November 12, 2018


CHEOPS (CHaracterising ExOPlanet Satellite) is set to study planets orbiting stars other than our Sun but sufficiently close to our solar system to be able to determine their properties very precisely. These data, complemented by other ground- and space-based measurements, will illustrate the diversity of small planets out there and shed new light on how they formed and evolved.

Since the first exoplanet was detected in 1995, more than 3,500 have been recorded but their properties remain largely unknown. Unlike stars, planets emit very little or no radiation that can be seen by telescopes. As a result, they’re mostly detected by the perturbing effects they induce on their star. For example, the slight dip in brightness that can be observed when they cross in front of their star—an occurrence called a ‘transit’—is directly related to the star’s geometric properties, making it possible to deduce the planet’s diameter. This is how three-quarters of all know exoplanets have been identified.

However, to identify transits of Earth-size planets, we need to be able to measure the tiniest variations in light from their host star—and that can only be done from space. This is where the CoRoT and Kepler missions have succeeded spectacularly. Both missions looked for planets mostly orbiting stars a very long way from the solar system.

Unlike CoRoT and Kepler, CHEOPS is not designed to discover new planets but rather to study those already identified in greater detail using the transit method or the radial velocity method. These two methods tell us different things about detected exoplanets. The radial velocity method gives an indication of a planet’s mass, while the transit method gives us a first estimate of its diameter. Both provide indications about the distance of a planet from its star and about the shape of its orbit. A planet’s density can be inferred by combining the two, as the mass and diameter reveal its overall composition, telling us for example whether it’s made mostly of rock or iron, or whether it might have oceans.

CHEOPS will therefore round out the range of measurements available for determining the structure of exoplanets. The most promising are what we call super-Earths, planets with a mass somewhere between that of our own Earth and that of Neptune (17 times more massive). No such planets exist in our solar system.

With CHEOPS, scientists hope to identify a new kind of planet in the Earth-to-Neptune mass range, in other words giant ocean planets, mini-Neptunes or dwarf gas planets. Credits: ESA, 2013

In certain systems with several planets in very close orbits, the planets affect one another’s orbits and transit times are thus altered very slightly. CHEOPS will have the ability to acquire successive measurements to help precisely determine the complex motions of planets. Planets’ masses will be derived with greater precision from mathematical models. Here again, CHEOPS will provide a valuable complement to measurements obtained from other methods.

The scientific community also hopes with CHEOPS’ greater precision to identify types of planets thought theoretically possible but never yet observed—planets like mini-Neptunes (small gas planets) and giant ocean planets. Such discoveries would challenge our assumptions about how they formed, like for example the mass of the solid core required for surrounding gas to be captured and form a thick atmosphere of varying composition.

Results from CHEOPS will provide a collection of target planets for detailed study of their physical properties and atmospheric composition. Other telescopes will also be able to attempt to analyse the light reflected by their atmospheres to determine their composition:

- In the United States, with the James Webb Space Telescope (JWST) developed by NASA in partnership with the Canadian and European space agencies, set for launch in 2021.

- In Europe, with:

  • On the ground, ESO’s European Extremely Large Telescope (EELT), on which construction has now started, expected to enter service in 2024.
  • ESA’s ARIEL space telescope, scheduled to launch in 2028.

Over its planned three-and-a-half-year lifetime, CHEOPS is expected to observe between 500 and 1,000 stars hosting at least one exoplanet. But the mission has enough fuel reserves to be extended a year and a half for a total duration of five years.

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