Astronomers may have finally figured out why a distant exoplanet with the density of a microwaved marshmallow has such a fluffy atmosphere.
The planet, named WASP-107 b, is a gas giant situated around 200 light-years from Earth, and is around the same size as Jupiter but has only about one-tenth of its mass.
The mystery of how this planet gained such a low-density "floofy" atmosphere may have been solved, according to two new papers in the journal Nature, and it might be because of the planet's core being a lot warmer than expected.
WASP-107 b was first spotted in 2017 orbiting its star at a distance of about 5.1 million miles—rather close compared to Earth's 94 million-mile orbit—completing an orbit around the star once every 5.7 days. WASP-107 b is only slightly smaller than Jupiter, and has a mass of 30.5 Earths, while Jupiter itself has a mass of 318 Earths.
This means that the planet is one of the least dense ever discovered.
"Based on its radius, mass, age, and assumed internal temperature, we thought WASP-107 b had a very small, rocky core surrounded by a huge mass of hydrogen and helium," Luis Welbanks, a researcher at Arizona State University (ASU) and co-author of one of the papers, said in a statement. "But it was hard to understand how such a small core could sweep up so much gas, and then stop short of growing fully into a Jupiter-mass planet."
According to the papers, WASP-107 b may have grown to such a large size despite its lower mass because of having a hotter-than-expected core, which is indicated by it possessing less methane in its atmosphere than first thought.
"We want to look at planets more similar to the gas giants in our own solar system, which have a lot of methane in their atmospheres," David Sing, a professor of Earth and Planetary Sciences at Johns Hopkins University, and co-author of the other paper, said in another statement. "This is where the story of WASP-107 b got really interesting, because we didn't know why the methane levels were so low."
According to data collected by the James Webb Space Telescope, WASP-107 b has 1,000 times less methane than expected, which indicates that the planet is intensely hot inside.
"This is evidence that hot gas from deep in the planet must be mixing vigorously with the cooler layers higher up," said Sing. "Methane is unstable at high temperatures. The fact that we detected so little, even though we did detect other carbon-bearing molecules, tells us that the interior of the planet must be significantly hotter than we thought."
Additionally, the lower methane levels and higher temperatures indicate that WASP-107 b has an exceptionally large core—12 times the mass of Earth's core—marking the first time that an exoplanet's core mass has been measured from afar.
"Looking into the interior of a planet hundreds of light-years away sounds almost impossible, but when you know the mass, radius, atmospheric composition, and hotness of its interior, you've got all the pieces you need to get an idea of what's inside and how heavy that core is," said Sing. "This is now something we can do for lots of different gas planets in various systems."
The planet's high internal temperature would have allowed it to puff up much more, contributing to its unexpectedly large size. This heat may be generated by tidal heating as a result of its elliptical and close orbit with its star, with the gravity from its sun stretching and squishing its core and heating it up.
"The Webb data tells us that planets like WASP-107 b didn't have to form in some odd way with a super small core and a huge gassy envelope," co-author Mike Line, a researcher from ASU, said in the statement. "Instead, we can take something more like Neptune, with a lot of rock and not as much gas, just dial up the temperature, and poof it up to look the way it does."
The researchers hope to use these techniques to examine the atmosphere and temperature of other exoplanets across the galaxy in the search for signs of extraterrestrial life.
"We had never been able to study this mixing process in an exoplanet atmosphere in detail, so this will go a long way in understanding how these dynamic chemical reactions operate," Sing said. "It's something we definitely need as we start looking at rocky planets and biomarker signatures."
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