The warm gas giant WASP-107 b, known for its unusually low density and moderate temperature, may have a puffy atmosphere due to tidal heating that warms its interior more than previously thought. (Concept artist.) Credit: SciTechDaily.com
The surprising lack of methane suggests that tidal heating inflated the atmosphere of the warm gas giant WASP-107b.
Why is the warm gas giant exoplanet WASP-107 b so, so bloated? With a mild temperature and ultra-low density comparable to the microwave probe, it appears to contradict standard theories of planet formation and evolution.
Two independent teams of researchers think they’ve figured it out. Data from Webb, combined with previous Hubble observations, show that WASP-107b’s interior must be much toastier than previously thought. The unexpectedly high temperature, thought to be caused by tidal forces that stretch the planet like silly putty, may explain how planets like WASP-107 b can be so wonky, and perhaps solve 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 in volume, but its mass is less than 10% that of Jupiter, making it one of the lowest-density exoplanets known. Acknowledgments: NASA, ESA, CSA, Ralf Crawford (STScI)
The Webb Space Telescope cracks the case of a bloated exoplanet
Why is the warm gas giant exoplanet WASP-107b so puffy? Two independent research teams now 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. This suggests 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 the result of tidal heating caused by the planet’s slightly non-circular orbit, and may explain how WASP-107 b can be so bloated without resorting to extreme theories about how it formed.
The results, 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 to solve a long-standing mystery in exoplanet science.
Problem with WASP-107 b
At more than three-quarters volume Jupiter but less than one-tenth the weight, “warm NeptuneThe exoplanet WASP-107b is one of the lowest-density planets known. While bulge planets are not uncommon, most are hotter and more massive and therefore easier to explain.
“Based on its radius, mass, age and predicted internal temperature, we believed that WASP-107 b has a very small, rocky core surrounded by a huge mass of hydrogen and helium,” explained Luis Welbanks of Arizona State University (ASU). lead author of an article published on May 20 in the journal Nature. “However, it was difficult to understand how such a small core could absorb so much gas and then stop before fully growing into a Jupiter-mass planet.”
This transmission spectrum, captured using NASA’s Hubble and James Webb Space Telescopes, shows the abundance of different wavelengths (colors) of starlight blocked by the atmosphere of the gas giant exoplanet WASP-107 b.
The spectrum includes light collected during 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 a planet’s atmosphere (transmitted light) to 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 limit the amount 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 exoplanet atmospheres.
Credits: NASA, ESA, CSA, Ralf Crawford (STScI), Luis Welbanks (ASU), JWST MANATEE Team
If WASP-107 b instead has more of its mass in its core, the atmosphere should contract as the planet has cooled over time since its formation. Without a heat source to re-expand the gas, the planet should be much smaller. Although WASP-107 b has an orbital distance of only 5 million miles (seventh the distance between Mercury and the Sun), it does not receive enough energy from its star to be so inflated.
“WASP-107 b is such an interesting target for Webb because it is significantly cooler and more similar in mass to Neptune than many of the other low-density, hot Jupiter planets we have studied,” said Johns’ David Sing. 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 can give us information about its chemistry and internal dynamics that we can’t get from a warmer planet.”
A wealth of previously undetectable molecules
WASP-107 b’s giant radius, extended atmosphere, and orbit make it ideal for transmission spectroscopy, a method used to identify various gases in exoplanet atmospheres 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 create a broad spectrum of absorbed light from 0.8 to 12.2 microns. by the atmosphere of WASP-107 b. Using Webb’s NIRSpec (Near-Infrared Spectrograph), Sing’s team produced an independent spectrum covering 2.7 to 5.2 microns.
The accuracy of the data makes it possible to not only detect, but actually measure the abundance of molecules, including water vapor (H2O), methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2) and ammonia (NH3).
This transmission spectrum, captured with Webb’s NIRSpec (Near-Infrared Spectrograph), shows the abundance of different wavelengths (colors) of near-infrared light from stars blocked by the atmosphere of the gas giant exoplanet WASP-107b.
The spectrum was created 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 a planet’s atmosphere (transmitted light) to 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 limit the amount 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 planet’s internal temperature and mass. core.
Acknowledgments: NASA, ESA, CSA, Ralf Crawford (STScI), David Sing (JHU), NIRSpec GTO Transiting Exoplanet Team
Rolling throttle, 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 its predicted temperature.
“This is evidence that the hot gas from deep within the planet must be mixing intensively with the cooler layers above,” Sing explained. “Methane is unstable at high temperatures. The fact that we detected so little, even though we detected other carbon-bearing 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 changes as well, stretching and heating the planet.
Researchers have previously suggested that tidal heating might be the cause of WASP-107 b’s swelling, but there was no evidence until Webb’s results were available.
Once the teams found that the planet had enough internal heat to thoroughly churn up the atmosphere, they 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 proportion of the planet is made up of heavier elements like carbon, nitrogen, oxygen and sulfur versus hydrogen and helium, we can calculate how much mass the core must have,” explained Daniel Thorngren of JHU.
The core turns out to be at least twice as massive as originally thought, 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 special way with a super-small core and a huge gaseous envelope,” explained ASU’s Mike Line. “Instead, we can take something like Neptune, with a lot of rock and not as much gas, and just dial up the temperature and set it up to look the way it does.”
References: “High Internal Heat Flow and a Large Core in a Warm Neptunian 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 The James Webb Space Telescope is the world’s leading space science observatory. Webb solves mysteries in our solar system, looks further to distant worlds around other stars, and explores the mysterious structures and origins of our universe and our place in it. Webb is an international-led program NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).