIR detector Replace the CCD detector by a Hawaii-2RG near infrared device. A beam splitter will provide two images on the same detector with wavelength ranges 0.4–1.05 mu and 1.05–1.7 mu. Characterization of hot Jupiter atmospheres will become possible, including direct detection and analysis of phase curves resulting in constraints on atmospheric circulation. The Hawaii-2RG device has not been fully qualified at the required photometric precision. Lab studies are currently underway to assess this. Passive cooling to 160 K required. Impact on cost likely to be significant.

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