It’s been suggested by astrobiologists and science-fiction authors that there may be evolutionary parallels to Earth on distant worlds because creation tends to be economical, but never, CHEOPS mission data reveals, on the most extreme planet known in the universe –an alien world 20 times closer to its host star than Earth is to the Sun, a star so hot –more than 2000 degrees Celsius hotter than our sun–it appears blue and not yellow-white.
A Three-Day Orbit
The target of the CHEOPS observations, WASP-189b, is an exoplanet orbiting the star HD 133112, one of the hottest stars known to have a planetary system. “The WASP-189 system is 322 light years away,” explains astrophysicist Monika Lendl, from the University of Geneva, and member of the National Center of Competence in Research PlanetS. “WASP-189b is especially interesting because it is a gas giant that orbits very close to its host star. It takes less than three days for it to circle its star. The planet is more than 1.5 times as large as Jupiter, the largest planet of the solar system.
The CHEOPS mission –a joint mission by the European Space Agency (ESA) and Switzerland–dedicated to characterizing known exoplanets, those orbiting stars outside the solar system. Under the leadership of the University of Bern and ESA, a consortium of more than 100 scientists and engineers from 11 European states was involved in constructing the satellite over five years. The Science Operations Center of CHEOPS is located at the observatory of the University of Geneva.
An Exotic Object
Planetary objects like WASP-189b are very exotic, observes Lendl: with a permanent day side, which is always exposed to the light of the star, and, accordingly, a permanent night side, which means that its climate is completely different from that of the gas giants Jupiter and Saturn in our solar system.
“Based on the observations using CHEOPS, we estimate the temperature of WASP-189b to be 3,200 degrees Celsius. Planets like WASP-189b are called “ultra-hot Jupiters. Iron melts at such a high temperature, and even becomes gaseous. This object is one of the most extreme planets we know so far,” says lead author Lendl. Is it the most extreme planet in the cosmos? Perhaps. But our intuition suggests worlds orbiting pulsars and neutron star hold even greater potential for weirdness beyond comprehension
“We cannot see the planet itself as it is too far away and too close to its host star, so we have to rely on indirect methods,” explains Lendl.: “Because the exoplanet WASP-189b is so close to its star, its dayside is so bright that we can even measure the ‘missing’ light when the planet passes behind its star; this is called an occultation. We have observed several such occultations of WASP-189b with CHEOPS. It appears that the planet does not reflect a lot of starlight. Instead, most of the starlight gets absorbed by the planet, heating it up and making it shine.”
The researchers suggest that the planet is not very reflective because there are no clouds present on its dayside. “This is not surprising, as theoretical models tell us that clouds cannot form at such high temperatures,” says Lendl.
“We also found that the transit of the gas giant in front of its star is asymmetrical,” observes Benz. “This happens when the star possesses brighter and darker zones on its surface. Thanks to CHEOPS data, we can conclude that the star itself rotates so quickly that its shape is no longer spherical, but ellipsoidal. The star is being pulled outward at its equator.”
Source: M. Lendl et al, The hot dayside and asymmetric transit of WASP-189 b seen by CHEOPS, Astronomy & Astrophysics (2020). DOI: 10.1051/0004-6361/202038677
Image credit top of page: ESA