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

For the first time, JWST has given us a detailed breakdown of the interior of a world outside our solar system.

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

WASP-107b was previously thought to have a fairly small core surrounded by a huge bloated envelope of hydrogen and helium – which would require some changes in our understanding of how planets form and evolve. The new results mean that the exoplanet can be explained using existing models without the need for a radical revision.

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

“Instead, we can take something like Neptune, with lots of rock and not so much gas, just turn up the temperature and inflate it to make it look like [WASP-107b] does.”

When the discovery of WASP-107b was announced in 2017, we already knew something was strange about the exoplanet. By carefully studying how the exoplanet affected its host star, astronomers were able to deduce its mass and radius, revealing that it has an incredibly low density.

Further analysis revealed that the density is so low that the world can be classified as a “super-puff” – just 0.13 grams per cubic centimeter. By comparison, Jupiter’s average density is 1.33 grams per cubic centimeter, and Earth’s density is 5.51 grams.

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

While that may seem short to us here in the Solar System, it’s quite a loop for the bloated gas giants, making WASP-107b cooler than its peers—the hot Jupiters with much shorter orbital periods, whose expanded atmospheres can be explained by heat radiating from their star. WASP-107b’s “distant” orbit and relatively low temperature made its inflation difficult to explain.

So two teams of astronomers, one led by Singh and the other led by ASU’s Luis Welbanks, recruited JWST to 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 light from a star with and without an exoplanet and looking for brighter and fainter wavelengths in the spectrum, astronomers can identify the fingerprints of specific molecules in the exoplanet’s gas mantle.

While it is surprising that WASP-107b’s atmosphere contains very little methane, it does offer this an explanation of how an exoplanet came to be what it is.

“This is evidence that the hot gas from deep within the planet must be mixing intensively 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. The next section includes the rest of what scientists found in WASP-107b’s atmosphere — including sulfur dioxide, water vapor, carbon dioxide and carbon monoxide with higher heavy element content than Neptune or Uranus.

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

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

As to what causes the core to be so hot, that will require further investigation. An exoplanet’s orbit around its host star is slightly elliptical, which creates a changing gravitational stress on the interior of the planet, heating it from the inside. Scientists believe this is likely the source of heat that makes WASP-107b so hot.

These two documents were published in Nature. They can be found here and here.