Astronomers discover rugby ball-shaped exoplanet

Planets are usually round bodies in space that orbit stars, at least most of the time anyway: we just got our first glimpse of an exoplanet that has been warped by the gravity of its star into a body similar to a rugby ball.

Although it sounds strange, it is not unexpected, but it is the first time that we have seen something like this in the universe. The exoplanet, detected by ESA Cheops exoplanet hunting mission, orbiting WASP-103 in the constellation of Hercules.

Cheops finds exoplanets by measuring starlight and watching for telltale drops in brightness as a possible exoplanet passes between us and the star. Dips that occur at regular and precise intervals are very strong evidence of an exoplanet.

This exoplanet, WASP-103b, is a gas giant about twice the size of Jupiter at 1.5 times its mass. But rather than orbiting within the confines of its solar system, WASP-103b is what is called a “hot Jupiter”.

They are gas giants that orbit very close to their star, often much closer than even Mercury orbits our Sun. As a result, these exoplanets typically orbit their stars much faster than what we’re used to seeing, but WASP-103b orbits its star – which is about 1.7 times the size of our sun – in less than a day.

This puts it extremely close to WASP-103; so close, in fact, that the gravity exerted on the star-facing side of the planet is much greater than that exerted on the opposite side of the star.

This difference in gravity, known as the tidal force, extends the exoplanet out of the typical spheroid shape. We experience something similar with the moon as it produces ocean tides on Earth, hence its name. Until now, however, we have never seen these forces actually warp a planet.

The astronomers of Cheops succeeded in detecting the deformation thanks to the very fast orbital period of the exoplanet. This gave astronomers plenty of opportunities to take measurements and observe the exoplanet passing through the star, and the data they saw in the star’s light curve revealed the planet’s unusual shape.

“It’s incredible that Khéops was able to reveal this tiny deformation”, Jacques Laskar, of the Paris Observatory, Université Paris Sciences et Lettres, and co-author of the study detailing the results of this week Astronomy & Astrophysics newspaper, said in a ESA statement.

“This is the first time that such an analysis has been carried out, and we can hope that the observation over a longer time interval will reinforce this observation and lead to a better knowledge of the internal structure of the planet.”

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Analysis: why has our own Jupiter not become “hot”?

When we first started looking at exoplanets, we expected to see them laid out as they are here in our own solar system, with rocky inner worlds and large gas giants in the outer regions.

We did, however, find a surprising number of hot Jupiters, including the very first exoplanet ever identified around a main sequence star (i.e. one that was not a stellar corpse), Pegasus 51b.

While a majority of gas giant exoplanets that we have identified mirror the configuration of our own solar system, it begs the question of what causes a gas giant to migrate rapidly to such a close orbit around a star, and which prevented the original Jupiter from doing the same.

We don’t really know, honestly. This is one of the reasons astronomers are so eager to study exoplanets – it’s our real best hope of understanding how our own solar system formed.