The first look inside the strange planet reveals what makes it so bloated

For the first time, JWST has given us a detailed overview of the interior of a world outside our Solar System.

The extremely strange exoplanet WASP-107b has an atmosphere surprisingly low in methane, which suggests that the exoplanet’s interior must be significantly hotter than we thought, and its core also more massive. This ultimately helps explain the cotton candy-like density of WASP-107b.

Previously, WASP-107b was thought to have a fairly small core, surrounded by a large puffy envelope of hydrogen and helium, which would have required some changes to our understanding of how planets form and evolve. The new results mean that the exoplanet can be explained with existing models, without the need for radical revision.

“The Webb data tell us that planets like WASP-107 b did not have to form in a strange way with a super-small core and a large gas envelope,” says astronomer Mike Line of Arizona State University (ASU ).

“Instead, we could get something more like Neptune, with lots of rocks and not so much gas, just turn up the temperature and sit down to look at the path. [WASP-107b] does.”

Even when the discovery of WASP-107b was announced in 2017, we knew there was something strange about the exoplanet. By carefully studying how the exoplanet impacted its host star, astronomers were able to deduce its mass and radius, which revealed that it had an incredibly low density.

Further analysis revealed that the density is so low that the world can be classified as a ‘super-bloat’ at just 0.13 grams per cubic centimeter. Jupiter’s average density, by comparison, is 1.33 grams per cubic centimeter, and Earth’s is 5.51 grams.

We also know from those earlier studies that the giant exoplanet orbits a star about 200 light-years away, with an orbital period of 5.7 days.

While this may seem short to us here in the Solar System, for the bloated gas giants it’s a loop that happens to make WASP-107b cooler than its fellow hot Jupiters with much shorter orbital periods, extended atmospheres of which can be explained by the heat radiating from the star. WASP-107b’s ‘distant’ orbit and relatively cool temperature made its swelling challenging to explain.

So two teams of astronomers, one led by Sing and the other led by ASU’s Luis Welbanks, recruited JWST to take a look at the exoplanet’s atmosphere.

As WASP-107b passes between us and its host star, some of the star’s light is absorbed or amplified by molecules in the exoplanet’s atmosphere. By studying the difference in starlight with and without exoplanets, and looking for brighter and dimmer wavelengths in the spectrum, astronomers can identify the fingerprints of specific molecules in an exoplanet’s gas blanket.

An artist’s impression of WASP-107b passing in front of its host star, with its atmosphere glowing from behind. (ESA/Hubble, NASA, M. Kornmesser)

While it is surprising that WASP-107b’s atmosphere contains very little methane, it provides an explanation for how the exoplanet came to be the way it is.

“This is evidence that hot gas from deep within the planet must be mixing strongly with the cooler layers above,” says Sing.

“Methane is unstable at high temperatures. The fact that we detected so little, even though we detected other carbon-containing molecules, tells us that the interior of the planet must be significantly hotter than we thought.”

This is one piece of the puzzle. Another part includes the rest of what researchers found in WASP-107b’s atmosphere, including sulfur dioxide, water vapor, carbon dioxide and carbon monoxide, with a higher content of heavy elements than Neptune or Uranus .

By combining the ratios of heavier to lighter elements with how much energy is inside the exoplanet based on the amount of heat it generates, the researchers determined the size of WASP-107b’s core. And they found that it was much larger than we thought, 12 times the mass of the Earth’s core and at least twice as large as originally thought.

This means we don’t need strange models of planetary formation to explain its existence.

As for what is causing the core to be so hot, that will require further investigation. The exoplanet’s orbit around its host star is slightly elliptical, which puts changing gravitational stress on the planet’s interior, heating it up from the inside. Researchers believe this is probably the source of heat that makes WASP-107b so hot.

The two papers were published in Nature. They can be found here and here.

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