Why is the warm gas giant exoplanet WASP-107 b so swollen? Two independent research teams have an answer.
Data collected by NASA’s James Webb Space Telescope, combined with previous observations from NASA’s Hubble Space Telescope, show surprisingly little methane (CH).4) in the planet’s atmosphere, suggesting that the interior of WASP-107 b must be significantly hotter and the core much more massive than previously thought.
The unexpectedly high temperature is thought to be due to tidal heating caused by the planet’s slightly non-circular orbit, and can explain how WASP-107 b can become so inflated without resorting to extreme theories about its formation .
The results, made possible by Webb’s extraordinary sensitivity and accompanying ability to measure light passing through the atmospheres of exoplanets, could explain the swelling of dozens of low-density exoplanets and help solve a long-standing mystery of 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 dense planets we know of. While puffy planets are not uncommon, most are hotter and more massive and therefore easier to explain.
“Based on its radius, mass, age and assumed internal temperature, we assumed that WASP-107 b had a very small, rocky core surrounded by a huge mass of hydrogen and helium,” explained Luis Welbanks from the Arizona State University (ASU). Lead author of an article published today in Nature. “But it was difficult to understand how such a small core could entrain so much gas and then not fully grow into a Jupiter-mass planet.”
If WASP-107 b had more of its mass in its core instead, the atmosphere would have contracted as the planet has cooled over time since its formation. Without a heat source to expand the gas again, the planet would have to 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 does not receive enough energy from its star to inflate that much.
“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, called Jupiters, that we have studied,” said David Sing of the Johns Hopkins University (JHU), lead author of a parallel study also published today Nature. “This should allow us to detect methane and other molecules, which can give us information about its chemistry and internal dynamics that we cannot get from a hotter planet.”
A wealth of previously undetectable molecules
WASP-107 b’s huge radius, extended atmosphere and lateral orbit make it ideal for transmission spectroscopy, a method for identifying the different gases in an exoplanet’s atmosphere based on their effect on 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), Welbank’s team was able to detect a broad spectrum of absorbed light with a wavelength of 0.8 to 12 .2 micrometers build up through the atmosphere of WASP-107 b. Using Webb’s NIRSpec (Near-Infrared Spectrograph), Sing’s team created an independent spectrum covering 2.7 to 5.2 micrometers.
The precision of the data makes it possible to not only detect but actually measure the abundance of a plethora of molecules, including water vapor (H).2O), methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2) and ammonia (NH3).
Bubbling gas, hot interior and massive core
Both spectra show a surprising lack of methane in WASP-107 b’s atmosphere: one-thousandth of the amount expected based on the assumed temperature.
“This is evidence that hot gas from deep in the planet needs to mix vigorously with the cooler layers higher up,” Sing explained. “Methane is unstable at high temperatures. The fact that we have discovered so little, even though we have discovered other carbon-containing molecules, tells us that the interior of the planet must be significantly hotter than we thought.”
A likely source of WASP-107 b’s additional internal energy is tidal heating caused by its slightly elliptical orbit. As the distance between the star and planet changes continuously during the 5.7-day orbit, the gravitational pull also changes, stretching and heating the planet.
Researchers had previously suggested that tidal warming could be the cause of WASP-107 b’s swellings, but until the Webb results came in, there was no evidence.
After determining that the planet had enough internal heat to thoroughly churn the 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 what the proportion of heavier elements such as carbon, nitrogen, oxygen and sulfur are on the planet compared to the proportion of hydrogen and helium, we can calculate how much mass “The core must be included in it,” explained Daniel Thorngren from the JHU.
It turns out that the core is at least twice as massive as originally thought, which makes more sense in terms of planet formation.
All in all, WASP-107 b is no longer as mysterious as it once seemed.
“The Webb data shows us that planets like WASP-107 b do not have to have formed in some strange way with a super-small core and a huge gas envelope,” explained Mike Line of ASU. “Instead we can take something more like Neptune, with a lot of rock and not so much gas, just increase the temperature and spice it up to make it look the way it does.”
The James Webb Space Telescope is the world’s leading space observatory. Webb solves mysteries in our solar system, looks beyond the distant worlds around other stars, and explores 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).