Webb Detects Methane in Atmosphere of Exo-Saturn TOI-199b
Using spectral data from the Near Infrared Spectrograph (NIRSpec) aboard the NASA/ESA/CSA James Webb Space Telescope, astronomers analyzed the atmosphere of TOI-199b, a distant Saturn-mass world that is neither frozen nor hot.
Artist’s impression of a gas giant exoplanet. Image credit: Sci.News.
TOI-199 is a G-type star located about 330 light years away in the constellation Dorado.
It hosts at least two massive planets: TOI-199b and TOI-199c.
The inner planet orbits its host star every 105 days, receiving 2.5 times the irradiation of Earth, which corresponds to a temperature of 352 K (79 degrees Celsius or 174 degrees Fahrenheit).
Its mass (0.17 Jupiter masses) and radius (0.81 Jupiter radii) suggest a Saturn-like internal structure with an atmosphere dominated by hydrogen.
“TOI-199b stands out as one of the most favorable low-temperature giant planets for atmospheric characterization,” said Penn State astronomer Renyu Hu and colleagues.
To characterize TOI-199b’s atmosphere, astronomers used a technique called transmission spectroscopy to analyze light from the star passing through the planet’s atmosphere.
“When we compared the spectra during the transit to the baseline, we found that the atmosphere blocked the wavelengths of starlight absorbed by the methane,” said Dr. Aaron Bello-Arufe, a postdoctoral researcher at NASA’s Jet Propulsion Laboratory.
“Models of the composition of temperate gas giant exoplanets had predicted that they would contain methane, so it is good to get confirmation that our theories are correct.”
In addition to methane, Webb’s observations suggested that the atmosphere also contained ammonia and carbon dioxide.
“Through additional observations of this planet, we could establish the relative abundance of these different gases in its atmosphere,” Dr. Hu said.
“This more complete picture of the atmosphere of a temperate gas giant can then be used to improve our models and potentially better understand how planets and their atmospheres form and evolve, including for Earth.”
“The success of this first study of the atmosphere of a giant temperate planet also gives us the confidence to devote more resources and observation time to the study of other similar planets.”
“We will then be able to see if this planet is unique or if there are general characteristics common to this type of planet.”
The team’s results were published in the May 20 issue of Astronomical Journal.
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Aaron Bello-Arufe and others. 2026. Methane on temperate Exo-Saturn TOI-199b. A.J. 171, 354; doi: 10.3847/1538-3881/ae4fba
