Why is Cotton-Candy Exoplanet WASP-107b So Puffy?

A surprisingly low amount of methane and a super-sized core hide within WASP-107b, a super-Neptune exoplanet approximately 212 light-years away in the constellation of Virgo, according to two new papers published in the journal Nature.

This artist’s concept shows what the exoplanet WASP-107b could look like based on recent data gathered by the NASA/ESA/CSA James Webb Space Telescope along with previous observations from the NASA/ESA Hubble Space Telescope and other observatories. Image credit: NASA / ESA / CSA / Ralf Crawford, STScI.

WASP-107 is a highly active K-type main sequence star located about 212 light-years away in the constellation of Virgo.

First discovered in 2017, WASP-107b is one of the least dense exoplanets known — a type that astrophysicists have dubbed ‘super-puff’ or ‘cotton-candy’ planets.

The planet orbits very close to the star — over 16 times closer than the Earth is to the Sun — once every 5.7 days.

It has one of the coolest atmospheres of any of the exoplanets discovered, although at 500 degrees Celsius (932 degrees Fahrenheit) is still radically hotter that Earth.

The high temperature is thought to be a result of tidal heating caused by the planet’s slightly non-circular orbit, and can explain how WASP-107b can be so inflated without resorting to extreme theories of how it formed.

“Based on its radius, mass, age, and assumed internal temperature, we thought WASP-107b had a very small, rocky core surrounded by a huge mass of hydrogen and helium,” said Arizona State University’s Dr. Luis Welbanks, lead author of the first paper.

“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.”

“If WASP-107b instead has more of its mass in the core, the atmosphere should have contracted as the planet cooled over time since it formed.”

“Without a source of heat to re-expand the gas, the planet should be much smaller.”

“WASP-107b is such an interesting target for Webb because it’s significantly cooler and more Neptune-like in mass than many of the other low-density planets, the hot Jupiters, we’ve been studying,” said Johns Hopkins University’s Dr. David Sing, lead author of the second paper.

“As a result, we should be able to detect methane and other molecules that can give us information about its chemistry and internal dynamics that we can’t get from a hotter planet.”

WASP-107b’s giant radius, extended atmosphere, and edge-on orbit make it ideal for transmission spectroscopy, a method used to identify the various gases in an exoplanet atmosphere based on how they affect starlight.

Combining observations from Webb’s Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI), and Hubble’s Wide Field Camera 3 (WFC3), Dr. Welbanks and colleagues were able to build a broad spectrum of 0.8- to 12.2-micron light absorbed by WASP-107b’s atmosphere.

Using Webb’s Near-Infrared Spectrograph (NIRSpec), Dr. Sing and colleagues built an independent spectrum covering 2.7 to 5.2 microns.

The precision of the data makes it possible to not just detect, but actually measure the abundances of a wealth of molecules, including water vapor, methane, carbon dioxide, carbon monoxide, sulfur dioxide, and ammonia.

Both spectra show a surprising lack of methane in WASP-107b’s atmosphere: one-thousandth the amount expected based on its assumed temperature.

“This is evidence that hot gas from deep in the planet must be mixing vigorously with the cooler layers higher up,” Dr. Sing said.

“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.”

A likely source of WASP-107b’s extra internal energy is tidal heating caused by its slightly elliptical orbit.

With the distance between the star and planet changing continuously over the 5.7-day orbit, the gravitational pull is also changing, stretching the planet and heating it up.

Astronomers had previously proposed that tidal heating could be the cause of WASP-107b’s puffiness, but until the Webb results were in, there was no evidence.

Once they established that the planet has enough internal heat to thoroughly churn up the atmosphere, the researchers realized that the spectra could also provide a new way to estimate the size of the core.

“If we know how much energy is in the planet, and we know what proportion of the planet is heavier elements like carbon, nitrogen, oxygen, and sulfur, versus how much is hydrogen and helium, we can calculate how much mass must be in the core,” said Johns Hopkins University’s Dr. Daniel Thorngren.

“It turns out that the core is at least twice as massive as originally estimated, which makes more sense in terms of how planets form.”

“All together, WASP-107b is not as mysterious as it once appeared.”

“The Webb data tells us that planets like WASP-107b didn’t have to form in some odd way with a super small core and a huge gassy envelope,” said Arizona State University’s Dr. Mike Line.

“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.”

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L. Welbanks et al. A high internal heat flux and large core in a warm Neptune exoplanet. Nature, published online May 20, 2024; doi: 10.1038/s41586-024-07514-w

D.K. Sing et al. A warm Neptune’s methane reveals core mass and vigorous atmospheric mixing. Nature, published online May 20, 2024; doi: 10.1038/s41586-024-07395-z

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