Generally, planets are roughly spherical in shape, their own gravity pulling them into this shape. But there are exceptions – one of them is the exoplanet WASP-103b in the constellation Hercules. The planet orbits a bright star very closely and takes only one day to complete its orbit. Astronomers have now for the first time directly observed its results using the CHEOPS space telescope. The light curves of WASP-103b’s transit in front of its star reveal that the planet is severely deformed – the tidal forces of a nearby star tore it apart like a rugby ball. The extent of this deformation also allows to draw conclusions about its internal structure.
We experience the effects of tidal forces on Earth every day: the Moon’s gravitational pull acts on the waters of the oceans and creates ups and downs. The effect of the Sun’s gravity also comes into play, albeit to a lesser extent because of its great distance. However, it is far more extreme when a planet orbits very close to its star, as is the case with many so-called hot Jupiters – large gas planets that have orbits very close to the star. Astronomers have long suspected that this proximity leads to extreme tidal forces on the planet. They can be so strong that they elliptically spread and deform the entire planet. Until now, however, it has never been possible to directly observe this effect.
light curves reveal distortion
To change that, an international team of astronomers led by Susana Barros from the University of Porto in Portugal took a closer look at some hot Jupiters with the CHEOPS space telescope. This telescope is designed to record the light curves of planetary transits in high resolution. Using light curves, astronomers can estimate the essential properties of exoplanets. One of the targets of the measurement campaign was the planet WASP-103b, a hot Jupiter in the constellation Hercules, about twice as large and 1.5 times as massive as Jupiter. It orbits a star that is about 200 degrees hotter and 1.7 times larger than our Sun – but takes only a day to complete one orbit. WASP-103b is about fifty times closer to the Sun than its star is to Earth.
“Because of its proximity to its star, we already suspected that the planet would have very large tides. So far, we haven’t been able to prove it,” explains co-author Yann Elibert of the University of Bern. But when the team analyzed twelve transits of the planet with CHEOPS and also combined data from the Hubble and Spitzer space telescopes, this changed: From the light curves, they could see that WASP-103b must be deformed. As a result of the strong tidal forces, this one The elliptical looks more like a rugby ball than a sphere. “It’s unbelievable that CHEOPS was able to detect this tiny deformity,” says co-author Jacques Lasker of the observatory at the Sorbonne University in Paris. “This is the first time When this kind of analysis has been done.”
love numbers and orbital changes
The results allow us to draw conclusions not only about the size of WASP-103b, but also about its interior. The team was also able to derive the so-called Lav number from the planet’s transit light curve – a parameter that indicates how mass is distributed within the planet. “A material’s resistance to deformation depends on its structure,” explains co-author Babatunde Akinsanami from the University of Geneva. “By measuring how deformed the planet is, we can determine how much rock, gas or water it contains.” The evaluation showed that WASP-103b has a love number similar to that of Jupiter. So the composition of the exoplanet should be similar to that of the gas giant in our solar system, but it is more bloated. Exactly what caused this swelling is still unclear.
The effect of stellar gravity on WASP-103b’s orbit is also unclear. According to the theory, a planet at such a short distance from its star would have to be attracted so strongly that its orbit would continue to narrow in a spiral. At some point the planet will come so close to its star that it will be torn apart by its own gravity. In the case of WASP-103b, however, the team of astronomers found no clear indication of such a contraction of the orbit – quite the contrary. “There is evidence for a longer period of orbital period, which is contrary to expectations,” Barros and colleagues explain. Whether the exoplanet is actually moving away from its star or whether the data is due to statistical artifacts and other measurement-related distortions remains to be clarified.
Quayle: Susan Barros (University of Porto) et al., Astronomy and Astrophysics, doi:10.1051/0004-6361/02142196
