Using spectral data from the Near-Infrared Spectrograph (NIRSpec) onboard 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 scorching hot.
An artist’s impression of a gas-giant exoplanet. Image credit: Sci.News.
TOI-199 is a G-type star located around 330 light-years away in the constellation of 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 Earth’s irradiation, which corresponds to a temperature of 352 K (79 degrees Celsius, or 174 degrees Fahrenheit).
Its mass (0.17 Jupiter mass) and radius (0.81 Jupiter radii) suggest a Saturn-like internal structure with a hydrogen-dominated atmosphere.
“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, the astronomers used a technique called transmission spectroscopy to analyze light from the star that passes through the planet’s atmosphere.
“When we compared the spectra during the transit to the baseline, we saw that the atmosphere blocked the wavelengths of starlight absorbed by methane,” said Dr. Aaron Bello-Arufe, a postdoctoral researcher at NASA’s Jet Propulsion Laboratory.
“Models for 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 accurate.”
In addition to methane, the Webb observations gave hints that the atmosphere also contained ammonia and carbon dioxide.
“With additional observations of this planet, we could establish the relative abundance of these various gases in its atmosphere,” Dr. Hu said.
“This more complete picture of a temperate gas giant’s atmosphere 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 a temperate giant planet’s atmosphere also gives us confidence to dedicate more resources and observation time to study other similar planets.”
“We can then see if this planet is unique or if there are general shared characteristics for this type of planet.”
The team’s results were published in the May 20 issue of the Astronomical Journal.
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Aaron Bello-Arufe et al. 2026. Methane on the Temperate Exo-Saturn TOI-199b. AJ 171, 354; doi: 10.3847/1538-3881/ae4fba
