Called wasp-103b and orbits a star 1,7 times larger and about 200 degrees hotter than the Sun. This exoplanet, studied by an international team led by the researcher at Institute of Astrophysics and Space Science (IAstro) Susana Barros, has the particularity of having approximately the shape of a rugby ball. The study, based on new observations made by the space mission CHEOPS, of the European Space Agency (ESA), has now been published in the journal Astronomy & Astrophysics.
For Susana Barros (IAstro & Dep. Physics and Astronomy – Faculty of Sciences of the University of Porto): “This result is the result of several years of our work at IAstro, to develop models of planet deformation and data analysis models of extreme precision. This allowed us to lead this study within the CHEOPS consortium, whose extreme precision allowed us to detect for the first time the deformed shape of an exoplanet.”
Just as our Moon's gravity pulls on our planet (which gives rise to the movement of the oceans known as tides), so does the Sun's gravity pull on our planet. Fortunately, due to the great distance it is from us, the Sun's gravity is insufficient to cause a very significant deformation of the surface of our planet.
But it is not the case of wasp-103b – this exoplanet is so close to its star that it takes just one day to complete one orbit. With such extreme proximity, astronomers had long suspected that the massive tidal forces caused by the star would result in a massive deformation of the planet, which until now had not been possible to confirm.
Thanks to the combination of observations of transits of the exoplanet, carried out by CHEOPS, with data already known from the Hubble Space Telescope (NASA/ESA) and the Spitzer Space Telescope (NASA), the team was able to confirm that the planet is, in fact, wider at the equator than at the poles, having a shape roughly similar to a rugby ball. But the great precision of the CHEOPS observations can be used to reveal even new information about the internal structure of this warped planet.
Susana Barros also explains: “Here on Earth we have tides caused by the Moon and the Sun, but we only see them in the movement of the oceans – the rocky part of the Earth practically does not move. Measuring how much the planet deforms allows us to determine which parts of it are rocky, gaseous or water, as the resistance of a material to being deformed depends on its composition.”
This calculation is done by determining the Love number, a physical parameter that measures how mass is distributed across the planet. In the case of Wasp-103b, the Love number has a similar value to that of the largest planet in the Solar System, which seems to suggest a similar internal composition even though the planets are in very different environments.
“In principle, you would expect an exoplanet with 1,5 times the mass of Jupiter to be about the same size,” explains Barros. However, the exoplanet is twice the diameter of Jupiter. The researcher adds: “Due to its proximity to its star, WASP-103b is very hot, with a temperature of 2500 K. This is thought to be the cause of the planet being bloated relative to Jupiter, with a diameter 2 times larger. . So it is quite interesting that the internal structure of the two planets is similar.”
Due to the poor precision in the calculation of the Love number, more observations with CHEOPS will be necessary, complemented with observations in the infrared band with the recently launched JWST – James Webb Space Telescope (NASA/ESA), which will further improve measurements of tidal deformation of exoplanets. Barros explains: “If we can determine its structure with more observations, we can better understand why the planet is so swollen. Knowing the size of its core will still be important to understand how this exoplanet formed.”
Nuno Cardoso Santos (IAstro & DFA-FCUP), the principal investigator of the IAstro Planetary Systems team comments: “This result illustrates well the potential of the CHEOPS mission and the ability of the IA team to do innovative science. But this is just a first step. More observations from the CHEOPS mission, as well as data that will be collected by future missions, such as PLATO, will give us the possibility to study deformation on more exoplanets, and thus make further leaps forward in their detailed study.”
There is yet another mystery to solve about WASP-103b, discovered by the team: tidal forces on a planet so close to its star normally cause its orbit to slow down, causing the planet's orbit to decay, until it is eventually swallowed by the star. . However, WASP-103b's orbital speed appears to be increasing, with the planet moving away from the star.
The team tested several scenarios to explain this behavior, such as the presence of a companion star or even the exoplanet's orbit being more elliptical than predicted, but the data does not allow us to conclude which option is correct. Only with additional data will it be possible to clarify whether this increase in speed actually exists, and if so, what causes it.
The CHEOPS consortium is led by Switzerland and ESA. It has the participation of 11 European countries, and in Portugal the scientific participation is led by the IA. The participation of the IA in the CHEOPS consortium is part of a broader strategy to promote research on exoplanets in Portugal, through the construction, development and scientific definition of various instruments and space missions, such as the CHEOPS or the spectrograph EXPRESS, already in operation at the Paranal Observatory (ESO).
Sérgio Sousa (IAstro & Universidade do Porto), adds: “our leadership role in these missions and projects allows us to be at the forefront of these results. In the case of CHEOPS, in addition to having developed software for the mission, we have researchers, such as Susana Barros, who lead working groups to explore the data”.
IAstro's strategy will continue over the next few years, with the launch of the PLATO Space Telescope (ESA) in 2026, the ARIEL (ESA) mission in 2029, and the installation of the HIRES spectrograph on the largest next-generation telescope, the ELT (ESO), scheduled to become operational in 2030.
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