JWST finds methane on Saturn-sized planet

Penn State and NASA’s Jet Propulsion Laboratory said on May 20 that researchers using the James Webb Space Telescope detected methane in the atmosphere of TOI-199b, a gas giant about the size of Saturn with a temperature closer to Earth’s than to the ultra-hot giant exoplanets commonly studied. The finding was described in a university release and in a paper published the same day in the *Astronomical Journal*. TOI-199b is more than 330 light-years from Earth and circles a G-type star about every 100 days, according to Penn State and the paper’s abstract. The researchers said the planet is one of only a small number of known temperate giant exoplanets and the first such world whose atmosphere has been studied in detail. ### Which planet did Webb study, and why was it a useful target? TOI-199b has a Saturn-like mass and an equilibrium temperature of about 350 kelvin, or roughly 175 degrees Fahrenheit, placing it between the cold gas giants of the solar system and the much hotter “hot Jupiters” often observed around other stars. (eurekalert.org) Penn State said that combination makes it a rare atmospheric target. Renyu Hu, an associate professor of astronomy and astrophysics at Penn State and leader of the research team, said the value of exoplanet work is that it lets astronomers study planetary types not present in the solar system. (eurekalert.org) Hu said only a few giant, temperate exoplanets are known, and TOI-199b is the first of them to have its atmosphere characterized in detail. ### How did Webb detect methane there? (eurekalert.org) JWST observed a single transit of TOI-199b with its NIRSpec G395M mode, according to the paper’s abstract. In transit spectroscopy, astronomers measure how starlight filters through a planet’s atmosphere as the planet passes in front of its star. The paper said Bayesian retrievals found methane, or CH4, with a Bayes factor of about 700 in a cloudy-atmosphere model. The authors also reported lower-than-nominal precision because of a pointing misalignment during the observation, but said the methane signal was still recovered. (eurekalert.org) ### Why does methane matter on a temperate gas giant? Methane is expected to be important in cooler hydrogen-dominated atmospheres, and the paper said the detection supports an emerging pattern in which temperate low-molecular-weight atmospheres show methane features in transmission spectra. (arxiv.org) That makes TOI-199b a data point for testing atmospheric chemistry outside the hotter exoplanet population that has dominated earlier Webb studies. Penn State said the atmospheric composition could help refine models of planetary formation and evolution. The university also said studying a temperate giant planet can improve scientists’ understanding of how atmospheres behave under conditions closer to those seen in parts of the solar system than on extreme hot Jupiters. ### Did the team find anything besides methane? The paper reported an increase in transit depth near 3 microns that the researchers attributed either to ammonia, NH3, or less likely hydrogen cyanide, HCN. (arxiv.org) The authors said they tested several haze prescriptions, including Titan-like tholin and soot, but found only weak evidence favoring haze over a clearer atmosphere. The study also examined the broader TOI-199 system. (eurekalert.org) The authors said strong transit timing variations point to an outer non-transiting giant planet, and their analysis reduced the mass uncertainty of TOI-199 c by 50%. ### What comes next for TOI-199b? The researchers said follow-up observations will be needed to separate ammonia from hydrogen cyanide in the 3-micron feature and to better constrain the planet’s vertical mixing regime. (arxiv.org) The paper, titled “Methane on the temperate exo-Saturn TOI-199b,” lists Aaron Bello-Arufe as first author and includes Hu, Heather A. Knutson, David K. Sing and Xi Zhang among the co-authors.