Researchers use NASA’s JWST to crack case of inflated exoplanet

Why is the warm gas-giant exoplanet WASP-107 b so puffy? Two independent teams of researchers, including The Open University's (OU) Dr Joanna Barstow, have an answer, published today in Nature.

Data collected using NASA's James Webb Space Telescope (JWST), combined with prior observations from the Hubble Space Telescope, show surprisingly little methane (CH₄) in the planet's atmosphere, indicating that the interior of WASP-107 b must be significantly hotter and the core much more massive than previously estimated.

The unexpectedly 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-107 b can be so inflated without resorting to extreme theories of how it formed.

The results, which were made possible by Webb's extraordinary ability to analyse exoplanet atmospheres, may explain the puffiness of dozens of relatively cool, low-density exoplanets, helping solve a long-standing mystery in exoplanet science.

The Problem with WASP-107 b

At more than three-quarters the volume of Jupiter but less than one-tenth the mass, the "warm Neptune" exoplanet WASP-107 b is one of the least dense planets known. While puffy planets are not uncommon, most are hotter and more massive, and therefore easier to explain.

If WASP-107 b instead has a more massive core, the atmosphere should have contracted as the planet cooled over time. Without a source of heat to re-expand the gas, it should be much smaller. Although WASP-107 b has an orbital distance of just 5 million miles (one-seventh the distance between Mercury and the Sun), it doesn't receive enough energy from its star to be so inflated.

A Wealth of Previously Undetectable Molecules

WASP-107 b'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.

Dr Joanna Barstow, OU Research Fellow and co-author of the research, said:

"WASP-107b is a great target for transit spectroscopy because it's one of the 'fluffiest' exoplanets we know of - its low density means the spectral features we're looking for are nice and large."

The remarkable precision of the data makes it possible to not just detect, but actually measure the abundances of a wealth of molecules.

Roiling Gas, Hot Interior, and Massive Core

The research showed a surprisingly lack of methane in WASP-107 b's atmosphere: one-thousandth the amount expected for a planet this cool.

"This is evidence that hot gas from deep in the planet, perhaps down to the core, must be mixing vigorously with the cooler layers higher up," explained David Sing from the Johns Hopkins University (JHU). "Methane is unstable at high temperature. The fact that we detected so little, even though we did detect other carbon molecules, tells us that the interior of the planet must be significantly hotter than we thought."

A likely source of WASP-107 b'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.

Researchers had previously proposed that tidal heating could be the cause of WASP-107 b'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 teams realised that the spectra could also provide a new way to estimate the size of the core.

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

All together, it turns out that WASP-107 b is not as mysterious as it once appeared.

Dr Barstow added:

"Being able to precisely measure the abundance of gases like methane provides us with really exciting insights into atmospheric dynamics on planets like WASP-107b, and JWST observations are teaching us a great deal about how planetary atmospheres work."

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