Rugby ball-shaped exoplanet surprises astronomers

Artist’s concept of the rugby ball-shaped exoplanet WASP-103b. Strong tidal forces from its star cause the planet’s unusual shape. Image via ESA/ University of Bern.

Planets are round, right? While many are a bit compressed at the poles (as is Earth), all the planets in our solar system are in fact spheres. The same goes for the many exoplanets that have been found so far. But on January 11, 2022, astronomers announced that they found a giant world that defies the norm and looks like a… rugby ball.

An international team of researchers studied the oddly shaped planet using the CHEOPS space telescope. The astronomers published their peer-reviewed findings in the journal Astronomy and Astrophysics on January 11

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A rugby ball-shaped exoplanet bigger than Jupiter

The planet, WASP-103b, is a giant nearly twice the size of Jupiter. The star it orbits is in the direction of the constellation Hercules. The planet is also one and a half times the mass of Jupiter and orbits its star about 50 times closer than Earth is to our sun. It completes a job in just one day! This means that it is a type of planet called a hot Jupiter. That is, it is a gas giant planet orbiting close to its star. And it turns out it’s shaped like a rugby ball. It is located 1,225 light-years from Earth and was first discovered in 2014.

The Hubble Space Telescope and Spitzer Space Telescope also looked at WASP-103b. With the additional observations from CHEOPS, the astronomers were now able to measure the tidal deformation of the planet. Tidal forces “drag” on a planet, much like Earth’s moon pulls on our world’s oceans, creating the tides. For WASP-103b, these forces were suspected, but not confirmed. Co-author Yann Alibert said:

Due to the close proximity of its star, we already suspected that very large tides are caused on the planet. But we hadn’t been able to verify that yet.

These strong tidal forces cause the planet to “stretch” in the middle, making the planet more elongated than round.

Poster with many photos related to location, distance, size and mass of explanet with labels.
Infographic detailing the discovery of WASP-103b. It orbits its star in just one day and is probably similar to Jupiter inside. However, it is larger, more massive and hotter than Jupiter. Image via ESA/ University of Bern.

Measuring Tide Distortion

The researchers measured the planet’s slight but noticeable tidal shifting. They did this by observing the transitions of the planet as it passed in front of its star as seen from Earth. Co-author Babatunde Akinsanmi, a researcher at the University of Geneva, said:

After observing several such so-called “transits”, we were able to measure the distortion. It’s unbelievable that we were able to do this; it is the first time such an analysis has been done.

So the new observations tell scientists the shape of the planet. But they also give clues about the inside of this world. How is that possible? Basically, the researchers were able to derive a parameter known as the “Love Number” from the transit light curves. This parameter is named after the British mathematician Augustus Edward Hough Love. It provides information about how mass is distributed within a planet. Consequently, this can also give hints about the inner structure of the planet. As Akinsanmi noted:

The resistance of a material to deformation depends on its composition. We can only see the tides on Earth in the oceans. The rocky part doesn’t move that much. Therefore, by measuring how much the planet has been deformed, we can determine how much of it is made up of rock, gas or water.

Smiling young woman standing with her arms crossed and pictures on the wall behind her.
Monika Lendl of the University of Geneva is one of the co-authors of the new study. Image via Fabien Scotti/ University of Geneva/ University of Bern.

Jupiter-like inside

Despite the size difference, the interior of the WASP-103b is believed to be similar to Jupiter. Co-author Monika Lendl of the University of Geneva said:

In principle, we would expect a planet with 1.5 times the mass of Jupiter to be about the same size. Therefore, WASP-103b must be highly inflated due to heating from the nearby star and perhaps other mechanisms.

The results so far gave a good idea about the shape and interior of the planet, but further observations are still needed. These include observations by both CHEOPS and the newly launched James Webb Space Telescope. Lendl added:

This would improve our understanding of these so-called “hot Jupiters” and allow for a better comparison between them and giant planets in the solar system.

Bright star with three planets and a spacecraft.
The main scientific goal of CHEOPS is to study the structure of exoplanets in the size range from super-Earth to Neptune that orbit bright stars with an orbital period of less than 50 days. It is the 1st of 3 special exoplanet missions by the European Space Agency (ESA). Image via ESA.

In short, an international team of astronomers has discovered that one giant exoplanet is unlike any other before. It is about twice the size of Jupiter and its unusual elongated shape makes it resemble a rugby ball. It orbits its star 50 times closer than the Earth does to the sun.

Source: Detection of the Tide Deformation of WASP-103b at 3 Sigma with CHEOPS

Via University of Bern

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