Abstract

The CHaracterising ExOPlanet Satellite (CHEOPS) was selected on October 19, 2012, as the first small mission (S-mission) in the ESA Science Programme and successfully launched on December 18, 2019, as a secondary passenger on a Soyuz-Fregat rocket from Kourou, French Guiana. CHEOPS is a partnership between ESA and Switzerland with important contributions by ten additional ESA Member States. CHEOPS is the first mission dedicated to search for transits of exoplanets using ultrahigh precision photometry on bright stars already known to host planets. As a follow-up mission, CHEOPS is mainly dedicated to improving, whenever possible, existing radii measurements or provide first accurate measurements for a subset of those planets for which the mass has already been estimated from ground-based spectroscopic surveys. The expected photometric precision will also allow CHEOPS to go beyond measuring only transits and to follow phase curves or to search for exo-moons, for example. Finally, by unveiling transiting exoplanets with high potential for in-depth characterisation, CHEOPS will also provide prime targets for future instruments suited to the spectroscopic characterisation of exoplanetary atmospheres. To reach its science objectives, requirements on the photometric precision and stability have been derived for stars with magnitudes ranging from 6 to 12 in the V band. In particular, CHEOPS shall be able to detect Earth-size planets transiting G5 dwarf stars (stellar radius of 0.9R⊙) in the magnitude range 6 ≤ V ≤ 9 by achieving a photometric precision of 20 ppm in 6 hours of integration time. In the case of K-type stars (stellar radius of 0.7R⊙) of magnitude in the range 9 ≤ V ≤ 12, CHEOPS shall be able to detect transiting Neptune-size planets achieving a photometric precision of 85 ppm in 3 hours of integration time. This precision has to be maintained over continuous periods of observation for up to 48 hours. This precision and stability will be achieved by using a single, frame-transfer, back-illuminated CCD detector at the focal plane assembly of a 33.5 cm diameter, on-axis Ritchey-Chrétien telescope. The nearly 275 kg spacecraft is nadir-locked, with a pointing accuracy of about 1 arcsec rms, and will allow for at least 1 Gbit/day downlink. The sun-synchronous dusk-dawn orbit at 700 km altitude enables having the Sun permanently on the backside of the spacecraft thus minimising Earth stray light. A mission duration of 3.5 years in orbit is foreseen to enable the execution of the science programme. During this period, 20% of the observing time is available to the wider community through yearly ESA call for proposals, as well as through discretionary time approved by ESA’s Director of Science. At the time of this writing, CHEOPS commissioning has been completed and CHEOPS has been shown to fulfill all its requirements. The mission has now started the execution of its science programme.

Highlights

  • In March 2012, the European Space Agency (ESA) issued a call for a small mission opportunity

  • The CHEOPS (CHaracterising ExOPlanet Satellite)1 proposal was submitted in response to the call by a Consortium of research institutes located in ESA member states

  • At 8:54:20 UT on December 18, 2019, CHEOPS was successfully launched as a secondary passenger on a Soyuz-Fregat rocket from Kourou in French Guiana

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Summary

Introduction

In March 2012, the European Space Agency (ESA) issued a call for a small mission opportunity. The follow-up nature of CHEOPS, with a single star being targeted at a time, makes this transit mission unique compared to its successful precursors COROT [3], Kepler [23], and TESS [35], or its successor PLATO [34] This difference is the basis for an original science programme (see Section 2) in which the focus is not on the discovery of additional exoplanets, but rather on the characterisation of a set of most promising objects for constraining planet formation and evolution theories and for further studies by future large infrastructures (e.g. JWST, Ariel, ELTs).

The CHEOPS science programme
The guaranteed time observing programme science
Community access to CHEOPS: the guest observers programme
Requirements and estimated performances
Photometric precision and stability
Estimated performances
Instrumental noises:
Orbit and sky coverage
Project implementation approach
Mission design
Spacecraft design
The CHEOPS payload
Nominal observations
On-board science data processing
Model philosophy and verification approach
The ground segment
11.2 Photometric precision and stability
11.3 KELT-11b: radius determination of a bloated planet
Findings
12 Summary and conclusions
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