CHEOPS reveals rugby ball-shaped exoplanet

With the help of the CHEOPS space telescope, an international team comprising researchers from the universities of Bern and Geneva as well as the National Center for Research Competence (NCCR) PlanetS, was able to detect for the first time the deformation of an exoplanet. Due to the strong tidal forces, the appearance of the planet WASP-103b resembles a rugby ball rather than a sphere.

On the coasts, the tides determine the rhythm of events. At low tide, the boats stay ashore; at high tide, the exit from the sea is again clear to them. On Earth, the tides are mainly generated by the Moon. Its gravitational pull causes water to accumulate in the ocean region below, which is then lacking in the surrounding regions and thus explains the low tide. Although this deformation of the ocean causes striking differences in level in many places, it is hardly recognizable from space.

On planet WASP-103b, the tides are much more extreme. The planet orbits its star in a single day and is warped by strong tidal forces so drastically that its appearance resembles a rugby ball. This is shown by a new study involving researchers from the universities of Bern and Geneva as well as the National Center of Research Competence (NCCR) PlanetS, published today in the scientific journal Astronomy & Astrophysics. This discovery was made possible thanks to observations with the CHEOPS Space Telescope. CHEOPS is a joint mission of the European Space Agency (ESA) and Switzerland, led by the University of Bern in collaboration with the University of Geneva.

A revolutionary measure

The planet WASP-103b is located in the constellation of Hercules, is almost twice the size of Jupiter, has a mass and a half times greater, and is about fifty times closer to its star than Earth is to the Sun. . “Due to its close proximity to its star, we had already suspected that very high tides were occurring on the planet. But, we had not yet been able to verify it”, explains the co-author of the study Yann Alibert , professor of astrophysics at the University of Bern and member of PRN PlanetS.

The NASA / ESA Hubble Space Telescope and NASA’s Spitzer Space Telescope had previously observed the planet. Combined with the high precision and pointing flexibility of CHEOPS, these observations allowed researchers to measure the tiny tidal deformation signal of the planet light years away. In doing so, they took advantage of the fact that the planet dims the star’s light slightly each time it passes in front of it. “After observing several of these so-called ‘transits’, we were able to measure the deformation. It’s amazing that we were able to do this – this is the first time that such an analysis has been done,” Babatunde Akinsanmi reports. , researcher at the University of Geneva, co-author of the study and NCCR PlanetS associate.

The planet is swollen

The researchers’ results not only allow conclusions to be drawn about the shape of the planet, but also about its interior. Indeed, the team was also able to derive a parameter called “love number” (named after British mathematician Augustus EH Love) from the transit light curve of WASP-103b. It indicates how mass is distributed within the planet and thus also gives clues about its internal structure. “The resistance of a material to deformation depends on its composition,” Akinsanmi explains. “We can only see the tides on Earth in the oceans. The rocky part doesn’t move much. Therefore, by measuring how distorted the planet is, we can determine how much rock, gas, or water is made up of it. water .”

The love number of WASP-103b is like that of Jupiter, the largest gas giant in our solar system. This suggests that the internal structures of WASP-103b and Jupiter are similar, even though WASP-103b is twice as large. “In principle, we would expect a planet with 1.5 times the mass of Jupiter to be roughly the same size. Therefore, WASP-103b must be heavily inflated due to heating by its neighboring star, and perhaps other mechanisms, ”explains Monika Lendl, professor of astronomy at the University of Geneva and co-author of the study.

However, since the measurement uncertainty of the Love number is still quite high, future observations with CHEOPS and the James Webb Space Telescope will be needed to decipher the details of the tidal deformation and internal structure of WASP- 103b and comparable exoplanets. “This would improve our understanding of these so-called ‘hot Jupiters’ and allow a better comparison between them and the giant planets in the solar system,” Lendl concludes.

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