The orbiting observatory Cheops has discovered an exoplanet with a truly peculiar shape: WASP-103b, turned into a Jupiter-sized rugby ball by the gravitational pull of its star. This is the first time that the deformation of an exoplanet has been detected.
The odd shape is created by the strong tidal forces the planet experiences. Its star is 1.7 times larger than our Sun (and a bit hotter), and WASP-103b orbits very close to it – one year on that planet is equivalent to one day on Earth.
The planet was discovered thanks to the transit method back in 2014. As it passes in front of the star, it blocks a bit of its light, and repeated periodical dips are hallmarks of a planetary presence. Cheops can follow these light signals with such precision that it could establish the planet’s shape, long suspected to be altered by the strong pull of the star. The findings are reported in the journal Astronomy & Astrophysics.
"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," co-author Babatunde Akinsanmi, a researcher at the University of Geneva, said in a statement.
Cheops is a joint mission of the European Space Agency and Switzerland. Its observation of this system allowed researchers to measure the planet’s Love number. This parameter is named after British mathematician Augustus E. H. Love, and indicates how mass is distributed in the planet. From that, scientists can work out the internal structure.
“The resistance of a material to being deformed depends on its composition,” lead author Susana Barros of Instituto de Astrofísica e Ciências do Espaço and University of Porto, said in a statement.
“For example, here on Earth we have tides due to the Moon and the Sun but we can only see tides in the oceans. The rocky part doesn’t move that much. By measuring how much the planet is deformed we can tell how much of it is rocky, gaseous or water.”
WASP-103b’s Love number is similar to Jupiter. This suggests that its internal structure is similar, despite having 8 times the volume but only 1.5 times the mass. This is certainly an inflated planet.
“In principle we would expect a planet with 1.5 times the mass of the Jupiter to be roughly the same size, so WASP-103b must be very inflated due to heating from its star and maybe other mechanisms,” added Barros.
The uncertainty on the Love number for WASP-103b is still quite high, but follow-up observations from Cheops and work by the freshly launched JWST will allow a better understanding of what’s going on with this rugby-ball-shaped planet.