For the first time, the JWST has given us a detailed view of the interior of a world outside our solar system.
The extremely strange exoplanet WASP-107b has an atmosphere that is surprisingly low in methane, which suggests that the exoplanet’s interior must be considerably hotter than we thought – and that its core is also more massive. This finally helps explain WASP-107b’s cotton candy-like density.
WASP-107b was previously thought to have a fairly small core surrounded by a huge swollen shell of hydrogen and helium – which would have required some changes in our understanding of how planets form and evolve. The new results mean the exoplanet can be explained using existing models, without the need for radical revision.
“The Webb data tells us that planets like WASP-107 b didn’t have to form in a strange 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 that looks more like Neptune, with a lot of rock and not so much gas, just raise the temperature and turn it up to watch the way [WASP-107b] do.”
Even when the discovery of WASP-107b was announced in 2017, we knew something strange was going on with the exoplanet. By carefully studying how the exoplanet affected its host star, astronomers were able to infer its mass and radius, revealing that its density was incredibly low.
Further analysis showed that the density is so low that the world can be classified as a ‘super cloud’: just 0.13 grams per cubic centimeter. For comparison, Jupiter’s average density is 1.33 grams per cubic centimeter, and Earth’s is 5.51 grams.
We also know from those 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, for puffy gas giants it’s quite a loop that happens to make WASP-107b cooler than its peers – 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 it difficult to explain its puffiness.
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 the star’s light with and without the exoplanet, and looking for brighter and dimmer wavelengths in the spectrum, astronomers can identify the fingerprints of specific molecules in an exoplanet’s blanket of gas.
Although it is surprising that WASP-107b’s atmosphere contains very little methane, this does provide a solution 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 mix vigorously with the cooler layers higher up,” says Sing.
“Methane is unstable at high temperatures. The fact that we detected so little, even though we did detect other carbonaceous molecules, tells us that the interior of the planet must be significantly hotter than we thought.”
This is one piece of the puzzle. Another piece concerns the rest of what the 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 and lighter elements with the amount of energy contained in the exoplanet based on the amount of heat it generates, the researchers determined the size of WASP-107b’s core. And they discovered it was much bigger than we thought: twelve times the mass of the Earth’s core, and at least twice as heavy as first thought.
This means we don’t need any weird planetary formation models to explain its existence.
What causes the core to be so hot requires further research. The exoplanet’s orbit around its parent star is slightly elliptical, which exerts a varying gravitational pull on the planet’s interior, heating it from within. The researchers think this is likely the source of the heat that makes WASP-107b so hot.
The two articles were published in Nature. They can be found here and here.
