An exoplanet in the shape of a rugby ball: unheard of!

As part of ESA’s Cheops mission, an exoplanet twice the size of Jupiter has been detected orbiting extremely close to its star. Result, a tidal effect such that the planet, totally deformed, presents the shape of a rugby ball!

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[EN VIDÉO] Exoplanets
What is an exoplanet, where are they found and why are they so interesting? Video response!

Launched at the end of 2019, the satellite Cheops (CHaracterising ExOPlanet Satellite) of the European Space Agency’s main mission is to study stars neighboring our Sun, known to harbor exoplanets in their star systems. With high measurement precision, the mission aims, among other things, to precisely characterize the sizes and densities of exoplanets detected.

Among these subjects of study is WASP-103, a star located in the constellation of Hercules around which orbit WASP-103b, a exoplanet twice as large as Jupiter and 1.5 times as massive. When it was discovered in 2014, the astronomers were surprised by its close proximity to its star (WASP-103b achieves a revolution in just one day), and suspected that the planet was subjected to extreme effects of tide, without being able to measure them.

The transit method to observe exoplanets… but also to deduce their shape!

The Cheops satellite uses the transit method to observe and characterize exoplanets. By regularly measuring the brightness of a star, we can detect a periodic decrease in luminosity, associated with the passage of a celestial body in front of the star: this is called a transit. By comparing 12 observations of the planet’s transit, astronomers have detected tiny differences between the various transits, associated with the deformation of the planet. This is the first time that the deformation of an exoplanet has been detected in this way.

Monumental tidal effects

The tide is a well-known phenomenon on Earth: the masses of water on the surface of our Planet are subject to the gravitational pull exerted by the Moon. But for WASP-103b, the scale of the phenomenon is much larger: because of its extreme proximity to its star, the tidal effects are very pronounced, deforming the entire planet. Thus, the planet no longer presents a spherical shape, but rather that of a ballon the rugby !

To be sure, ESA astronomers estimated the number of Love, a parameter characterizing the rigidity of a planetary body and its susceptibility to being deformed under the effect of tidal forces. This parameter also makes it possible to estimate the composition of the planet: with a value very close to that of our Jupiter, WASP-103b very likely has a very similar composition and internal structure, with fluid behavior.

The researchers are surprised at the passage of the abnormally large size of the planet: in principle, a planet as massive as 1.5 times Jupiter should be of a relatively similar size; here, astronomers suspect that the planet is swollen due to the extreme heat prevailing near its star, but this has yet to be confirmed.

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The planet’s orbit raises new questions

Because of its proximity to its star, astronomers expected that tidal interactions, combined with the strong proximity to the star, would cause WASP-103b’s orbit to decrease in the region. the course of time and that the planet gradually falls towards its star. However, the measurements indicate the opposite, the orbital period of the planet slowly increasing: the planet is in fact slowly moving away from its star. Astronomers are currently looking for possible phenomena dominating the tidal effects, which could cause such a drift.

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