CHEOPS Observes One of Hottest Exoplanets Known to Date: WASP-189b | Astronomy

Because of its proximity to its parent star, the dayside equilibrium temperature of the ultrahot Jupiter WASP-189b reaches 3,162 degrees Celsius (5,724 degrees Fahrenheit), according to an analysis of data collected by ESA’s CHEOPS (CHaracterising ExOPlanet Satellite) mission.

An artist’s impression of an ultrahot Jupiter. Image credit:

Discovered in 2018, WASP-189b is a gas giant transiting the 730-million-year-old A-type star WASP-189.

Also known as HD 133112, the host star is larger and more than 2,000 degrees Celsius hotter than the Sun, and so appears to glow blue.

WASP-189b sits around 20 times closer to its star than Earth does to the Sun, and completes a full orbit in just 2.7 days.

“The WASP-189 planetary system is located 322 light-years away in the constellation of Libra,” said Dr. Monika Lendl, an astronomer at the University of Geneva.

“WASP-189b is especially interesting because it is a gas giant that orbits very close to its host star.”

“It takes less than 3 days for it to circle its star, and it is 20 times closer to it than Earth is to the Sun.”

“Only a handful of planets are known to exist around stars this hot, and this system is by far the brightest,” she added.

“WASP-189b is also the brightest hot Jupiter that we can observe as it passes in front of or behind its star, making the whole system really intriguing.”

Key parameters of the WASP-189 planetary system. Image credit: ESA.

Key parameters of the WASP-189 planetary system. Image credit: ESA.

First, Dr. Lendl and colleagues used CHEOPS to observe WASP-189b as it passed behind its star — an occultation.

“We used this to measure the planet’s brightness and constrain its temperature to a scorching 3,162 degrees Celsius,” she said.

Next, the astronomers watched as WASP-189b passed in front of its star — a transit.

Transits can reveal much about the size, shape, and orbital characteristics of a planet. This was true for WASP-189b, which was found to be larger than thought at almost 1.6 times the radius of Jupiter.

“We also saw that the star itself is interesting — it’s not perfectly round, but larger and cooler at its equator than at the poles, making the poles of the star appear brighter,” Dr. Lendl said.

“It’s spinning around so fast that it’s being pulled outwards at its equator! Adding to this asymmetry is the fact that WASP-189b’s orbit is inclined; it doesn’t travel around the equator, but passes close to the star’s poles.”

“This first result from CHEOPS is hugely exciting: it is early definitive evidence that the mission is living up to its promise in terms of precision and performance,” said CHEOPS project scientist Dr. Kate Isaak, an astronomer at ESA.

The findings will be published in the journal Astronomy & Astrophysics.


M. Lendl et al. 2020. The hot dayside and asymmetric transit of WASP-189 b seen by CHEOPS. A&A, in press; doi: 10.1051/0004-6361/202038677

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