JWST detects methane on TOI-199 b

Astronomers using NASA’s James Webb Space Telescope have reported methane in the atmosphere of TOI-199 b, a Saturn-mass exoplanet more than 330 light-years from Earth. The finding was described in a study led by Penn State and NASA’s Jet Propulsion Laboratory and publicized on May 21. The planet is unusual because it is neither a searing “hot Jupiter” nor a frigid giant like Jupiter and Saturn in the solar system. Researchers said that makes it one of the few known temperate giant planets whose atmosphere can be studied in detail. ### Why are astronomers paying attention to TOI-199 b in particular? TOI-199 b orbits a G-type star roughly every 100 days and has an equilibrium temperature of about 350 kelvin, or about 175 degrees Fahrenheit, according to the study and Penn State’s summary. The paper describes it as a Saturn-mass planet and “one of the most favorable low-temperature gas giants for atmospheric study.” (sciencedaily.com) Penn State said giant exoplanets found so far have often clustered at temperature extremes — either very hot because they orbit close to their stars, or very cold because they orbit far away. Renyu Hu, a Penn State astronomer and the research team leader, said only a few giant temperate exoplanets are known and that this is the first time astronomers have been able to study the atmosphere of one of them in such detail. (arxiv.org) ### How did JWST detect methane there? A single JWST transit observation with the NIRSpec G395M mode produced the transmission spectrum used in the analysis, the paper said. In that method, astronomers measure starlight filtering through the planet’s atmosphere as the planet passes in front of its star. The team said Bayesian retrievals showed the presence of methane, written chemically as CH4, with what the paper described as a Bayes factor of about 700 in a cloudy-atmosphere model. (sciencedaily.com) The same analysis did not detect carbon monoxide or carbon dioxide, and the authors said that absence disfavors very high atmospheric metallicities above roughly 50 times solar. ### What else did the spectrum show besides methane? (arxiv.org) The study reported an increase in transit depth near 3 microns. The authors said their self-consistent models attribute that feature either to ammonia, NH3, or, less likely, hydrogen cyanide, HCN. Several haze prescriptions were also tested, including Titan-like tholin, soot and water-rich tholin, but the paper said the preference for haze models over a clear case was weak. (arxiv.org) That leaves the methane result as the central atmospheric detection from the current dataset. ### Does methane mean this planet could support life? TOI-199 b is a gas giant, and the study presents the methane result as an atmospheric chemistry finding, not as evidence of biology. (arxiv.org) The planet’s value to researchers is that it fills a poorly sampled part of the exoplanet population between very hot giant planets and very cold ones. Nature noted in a separate 2023 analysis of JWST methane work that methane is a basic carbon-bearing molecule that can be used to determine atmospheric composition; in giant-planet studies like this one, scientists treat it as a tracer of chemistry and formation history rather than a standalone biosignature. (arxiv.org) That interpretation is an inference from how exoplanet atmosphere studies are framed in the literature. (sciencedaily.com) ### What does this add to the broader exoplanet picture? The paper said TOI-199 b “serves as the first data point” for studying clouds and hazes in temperate gas giants. Hu said exoplanet studies let astronomers examine planet types not represented in the solar system and use those comparisons to test ideas about how planetary systems form and evolve. The TOI-199 system also includes evidence of an outer non-transiting giant planet, called TOI-199 c, from transit timing variations. (nature.com) The new analysis reduced the mass uncertainty for that outer planet by 50%, the paper said, while favoring a slightly longer orbital period that remains within the system’s conservative habitable zone. ### What comes next for this system? The authors said follow-up observations will be needed to distinguish whether the 3-micron feature comes from ammonia or hydrogen cyanide. (arxiv.org) The paper, titled “Methane on the temperate exo-Saturn TOI-199b,” is available as a preprint, and Penn State said the study was published in The Astronomical Journal on May 20.