Exoplanet Satellite

CHEOPS delivers what it promises: Observations with the space telescope have revealed features of the exoplanet WASP-189b, one of the most outer planets discovered so far. The European Space Agency (ESA) and Switzerland are collaborating in the CHEOPS project, led by the University of Bern in partnership with the University of Geneva.

The first scientific paper using CHEOPS data from the satellite telescope was published eight months after the start of the space telescope’s mission. CHEOPS is the first ESA mission dedicated to characterizing known exoplanets, i.e. planets orbiting stars outside our solar system. Michel Mayor and Didier Queloz, two Swiss astronomers, discovered exoplanets in 1995 and received the Nobel Prize for this discovery last year. CHEOPS was developed as part of a collaboration between ESA and Switzerland. A consortium of more than 100 scientists and engineers from 11 European countries worked on the satellite for five years, led by the University of Bern and ESA. The CHEOPS Science Operations Center is housed in the Observatory of the University of Geneva.

Scientists recently conducted a detailed study of exoplanet WASP-189 using data from CHEOPS. The results have just been finalized for publication in Astronomy & Astrophysics. “These results indicate that CHEOPS fully meets the high expectations for its performance,” said Willy Benz, Professor of Astrophysics at the University of Bern and leader of the CHEOPS consortium.

One of the most extreme planets in the universe

WASP-189b is an exoplanet orbiting the star HD 133112, one of the hottest stars known to host a planetary system and is the subject of the CHEOPS observations. “The WASP-189 system is located in the constellation of Libra (the Scales) and is 322 light-years away,” says Monika Lendl, lead author of the study and member of the National Center of Competence in Research PlanetS.

“WASP-189b is particularly intriguing because it is a gas giant orbiting fairly closely to its host star. It orbits its star in less than three days and is 20 times closer to the star than Earth is to the sun,” adds Monika Lendl. The planet is more than 1.5 times the size of Jupiter, the largest planet in the solar system.

“They have a permanent day side, which is always exposed to the light of the star, and consequently also a permanent night side,” says Monika Lendl about planetary objects like WASP-189b. This means that its climate is radically different from that of our solar system’s gas giants, Jupiter and Saturn. “We estimate the temperature of WASP-189b to be 3,200 degrees Celsius based on CHEOPS measurements. “Ultra hot Jupiters” are planets like WASP-189b. At such a high temperature, iron melts and even turns into a gas. “This is one of the most extreme planets that we have discovered so far,” says Lendl.

Brightness measurements with the highest precision

“We can’t see the planet because it’s too far away and too close to its host star,” Lendl continues, “so we have to rely on indirect means.” CHEOPS does this through extremely accurate brightness measurements: when a planet passes in front of its star, the star appears dimmer than seen from Earth for a short time. Transit is a term for this phenomenon. “Because WASP-189b is so close to its star, its dayside is so bright that we can even quantify the missing light as the planet passes behind its star; this is referred to as coverage,” adds Monika Lendl. WASP-189b coverages with CHEOPS have been observed multiple times. The planet doesn’t seem to reflect much starlight, instead Earth absorbs most of the starlight, heating it up and causing it to glow.”

Because there are no clouds on the planet’s dayside, researchers believe it’s not extremely reflective. “That’s not surprising,” adds Lendl, “because theoretical models predict that no clouds can form at such high temperatures.”

“We also found that the transit of the gas giant in front of its star is asymmetrical,” explains Willy Benz. This happens when the star’s surface has lighter and darker zones. From CHEOPS data we can deduce that the star rotates so fast that its shape is no longer spherical but elliptical. At its equator, the star is pulled outward.”

CHEOPS as depicted by an artist. ESA / ATG medialab / ESA / ATG medialab / ESA / ATG medialab / E

WASP-189b orbits a star significantly different from the Sun. “The star is significantly larger un