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For the first time, astronomers are discovering the Rugby ball shape of a deformed exoplanet

In addition to the solar system, out there in the wider galaxy, a very strange subgroup of exoplanets exists. They are called hot Jupiters, and they are mostly sticky - they hug so close to their host stars that they are not only intensely hot, but probably also distorted by gravitational forces.

Now, for the first time, astronomers have actually discovered the crooked shape of one of these hot Jupiters. Unlike ordinary, spherical planets we are all used to, this one is pulled out of shape into something more like a rugby ball.

This achievement, thanks to the CHEOPS space telescope, could help us understand how these exoplanets are going to exist in such extreme orbits.

"It's incredible that CHEOPS was actually able to detect this small deformation," says astronomer Jacques Laskar of the Paris Observatory, Université Paris Sciences et Lettres in France.

"This is the first time such an analysis has been made, and we can hope that observation over a longer period of time will strengthen this observation and lead to better knowledge of the planet's internal structure."

Cheops reveals a rugby ball-shaped exoplanet infographic(TO)

The exoplanet is called WASP-103b, which orbits a star called WASP-103 about 1,800 light-years away. It's pretty much a textbook hot Jupiter. As the name suggests, these exoplanets are gas giants like Jupiter; unlike Jupiter, however, they orbit very close to their host stars, with orbital periods of less than 10 days. That's what makes them hot.

According to current models of planet formation, technically hot Jupiters should not exist. A gas giant can not form so close to their star because gravity, radiation and intense stellar winds should prevent the gas from clumping together.

They do exist, however; of the nearly 5,000 confirmed exoplanets discovered to date, over 300 could be hot Jupiters. It is believed that they form further out in their planetary systems, and then migrate inward toward the star. No matter how they are formed, they can tell us a lot about the interactions of gravity or tides between a planet and a star, so they are very interesting to study.

WASP-103b was first discovered in 2015 and it ticks nicely in the boxes. It is about 1.5 times the mass and twice the size of Jupiter and orbits its star so closely that it orbits once a day. This means that it is incredibly hot, about 20 times hotter than Jupiter.

Although relatively large, we can not measure the WASP-103b directly. The light from its home star greatly outshines it. However, we can measure its transits. This is when the exoplanet passes between us and the star, causing small changes in starlight; a slight decrease when the exoplanet passes in front of the star, and a much weaker one when it passes behind, called a light curve.

transit light curveAn exoplanet phase light curve. (ESA)

The European Space Agency's CHEOPS is designed to detect these light curves with high precision. It achieved multiple transits to WASP-103; these data enabled astronomers to calculate how the mass of the exoplanet is distributed internally, and obtain a set of parameters known as love numbers.

In turn, this gives clues as to the composition of the exoplanet. This is because the resistance of a material to deformation depends on what it is made of, the researchers explain.

For example, the Earth's oceans change in response to the Moon's tidal currents, but the continents do not, at least not nearly as much. So the deformation of a planet can reveal what it is made of - whether it is a solid, a liquid or a gas.

According to their analysis, WASP-103b is not just roughly Jupiter-sized, it has a similar composition and structure. Maybe it's a little more flimsy though. Although its mass is 1.5 times the mass of Jupiter, its size is about twice that. This suggests that the exoplanet is inflated, probably due to the heat of the star.

"If we can confirm the details of its inner structure with future observations, we may be able to better understand what makes it so inflated. Knowing the size of the core of this exoplanet will also be important to better understand how it is formed, "explains Susana Barros of the Department of Astrophysics and Space Science and the University of Porto in Portugal.

More observations will help astronomers figure this out and perhaps solve another mystery along the way. Most hot Jupiters have orbital periods that get shorter as they spiral closer to their stars. The cycle of WASP-103bs appears to be longer, according to the CHEOPS observations.

It is impossible to say what may be the cause of this. Another body may interfere with the circulation, or the circulation may be eccentric, causing irregularities.

It is also possible that the measurements are slightly deviating or have not been correctly understood and that the circuit is actually decreasing. Studying the star and its strange exoplanet could further help solve this riddle.

"Future observations with the James Webb Space Telescope could help better limit the Love figure for WASP-103b and gain an unprecedented view of the interior of this hot Jupiter," the researchers write.

"This can help us better understand these extreme systems."

The research is published in Astronomy and astrophysics.

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