Monthly Archives: January 2022

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.