The warm gas giant WASP-107 b, known for its unusually low density and moderate temperature, may have its puffed-up atmosphere due to tidal heating that warms its interior more than previously thought. (Artist’s concept.) Credit: SciTechDaily.com
A surprising deficiency of methane suggests that tidal heating has puffed up the atmosphere of the warm gas giant WASP-107 b.
Why is the warm gas-giant exoplanet WASP-107 b so, so puffy? With a moderate temperature and an ultra-low density on par with a microwaved marshmallow, it seems to defy standard theories of planet formation and evolution.
Two independent teams of researchers think they’ve figured it out. Data from Webb, combined with prior observations from Hubble, show that the interior of WASP-107 b must be a lot toastier than previously estimated. The unexpectedly high temperature, which is thought to be caused by tidal forces that stretch the planet like silly putty, can explain how planets like WASP-107 b can be so floofy, possibly solving a long-standing mystery in exoplanet science.
This artist’s concept shows what the exoplanet WASP-107 b could look like based on recent data gathered by NASA’s James Webb Space Telescope, along with previous observations from Hubble and other space- and ground-based telescopes. WASP-107 b is a “warm Neptune” exoplanet orbiting a relatively small and cool star approximately 210 light-years from Earth, in the constellation Virgo. The planet is about 80% the size of Jupiter in terms of volume, but has a mass less than 10% of Jupiter, making it one of the least dense exoplanets known. Credit: NASA, ESA, CSA, Ralf Crawford (STScI)
Webb Space Telescope Cracks Case of Inflated Exoplanet
Why is the warm gas-giant exoplanet WASP-107 b so puffy? Two independent research teams now have an answer.
Data collected using NASA’s James Webb Space Telescope, combined with prior observations from NASA’s Hubble Space Telescope, show surprisingly little methane (CH4) in the planet’s atmosphere. This indicates 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 sensitivity and accompanying ability to measure light passing through exoplanet atmospheres, may explain the puffiness of dozens of 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.
“Based on its radius, mass, age, and assumed internal temperature, we thought WASP-107 b had a very small, rocky core surrounded by a huge mass of hydrogen and helium,” explained Luis Welbanks from Arizona State University (ASU), lead author on a paper published on May 20 in the journal Nature. “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.”
This transmission spectrum, captured using NASA’s Hubble and James Webb space telescopes, shows the amounts of different wavelengths (colors) of starlight blocked by the atmosphere of the gas-giant exoplanet WASP-107 b.
The spectrum includes light collected over five separate observations using a total of three different instruments: Hubble’s WFC3 (0.8–1.6 microns), Webb’s NIRCam (2.4–4.0 microns and 3.9–5.0 microns), and Webb’s MIRI (5–12 microns). Each set of measurements was made by observing the planet-star system for about 10 hours before, during, and after the transit as the planet moved across the face of the star.
By comparing the brightness of light filtered through the planet’s atmosphere (transmitted light) to unfiltered starlight, it is possible to calculate the amount of each wavelength that is blocked by the atmosphere. Since each molecule absorbs a unique combination of wavelengths, the transmission spectrum can be used to constrain the abundance of various gases.
This spectrum shows clear evidence for water (H2O), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), sulfur dioxide (SO2), and ammonia (NH3) in the planet’s atmosphere, allowing researchers to estimate the interior temperature and mass of the core.
This wavelength coverage from optical to mid-infrared is the broadest of any exoplanet transmission spectrum to date, and includes the first reported space telescope detection of ammonia in an exoplanet atmosphere.
Credit: NASA, ESA, CSA, Ralf Crawford (STScI), Luis Welbanks (ASU), JWST MANATEE Team
If WASP-107 b 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. 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.
“WASP-107 b 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 David Sing from the Johns Hopkins University (JHU), lead author on a parallel study also published today in Nature. “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.”
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.
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 build a broad spectrum of 0.8- to 12.2-micron light absorbed by WASP-107 b’s atmosphere. Using Webb’s NIRSpec (Near-Infrared Spectrograph), Sing’s team 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 (H2O), methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2), and ammonia (NH3).
This transmission spectrum, captured using Webb’s NIRSpec (Near-Infrared Spectrograph), shows the amounts of different wavelengths (colors) of near-infrared starlight blocked by the atmosphere of the gas-giant exoplanet WASP-107 b.
The spectrum was made by observing the planet-star system for about 8.5 hours before, during, and after the transit as the planet moved across the face of the star.
By comparing the brightness of light filtered through the planet’s atmosphere (transmitted light) to unfiltered starlight, it is possible to calculate the amount of each wavelength that is blocked by the atmosphere. Since each molecule absorbs a unique combination of wavelengths, the transmission spectrum can be used to constrain the abundance of various gases.
This spectrum shows clear evidence for water (H2O), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and sulfur dioxide (SO2) in the planet’s atmosphere, allowing researchers to estimate the interior temperature and mass of the core.
Credit: NASA, ESA, CSA, Ralf Crawford (STScI), David Sing (JHU), NIRSpec GTO Transiting Exoplanet Team
Roiling Gas, Hot Interior, and Massive Core
Both spectra show a surprising lack of methane in WASP-107 b’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,” explained Sing. “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-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 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,” explained Daniel Thorngren from 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 together, WASP-107 b is not as mysterious as it once appeared.
“The Webb data tells us that planets like WASP-107 b didn’t have to form in some odd way with a super small core and a huge gassy envelope,” explained Mike Line from ASU. “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.”
Reference: “A high internal heat flux and large core in a warm neptune exoplanet” by Luis Welbanks, Taylor J. Bell, Thomas G. Beatty, Michael R. Line, Kazumasa Ohno, Jonathan J. Fortney, Everett Schlawin, Thomas P. Greene, Emily Rauscher, Peter McGill, Matthew Murphy, Vivien Parmentier, Yao Tang, Isaac Edelman, Sagnick Mukherjee, Lindsey S. Wiser, Pierre-Olivier Lagage, Achrène Dyrek and Kenneth E. Arnold, 20 May 2024, Nature.
DOI: 10.1038/s41586-024-07514-w
The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing 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).
