First results of the CHEOPS space telescope on an extreme exoplanet

The CHEOPS Space Telescope keeps its promises: its first observations reveal the face of the exoplanet WASP-189b – one of the most extreme planets known. CHEOPS is a mission of the European Space Agency (ESA) with the participation of Swiss and French laboratories including IMCCE/CNRS/Paris Observatory – PSL University/Sorbonne University.

Eight months after the CHEOPS Space Telescope left for its journey into orbit around the Earth, the first scientific publication using data provided by CHEOPS has just been published. CHEOPS is ESA’s first mission to explore previously discovered exoplanets. Exoplanets are planets revolving around stars other than the Sun, the first discovery of which in 1995 was awarded the 2019 Nobel Prize in Physics to the Swiss Michel Mayor and Didier Queloz. CHEOPS was designed as a collaboration between ESA, Switzerland and various European countries. A consortium of more than a hundred scientists and engineers from eleven European countries participated in the construction of the satellite for five years. IMCCE is part of the CHEOPS Science Team because of its expertise in the dynamics of planetary systems.

The study using data provided by CHEOPS, which has just been accepted for publication in the journal “Astronomy & Astrophysics”, describes the exoplanet WASP-189b. The results obtained show that the observations made by CHEOPS are as precise as expected.

View by artist Frederik Peeters representing CHEOPS in front of the day side of the planet, with the star behind.

One of the most extreme planets known

The object of observations made by CHEOPS is WASP-189b, an exoplanet orbiting a star outside our Solar System, called HD 133112. This one, bigger and hotter than our Sun, emits a bluish light. It is located 322 light years from Earth, in the constellation Libra. Astronomers consider the star to be bright (although invisible to the naked eye), which greatly facilitates CHEOPS's study of the planet. WASP-189b is twenty times closer to its star than Earth to the Sun; it goes around it in less than three days! In addition, this star is much hotter than the Sun. The planet therefore receives an enormous amount of radiation, which literally heats it red. It is a giant planet measuring one and a half times the size of Jupiter. It has an extreme temperature, which is why it is called an ultra-hot Jupiter.

Planetary objects such as WASP-189b are out of the ordinary: due to the very intense tidal effects resulting from their proximity to their star, their rotation slowed down to the point of being synchronized with their orbital period, as is the case for the Moon around the Earth. They have a permanent day side, always exposed to starlight, and a permanent night side, always in shadow. This is what distinguishes, for example, its climate from that of the gas giants Jupiter and Saturn in our solar system. Based on observations from CHEOPS, the study coordinated by Monika Lendl of the Geneva Observatory estimates the temperature of WASP-189b at 3,200 degrees Celsius, making WASP-189b one of the most extreme planets known.

Very high precision luminosity measurements

We cannot directly see a planet so distant from us and so close to its star, we have to resort to indirect methods. To this end, CHEOPS measures starlight with great precision: when a planet seen from Earth passes in front of its star, it masks a small part of it. This passage, called transit, is detected as a small drop in the star’s brightness. Surprisingly, when it is the planet’s turn to pass behind the star, an even smaller drop in light is also observed! The fact that the exoplanet WASP-189b is very close to its star makes its day side so bright that we can measure the “missing” light as the planet passes behind the star. CHEOPS was able to observe WASP-189b during several passes behind its star and researchers were able to deduce its luminosity. The team found that the planet does not reflect much of its star light. This is linked to the absence of clouds which cannot form at such high temperatures. As a result, the planet heats up strongly and also emits light: it glows red!

The star itself is also unique

CHEOPS’s measurements are so precise that they also allow us to learn more about the WASP-189b host star. Indeed, the decrease in light recorded by CHEOPS as the planet passes in front of the star is not constant. The study concludes that the star’s surface has darker areas than others. For researchers, this is what you would expect to see if the star spins so quickly on itself: it twists slightly and, flattens. The parts of the star farther from its center become cooler and therefore darker.

For Willy Benz, Principal Investigator of CHEOPS, this system will constitute a reference for future studies: “We only know today a handful of exoplanets around such hot stars and this system is by far the brightest”, and therefore one of the most valuable. CHEOPS will be able to observe new exoplanets as they are discovered and other spectacular results will be possible thanks to its observations. This study reflects the quality of the observations of the CHEOPS space telescope and the next publications are already in preparation.


“The hot dayside and asymmetric transit of WASP-189b seen by CHEOPS”, M. Lendl et al., Astronomy & Astrophysics, 2020, in press.


CHEOPS – In search of potentially habitable planets

The “CHEOPS” mission (CHaracterising ExOPlanet Satellite) is the first of ESA’s “S-class missions” with a budget much lower than that of large and medium-sized missions, and a shorter period between the start of the project and the launch.

CHEOPS measures the changes in a star’s luminosity as a planet passes in front of it. These measurements make it possible to deduce the size of the planet and then to determine its density when their mass is also known. This provides vital information about these planets - for example, whether they are predominantly rocky, whether they are made up of gas, or whether there are deep oceans there. This is an important step in determining whether a planet has favorable conditions for the development of life.

CHEOPS is an ESA space mission carried out in collaboration with Switzerland and a consortium of more than 100 scientists and engineers from eleven European countries including France. On Wednesday, December 18, 2019, CHEOPS began its journey into space aboard a Soyuz-Fregat rocket at the European Space Center in Kourou, Guyana. Since then, CHEOPS has been circling the Earth in about an hour and a half at a height of 700 kilometers.

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