CHEOPS mission discovers one of the most extreme planets ever

This is the telescope's first exoplanet discovery — and it is a hot one.


Over the past decade, exoplanet missions such as NASA's Kepler mission and TESS (Transiting Exoplanet Survey Satellite) have found more than 4,000 alien worlds across the vast universe. But a newcomer to the hunt is giving these two space telescopes a run for their money.

The European Space Agency's CHaracterising ExOPlanet Satellite (or CHEOPS) just made its first exoplanet discovery, and it is a doozy.

The discovery was detailed in a study published Monday in the journal Astronomy & Astrophysics.

An artist's impression of exoplanet WASP-189b orbiting its host star.ESA

CHEOPS launched in December, 2019 to observe nearby stars and the strange worlds that may orbit them.

One of these stars is 2,000 times hotter than our Sun, making it appear to glow bright blue.

“Only a handful of planets are known to exist around stars this hot," Monika Lendl, a researcher at the University of Geneva, Switzerland, and lead author of the new study, said in a statement.

The exoplanet, dubbed WASP-189 b, was caught speeding its way around this ultra hot star. WASP-189 b is a hot Jupiter planet, a gas giant similar to our own Solar System's Jupiter, but with more extreme temperatures.

The scorching exoplanet is around 20 times closer to its star than Earth is to the Sun, and completes a full orbit in just 2.7 days. Temperatures on WASP-189 b reach a sweltering 3200 degrees Celsius, the scientists report.

WASP-189 b is quite large, measuring in at almost 1.6 times the radius of the largest planet of the Solar System, Jupiter.

As it crosses in front of its star, WASP-189 b causes a noticeable dip in the star's light.

Key points of the WASP-189 system.ESA

“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," Lendl said.

CHEOPS' first find is one of the hottest and most extreme exoplanets ever found.

The planet's host star is also quite intriguing. The star is not perfectly round, but is instead larger and cooler at the equator than at the poles, making the poles of the star appear brighter, according to the researchers.

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

For a planet to develop this inclined orbit, it may have formed further out from its star and was pulled towards its host over time. This might have happened if another object, such as a second star, pushed the exoplanet closer to the star at some point during its past.

Studying an exoplanet such as WASP-189 b helps scientists better understand the history of hot Jupiter planets, and how these gas giants evolve.

"By tracking exoplanets on their orbits with CHEOPS, we can make a first-step characterization of their atmospheres and determine the presence and properties of any clouds present,” Kate Isaak, CHEOPS project scientist at ESA, said in a statement. “CHEOPS will not only deepen our understanding of exoplanets but also that of our own planet, Solar System, and the wider cosmic environment.”

Abstract: The CHEOPS space mission dedicated to exoplanet follow-up was launched in December 2019, equipped with the capacity to perform photometric measurements at the 20 ppm level. As CHEOPS carries out its observations in a broad optical passband, it can provide insights into the reflected light from exoplanets and constrain the short-wavelength thermal emission for the hottest of planets by observing occultations and phase curves. Here, we report the first CHEOPS observation of an occultation, namely, that of the hot Jupiter WASP-189 b, a MP ≈ 2MJ planet orbiting an A-type star. We detected the occultation of WASP-189 b at high significance in individual measurements and derived an occultation depth of dF = 87.9 ± 4.3 ppm based on four occultations. We compared these measurements to model predictions and we find that they are consistent with an unreflective atmosphere heated to a temperature of 3435 ± 27 K, when assuming inefficient heat redistribution. Furthermore, we present two transits of WASP-189 b observed by CHEOPS. These transits have an asymmetric shape that we attribute to gravity darkening of the host star caused by its high rotation rate. We used these measurements to refine the planetary parameters, finding a ∼ 25% deeper transit compared to the discovery paper and updating the radius of WASP-189 b to 1.619 ± 0.021RJ . We further measured the projected orbital obliquity to be λ = 86.4 +2.9 −4.4 deg, a value that is in good agreement with a previous measurement from spectroscopic observations, and derived a true obliquity of Ψ = 85.4 ± 4.3 deg. Finally, we provide reference values for the photometric precision attained by the CHEOPS satellite: for the V=6.6 mag star, and using a one-hour binning, we obtain a residual RMS between 10 and 17 ppm on the individual light curves, and 5.7 ppm when combining the four visits.