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 with a surprisingly low methane content, 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 puffy 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 by 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 some weird way with a super-small core and a huge gas envelope,” says Arizona State University (ASU) astronomer Mike Line.
“Instead, we can get something more like Neptune, with lots of rocks and not as much gas, just dial up the temperature and turn it up so it looks good [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 affected its host star, astronomers were able to derive its mass and radius, which revealed that it has an incredibly low density.
Further analysis revealed that the density is so low that the world can be classified as “super-fluffy” – 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 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, for the fluffy gas giants it’s quite a loop, making WASP-107b colder than its peers—hot Jupiters with much shorter orbital periods whose extended atmospheres can are explained by heat emitted by their star. WASP-107b’s “distant” orbit and relatively cool temperature made its puffiness difficult to explain.
So two teams of astronomers, one led by Singh and the other led by ASU’s Louis Welbanks, hired JWST to look at the exoplanet’s atmosphere.
As WASP-107b passes between us and the 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 the exoplanet’s gaseous envelope.
While it is surprising that WASP-107b’s atmosphere contains very little methane, this suggests an explanation of how the exoplanet came to be the way it is.
“This is evidence that hot gas from deep within the planet must be vigorously mixing with the cooler layers higher up,” says Singh.
“Methane is unstable at high temperatures. The fact that we found so little, even though we found 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 have 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 how much heat it generates, the researchers 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 we don’t need strange models of planetary formation to explain its existence.
As for what causes the core to be so hot, that will require further investigation. The exoplanet’s orbit around its host star is slightly elliptical, which puts a changing gravitational stress on the planet’s interior, heating it up from the inside. Researchers believe this is likely the source of heat that makes WASP-107b so hot.
Both reports were published in Nature. You can find them here and here.
