Webb solves the problem of a bloated exoplanet :: NASANET

Why is gas giant exoplanet WASP-107 b so inflated? Two independent groups of researchers have the answer.

Data collected by 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 the interior of WASP-107 b should be significantly hotter, and the core is much more massive. than previously expected.

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 can inflate so much without resorting to extreme theories about how it formed.

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

Problem with WASP-107b

Exoplanet WASP-107 b, a “warm Neptune” occupying more than three-quarters of Jupiter’s volume but less than one-tenth its mass, is one of the least dense planets known. While bloated planets are not unusual, most are hotter and more massive and therefore easier to explain.

“Based on radius, mass, age and estimated internal temperature, we believe that WASP-107 b had a very small rocky core surrounded by a huge mass of hydrogen and helium,” explained Louis Welbanks of Arizona State University (ASU). lead author of the paper published today in the journal Nature. “But it was difficult to understand how such a small core could absorb so much gas and then not become a Jupiter-mass planet.”

If WASP-107 b had instead had more mass at its core, the atmosphere would have contracted as the planet has cooled 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 just over 8 million kilometers (one-seventh the distance between Mercury and the Sun), it does not receive enough energy from its star to inflate that much.

“WASP-107 b is a very interesting target for Webb because it is significantly cooler and more Neptune-like in mass than many of the other low-density, hot Jupiter planets we study,” said David Singh of Webb University. Johns Hopkins University (JHU) is the lead author of a parallel study also published today in the journal Nature. “As a result, we will be able to detect methane and other molecules, which can give us information about their internal chemistry and dynamics that we cannot get from a hotter planet.”

Large number of previously undetected molecules

WASP-107 b’s giant radius, extended atmosphere and marginal orbit make it ideal for transmission spectroscopy, a technique used to identify 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 plot a broad spectrum of absorbed light from 0.8 to 12.2 microns. . through the atmosphere of WASP-107 b. Using Webb’s NIRSpec (near-infrared spectrograph), Singh’s team constructed an independent spectrum spanning from 2.7 to 5.2 microns.

The accuracy of the data allows not only to detect, but also to measure the content of a large number of molecules, including water vapor (H2O), methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), sulfur. dioxide (SO2) and ammonia (NH3).

Turbulent gas, hot interior and massive core

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

“This is evidence that hot gas deep within the planet must be vigorously mixing with cooler layers above,” Singh explained. “Methane is unstable at high temperatures. “The fact that we found so little, even though we did find other carbon-containing molecules, tells us that the planet’s interior must be significantly hotter than we thought.”

A likely source of additional internal energy for WASP-107 b is tidal heating caused by its slightly elliptical orbit. As the distance between the star and the planet constantly changes during its 5.7-day orbit, the gravitational pull also changes, stretching the planet and warming it.

Previously, researchers had suggested that tidal heating might be the cause of WASP-107 b’s swelling, but there was no evidence until Webb’s results.

Once they established that the planet had enough internal heat to completely churning 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 there is on the planet, and we know what proportion of the planet is made up of heavier elements such as carbon, nitrogen, oxygen and sulfur, compared to the amount of hydrogen and helium, we can calculate how much mass there must be there. being in the center,” explained JHU’s Daniel Thorngren.

It turns out the core is at least twice as massive as originally thought, which makes more sense in terms of planet formation.

In general, WASP-107 b is not as mysterious as it seemed.

“Webb’s data tells us that planets like WASP-107 b shouldn’t have formed in some weird way with a super-small core and a huge gas envelope,” explained ASU’s Mike Line. “Instead, we can take something more like Neptune, with more rock and less gas, and just increase the temperature and raise it so that it looks like it is.”