Leveraging spectral data acquired via the Near-Infrared Spectrograph (NIRSpec) aboard the NASA/ESA/CSA James Webb Space Telescope, a cadre of astronomers undertook an in-depth examination of the atmospheric composition of TOI-199b, a celestial body comparable in mass to Saturn, situated at a considerable distance and exhibiting neither extreme frigidity nor scorching heat.
An artist’s impression of a gas-giant exoplanet. Image credit: Sci.News.
TOI-199 is identified as a G-type star, positioned approximately 330 light-years distant within the celestial expanse of Dorado.
This star system harbors a minimum of two substantial planets: TOI-199b and TOI-199c.
The planet situated in the inner orbit revolves around its parent star on a trajectory completed every 105 days. During this orbital period, it intercepts an amount of stellar radiation equivalent to 2.5 times that experienced by Earth, a level of irradiation corresponding to a thermal equilibrium of 352 Kelvin (79 degrees Celsius, or 174 degrees Fahrenheit).
Its measured mass (0.17 times that of Jupiter) and radius (0.81 times that of Jupiter) are indicative of an internal structure consistent with that of Saturn, featuring an atmosphere predominantly composed of hydrogen.
“TOI-199b presents itself as one of the most amenable low-temperature giant planets for the purpose of atmospheric characterization,” stated Penn State astronomer Renyu Hu and his research associates.
To meticulously define the characteristics of TOI-199b’s atmosphere, the research team employed a sophisticated methodology known as transmission spectroscopy. This technique involves the analysis of stellar light that traverses the planet’s atmospheric envelope.
“Upon juxtaposing the spectra obtained during the transit event with the baseline measurements, a discernible attenuation of specific wavelengths of starlight was observed, corresponding to absorption by methane,” remarked Dr. Aaron Bello-Arufe, a postdoctoral researcher affiliated with NASA’s Jet Propulsion Laboratory. “Theoretical models positing the composition of temperate, gas-giant exoplanets had foretold the presence of methane within their atmospheres. Consequently, this finding serves as a welcome substantiation of our theoretical frameworks.”
Beyond the detection of methane, the observational data acquired by the Webb telescope also provided indications suggesting the presence of ammonia and carbon dioxide within the atmosphere.
“With the acquisition of further observational data pertaining to this particular planet, it will become feasible to ascertain the relative proportions of these various gaseous constituents within its atmosphere,” Dr. Hu elaborated. “This more comprehensive depiction of a temperate gas giant’s atmospheric milieu can subsequently be instrumental in refining our predictive models and, potentially, enhancing our comprehension of planetary formation and atmospheric evolution, including insights applicable to Earth.”
“The successful execution of this inaugural study focusing on the atmosphere of a temperate giant planet instills considerable confidence, motivating the allocation of additional resources and observation time for the investigation of analogous celestial bodies.”
“This will allow us to discern whether this particular planet is an anomaly or if there exist generalized shared attributes among planets of this classification.”
The team’s findings were formally disseminated in the May 20th edition 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
