The NASA/ESA Hubble Space Telescope and NASA's Spitzer Space Telescope had already observed the planet. In combination with the high precision and pointing flexibility of CHEOPS, these observations enabled the researchers to measure the tiny signal of the tidal deformation of the planet light years away. In doing so, they took advantage of the fact that the planet dims the light of the star slightly each time it passes in front of it. "After observing several such so-called "transits," we were able to measure the deformation. It's incredible that we were able to do this -- it's the first time such an analysis has been done," reports Babatunde Akinsanmi, a researcher at the University of Geneva, co-author of the study and NCCR PlanetS associate.
The researchers' results not only allow conclusions to be drawn about the shape of the planet, but also about its interior. This is because the team was also able to derive a parameter called the "Love number" (named after the British mathematician Augustus E. H. Love) from the transit light curve of WASP-103b. It indicates how the mass is distributed within the planet and thus also gives clues about its inner structure. "The resistance of a material to deformation depends on its composition," explains Akinsanmi. "We can only see the tides on Earth in the oceans. The rocky part doesn't move that much. Therefore, by measuring how much the planet is deformed, we can determine how much of it is made up of rock, gas or water."
WASP-103b's Love number is like Jupiter's, our Solar System's biggest gas giant. It 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 about the same size. Therefore, WASP-103b must be highly inflated due to heating by its nearby star, and perhaps other mechanisms," says Monika Lendl, professor of astronomy at the University of Geneva and co-author of the study.