The warm gas giant WASP-107 b, known for its unusually low density and moderate temperature, may have its bloated atmosphere due to tidal heating that warms its interior more than previously thought. (Artist’s concept.) Credit: SciTechDaily.com
The surprising lack of methane suggests that tidal heating has inflated the atmosphere of the warm gas giant WASP-107 b.
Why the warm gas giant exoplanet WASP-107 b so so puffy? With a moderate temperature and ultra-low density, the equivalent of a microwaved marshmallow, it appears to defy standard theories of planet formation and evolution.
Two independent teams of researchers think they have it figured out. Webb’s data, combined with previous Hubble observations, indicate that WASP-107 b’s interior should be much hotter than previous estimates. The unexpectedly high temperature, thought to be caused by tidal forces stretching the planet like a goofy putty, could explain how planets like WASP-107 b can be so buoyant, possibly solving a long-standing mystery in exoplanet science.
This artist’s concept shows what the exoplanet WASP-107 b might look like based on recent data collected 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 by volume, but has a mass less than 10% that of Jupiter, making it one of the least dense exoplanets known. Courtesy: NASA, ESA, CSA, Ralf Crawford (STScI)
The Webb Space Telescope cracked a case of a bloated exoplanet
Why is the hot gas giant exoplanet WASP-107 b so fluffy? Two independent research teams now 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. This indicates that the interior of WASP-107 b should be significantly hotter and the core much more massive than previous estimates.
The unexpectedly high temperature is thought to be the result of tidal heating caused by the planet’s slightly non-circular orbit, and could explain how WASP-107 b could be so bloated without resorting to extreme theories about how it formed.
The results, made possible by Webb’s extreme sensitivity and accompanying ability to measure light passing through exoplanet atmospheres, may explain the puffiness of dozens of low-density exoplanets, helping to solve a long-standing mystery in exoplanet science.
The WASP-107 problem b
At more than three quarters of the volume of Jupiter but less than one-tenth of the mass, “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 the radius, mass, age and inferred internal temperature, we concluded that WASP-107 b has 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 an article published May 20 in the journal Nature. “But it was hard to understand how such a small core could absorb so much gas and then stop developing completely 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 in 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 micron) and Webb’s MIRI (5–12 micron). 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 the light filtered through the planet’s atmosphere (transmitted light) to the unfiltered starlight, it is possible to calculate the amount of each wavelength that is blocked by the atmosphere. Because each molecule absorbs a unique combination of wavelengths, the transmission spectrum can be used to constrain the abundances of different gases.
This spectrum shows clear evidence of 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 core temperature and mass.
This optical to mid-infrared wavelength coverage is the broadest of any exoplanet transmission spectrum to date and includes the first reported detection of ammonia by a space telescope in an exoplanet atmosphere.
Courtesy: 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 must have contracted as the planet has cooled over time since it formed. Without a heat source to re-expand the gas, the planet would have to be much smaller. Although WASP-107 b has an orbital distance of only 5 million miles (one-seventh the distance between Mercury and the Sun), it does not receive enough energy from its star to be so bloated.
“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, hot Jupiter planets we study,” said David Singh of Johns Hopkins University ( JHU), lead author of a parallel study 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 undetectable molecules
WASP-107 b’s giant radius, extended atmosphere and orbit make it ideal for transmission spectroscopy, a method used to identify the various gases in an exoplanet’s 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 construct a broad spectrum of absorbed light from 0.8 to 12, 2 microns from the atmosphere of WASP-107 b. 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 not just to detect but to actually measure the abundances of numerous 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 the light filtered through the planet’s atmosphere (transmitted light) to the unfiltered starlight, it is possible to calculate the amount of each wavelength that is blocked by the atmosphere. Because each molecule absorbs a unique combination of wavelengths, the transmission spectrum can be used to constrain the abundances of different gases.
This spectrum shows clear evidence of 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 core mass .
Courtesy: NASA, ESA, CSA, Ralf Crawford (STScI), David Sing (JHU), NIRSpec GTO Transiting Exoplanet Team
Rotating 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 the assumed temperature.
“This is evidence that hot gas from deep within the planet must be vigorously mixing with the cooler layers higher up,” Singh explained. “Methane is unstable at high temperatures. The fact that we found so little, despite finding other carbon-containing molecules, tells us that the interior of the planet 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 changes continuously during the 5.7-day orbit, the gravitational pull also changes, stretching the planet and heating it.
Researchers had previously suggested that tidal heating might account for WASP-107 b’s puffiness, but until Webb’s results came in, there was no evidence.
After establishing that the planet had enough internal heat to fully stir 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 on the planet, and we know how much 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 there 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 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 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 lots of rocks and not as much gas, just dial in the temperature and enhance it to look the way it does.”
Reference: “High internal heat flow and a large core in a warm Neptunian exoplanet” by Louis Welbanks, Taylor J. Bell, Thomas G. Beatty, Michael R. Line, Kazumasa Ono, Jonathan J. Fortney, Everett Schlavin, Thomas P. Green, Emily Rauscher, Peter McGill, Matthew Murphy, Vivienne Parmentier, Yao Tang, Isaac Edelman, Sagnik Mukherjee, Lindsey S. Weiser, Pierre-Olivier Lagage, Ahren 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 largest space science observatory. Webb solves mysteries in our solar system, looks beyond 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).
