Normally, planets are roughly spherical, their own gravity bringing them into this shape. But there are exceptions – one of them is the exoplanet WASP-103b in the constellation Hercules. This planet orbits very closely to a bright star and takes only one day to complete its orbit. The consequences of this have now been observed directly by astronomers for the first time with the help of the CHEOPS space telescope. The light curves of WASP-103b’s transits in front of its star reveal that the planet is severely deformed – the nearby star’s tidal forces have pulled it apart in a rugby ball shape. The extent of this deformation also allows conclusions to be drawn about its internal composition.
We experience the effects of tidal forces on earth every day: the gravitational pull of the moon acts on the water in the oceans and creates ebb and flow. The sun’s gravitational influence also comes into play, albeit to a lesser extent because of its great distance. It is far more extreme, however, when a planet orbits much closer to its star, as is the case with many so-called hot Jupiters – large gas planets whose orbits are very close to the star. Astronomers have long suspected that this proximity leads to extreme tidal forces on the planet. They could be so strong that they elliptically stretch and deform the entire planet. So far, however, it has never been possible to observe this effect directly.
Light curves reveal deformation
To change this, 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. Astronomers can then use the light curves to deduce the essential properties of the exoplanets. One of the targets of the measurement campaign was the planet WASP-103b, a hot Jupiter in the constellation Hercules, which is about twice the size and 1.5 times the mass of Jupiter. It orbits a star that is around 200 degrees hotter and 1.7 times larger than our sun – but takes just under a day to complete one orbit. WASP-103b is about fifty times closer to its star than Earth is to the sun.
“Because of its close proximity to its star, we had already suspected that very large tides are caused on the planet. However, we have not been able to prove this so far,” explains co-author Yann Alibert from the University of Bern. But after the team had analyzed twelve transits of the planet with CHEOPS and also used 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, it resembles an elliptical rugby ball rather than a sphere. “It’s incredible that CHEOPS was able to detect this small deformation,” says co-author Jacques Laskar from the Sorbonne University Observatory in Paris. “This is the first time such an analysis has been conducted.”
Love number and orbital changes
The results not only allow conclusions to be drawn about the shape of WASP-103b, but also about its interior. The team was also able to derive what is known as the Love 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 composition,” explains co-author Babatunde Akinsanmi from the University of Geneva. “By measuring how much the planet is deformed, we can therefore determine how much of it is made up of rock, gas or water.” The analysis showed that the Love number of WASP-103b is similar to that of Jupiter. The exoplanet must therefore have a similar structure to the gas giant in our solar system, but it is more inflated. What exactly caused this inflation is still unclear.
Also unclear is the influence of stellar gravity on WASP-103b’s orbit. According to the theory, a planet so close to its star should be so strongly attracted that its orbit spirals ever narrower. Eventually, the planet comes so close to its star that its gravity tears and engulfs it. In the case of WASP-103b, however, the team of astronomers could not find any clear evidence of such a narrowing of the orbit – rather the opposite. “There is evidence of an extension of the orbital period, which contradicts expectations,” explain Barros and her colleagues. However, whether the exoplanet is actually drifting away from its star or whether the data are due to statistical artifacts and other measurement-related distortions still needs to be clarified.
Source: Susan Barros (Universidade do Porto) et al., Astronomy & Astrophysics, doi: 10.1051/0004-6361/202142196