NASA’s Webb cracks case of blown-up exoplanet

Why is the warm gas giant exoplanet WASP-107 b so swollen? Two independent teams of researchers have an answer.

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

The unexpectedly high temperature is thought to be the result of tidal heating caused by the planet’s slightly non-circular orbit, and may explain how WASP-107 b could have become so blown up without resorting to extreme theories about how it formed.

The results, made possible by Webb’s extraordinary sensitivity and associated ability to measure light passing through the atmospheres of exoplanets, could explain the walls of dozens of low-density exoplanets and help solve a long-standing mystery in exoplanet science .

The problem with WASP-107 b

With more than three-quarters the volume of Jupiter, but less than a tenth the mass, the ‘warm Neptune’ exoplanet WASP-107 b is one of the least known low-density planets. Although puffy planets are not uncommon, most are hotter and more massive, and therefore easier to explain.

“Based on its radius, mass, age and presumed internal temperature, we thought WASP-107 b had a very small, rocky core surrounded by a huge mass of hydrogen and helium,” explains Luis Welbanks of Arizona State University (ASU). out. lead author of a paper published today in Nature. ‘But it was difficult to understand how such a small core could suck in so much gas and then not fully develop into a planet with the mass of Jupiter.’

If WASP-107 b instead had more of its mass in its core, its atmosphere should have contracted as the planet cooled over time since its formation. Without a heat source to re-expand the gas, the planet would be much smaller. Although WASP-107 b has an orbital distance of only 8 million kilometers (one-seventh the distance between Mercury and the Sun), it does not receive enough energy from its star to become so blown up.

“WASP-107 b is such an interesting target for Webb because it is significantly cooler and more Neptune-like in mass than many of the other low-density planets, the hot Jupiters, that we have studied,” says David Sing of the Johns. Hopkins University (JHU), lead author of a parallel study also published today in Nature. “As a result, we should be able to detect methane and other molecules that could give us information about its chemistry and internal dynamics that we can’t get from a hotter planet.”

A wealth of previously undetectable molecules

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

By combining observations from Webb’s NIRCam (Near-Infrared Camera), Webb’s MIRI (Mid-Infrared Instrument) and Hubble’s WFC3 (Wide Field Camera 3), Welbanks’ team was able to capture a broad spectrum from 0.8 to 12.2 microns absorbed building up light through the atmosphere of WASP-107 b. Using Webb’s NIRSpec (Near-Infrared Spectrograph), Sing’s team built an independent spectrum from 2.7 to 5.2 microns.

The accuracy of the data makes it possible to determine not only the abundance of molecules, including water vapor (H2O), methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2) and ammonia (NH3).

Boiling gas, hot interior and huge core

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

“This is evidence that hot gas from deep within the planet must mix vigorously with the cooler layers higher up,” Sing explains. “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.”

A likely source of WASP-107 b’s extra internal energy is tidal heating, caused by its somewhat elliptical orbit. As the distance between the star and the planet continually changes during its 5.7-day orbit, the force of gravity also changes, stretching and heating the planet.

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

Once they determined that the planet has enough internal heat to thoroughly shake up its atmosphere, the teams realized that the spectra could also provide a new way to estimate the size of the core.

“If we know how much energy is on the planet, and we know what part of the planet is made up of heavier elements like carbon, nitrogen, oxygen and sulfur, versus how much hydrogen and helium, we can calculate how much mass it must contain. the core,” explains Daniel Thorngren of JHU.

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 in all, WASP-107 b is not as mysterious as it once seemed.

“The Webb data tells us that planets like WASP-107 b didn’t have to form in some strange way with a super small core and a huge gaseous envelope,” explains ASU’s Mike Line. “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 crank it up to look like this.”

The James Webb Space Telescope is the world’s premier observatory for space science. Webb solves mysteries in our solar system, looks beyond to distant worlds around other stars and investigates the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners ESA (European Space Agency) and CSA (Canadian Space Agency).

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