Penn State: Saturn-sized planet atmosphere methane

Penn State researchers and collaborators at NASA’s Jet Propulsion Laboratory said on May 20 that NASA’s James Webb Space Telescope had detected methane in the atmosphere of TOI-199b, a Saturn-sized exoplanet with temperatures closer to Earth’s than to the hot giant planets Webb has often studied. The team said the result marks the first detailed atmospheric study of a temperate giant exoplanet. The findings were published May 20 in *The Astronomical Journal*. TOI-199b orbits a star more than 330 light-years from Earth. ### Which planet did Webb study? TOI-199b is a Saturn-mass planet orbiting a G-type star about every 100 days, according to the paper’s abstract. The researchers said its equilibrium temperature is about 350 kelvin, or roughly 175 degrees Fahrenheit, putting it in a much cooler regime than the “hot Jupiters” that dominate many atmospheric studies. Penn State said only a handful of temperate giant exoplanets are known. (eurekalert.org) Renyu Hu, a Penn State associate professor of astronomy and astrophysics and leader of the research team, said that made TOI-199b a useful target for studying a class of planet not represented in the solar system in the same way. ### How did the team find methane? (arxiv.org) The study used a transmission spectrum from a single transit observed with Webb’s NIRSpec G395M mode, the paper said. In that method, astronomers measure how starlight filters through a planet’s atmosphere as the planet passes in front of its host star. The paper said Bayesian retrievals found methane, or CH4, with a Bayes factor of about 700 in a cloudy-atmosphere model. (eurekalert.org) The same analysis found no detectable carbon monoxide or carbon dioxide, which the authors said ruled against very high atmospheric metallicities above roughly 50 times solar. ### Why does methane show up on this world? Penn State said methane is easier to preserve in cooler atmospheres than in hotter giant planets. (arxiv.org) That makes TOI-199b different from many previously studied gas giants, where higher temperatures can favor other carbon-bearing molecules over methane. The arXiv abstract said the detection “supports the emerging trend that temperate low-molecular-weight atmospheres display spectral features in transmission.” The researchers also said TOI-199b could serve as a first data point for studying clouds and hazes in temperate gas giants. (arxiv.org) ### Did the spectrum show anything besides methane? The paper said the spectrum showed an increase in transit depth near 3 microns. (eurekalert.org) The team’s self-consistent models attributed that feature either to ammonia, NH3, or, less likely, hydrogen cyanide, HCN. The researchers also tested several haze prescriptions, including Titan-like tholin, soot and water-rich tholin. (arxiv.org) The paper said those haze models were only weakly preferred over a clear-atmosphere case. ### What else did the study say about the system? The TOI-199 system shows strong transit timing variations caused by an outer non-transiting giant planet, according to the paper. (arxiv.org) The authors said their analysis reduced the mass uncertainty for TOI-199c by 50% and favored a slightly longer orbital period and higher eccentricity than previous estimates. May 20 is the key date for the next step in the record: Penn State’s release and the paper in *The Astronomical Journal* point readers to “Methane on the temperate exo-Saturn TOI-199b” for the full spectral analysis and follow-up plans to distinguish between ammonia and hydrogen cyanide. (eurekalert.org) (arxiv.org)