Tag Archives: Cheops

CHEOPS observes WASP-189b

ESA’s CHEOPS satellite was launched to produce high-quality light-curves of exoplanet systems. A new paper led by Adrien Deline of the University of Geneva now reports CHEOPS observations around the orbit of the ultra-hot-Jupiter WASP-189b. The figure shows the transit (planet passing in front of the star), the eclipse (the planet passing behind the star) and a slower variation caused by the varying visibility of the heated face of the planet.

One notable feature of the transit of WASP-189b is that it is distinctly asymmetrical. This is caused by gravity darkening, which occurs when a star is rapidly rotating. The centrifugal forces cause the equatorial regions to be pushed outwards, producing an equatorial bulge. Since the bulge is then further from the star’s centre, the surface gravity will be lower, and that means that the surface will be cooler and thus dimmer.

The illustrations below show the asymmetry, where the dashed line in the lowest panel shows the difference between a transit model both with and without gravity darkening. The right-hand panel illustrates the polar orbit of the planet.

Tidal deformation of WASP-103b

With ultra-hot Jupiters being so near to their star their shape is predicted to be distorted away from spherical by the tidal effects of the host-star’s gravity. The resulting “rugby-ball” shape (more technically called a “Roche lobe”) will then produce a transit profile that is slightly different from that produced by a spherical planet.

The CHEOPS team now report that they have detected this distortion in the case of WASP-103b. A press release presents the infographic:

The CHEOPS observations of transits of WASP-103b are shown below (grey points). The blue model is the expected profile for a deformed planet, while the green line (lowest panel) is the expected difference in transit profile between a deformed planet and a spherical planet. The CHEOPS team show statistically that the data prefer the deformed shape, at a confidence level of 3σ.

The authors, Susana Barros et al, explain that the degree of tidal deformation constrains the distribution of mass within the planet, since the gaseous hydrogen envelope is much easier to deform than the rocky core. ESA have produced an artist’s illustration showing the distorted shape of WASP-103b:

Following ESA’s press release, the work has been reported by CNN, Newsweek, the BBC, the Daily Mail, The Sun, The Independent and numerous other websites in multiple languages.

CHEOPS observations of WASP transits

CHEOPS, the CHaracterising ExOPlanet Satellite is ESA’s Small-class mission dedicated to recording transits of exoplanets. A new paper led by Luca Borsato presents some early observations of transits of WASP, KELT and HATnet planets.

Here, for example, are the lightcurves of two transits of WASP-8b, both plotted against phase.

The paper focuses on the transit timing, which can be as good as timing a transit to an accuracy of 13 to 16 seconds, depending on the brightness of the host star and the amount of transit covered by the observations.

One aim of such work is to look for variations in the timing of transits, caused by the gravitational perturbations of additional unseen planets in the system.

First results from ESA’s Cheops: WASP-189b

ESA’s Cheops satellite (the Characterising Exoplanet Satellite) started observing this year, and ESA has just put out a press release announcing its first science results. Cheops looked at transits and occultations of WASP-189b, an ultra-hot Jupiter in a polar orbit transiting a bright star.

“Only a handful of planets are known to exist around stars this hot, and this system is by far the brightest,” says Monika Lendl of the University of Geneva, Switzerland, lead author of the new study. “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.”

At a visual magnitude of V = 6.6, WASP-189 is the brightest host star of all the WASP planets. The discovery of the transiting hot Jupiter was announced in 2018 in a paper led by David Anderson. The exceptional nature of WASP-189 thus made it a prime target for Cheops.

The Cheops study shows 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,” says Dr Lendl. “It’s spinning around so fast that it’s being pulled outwards at its equator!”

“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,” says Kate Isaak, Cheops project scientist at ESA.

Press coverage has included articles in CNN, CTV, the International Business Times, The Sun, The Mirror, The Daily Mail, The Express and over 30 other news sites.