JWST detects methane in atmosphere of a temperate, Saturn-sized exoplanet

Penn State and NASA’s Jet Propulsion Laboratory said this week that the James Webb Space Telescope has detected methane in the atmosphere of TOI-199 b, a Saturn-mass exoplanet orbiting a Sun-like star more than 330 light-years away. The finding matters because TOI-199 b is not a blistering “hot Jupiter” and not a frozen outer-system giant, but a rarer middle case: a temperate gas giant with an estimated equilibrium temperature of about 350 Kelvin. The team reported the result in a study published May 20 in *The Astronomical Journal*, after analyzing a single transit with Webb’s NIRSpec G395M instrument. ### Why are astronomers calling this planet “temperate”? TOI-199 b circles a G-type star roughly every 100 days, according to the paper, placing it much farther from its star than the ultra-short-period giant planets Webb has often studied so far. The researchers define temperate gas giants as those with equilibrium temperatures below 400 Kelvin, and they give TOI-199 b a value of 350 Kelvin. (sciencedaily.com) Penn State said that makes the planet unusual in current exoplanet atmospheric work, which has focused heavily on giants at temperature extremes. In our own solar system, Jupiter and Saturn are much colder because they orbit far from the Sun, while many known giant exoplanets are much hotter because they orbit very close to their stars. ### What exactly did Webb see in the atmosphere? (arxiv.org) JWST observed TOI-199 b during a transit, when the planet crossed in front of its host star and a small fraction of starlight filtered through the planet’s atmosphere. By measuring how that filtered light changed with wavelength, the team reconstructed a transmission spectrum and looked for molecular fingerprints. The paper reports a methane signal, written chemically as CH4, with Bayesian retrievals favoring methane in a cloudy atmosphere by a Bayes factor of about 700. (sciencedaily.com) The authors also said they did not detect carbon monoxide or carbon dioxide, which helped rule out some higher-metallicity atmospheric scenarios. ### Why is methane on this kind of world getting attention? The authors describe temperate gas giants as an “unexplored frontier” in atmospheric spectroscopy, because few such planets are known and even fewer are suitable for detailed follow-up. (arxiv.org) TOI-199 b is one of the first temperate giant planets whose atmosphere has been studied this closely with Webb. Renyu Hu of Penn State said in the university release that exoplanet studies let astronomers compare many kinds of planets, including worlds not represented in the solar system, to learn how planetary systems form and evolve. (arxiv.org) Hu said only a few giant, temperate exoplanets are known and that this was the first time researchers had been able to study the atmosphere of one in such detail. ### Does “Earth-like temperatures” mean this planet could be habitable? The Penn State release said TOI-199 b has temperatures “surprisingly similar to those on Earth,” but that shorthand refers to temperature range, not to surface conditions or habitability. TOI-199 b is a gas giant about the size or mass class of Saturn, so it is not an Earth-like rocky planet. (sciencedaily.com) The paper frames the result as a way to study giant-planet chemistry, clouds and mixing under milder conditions than the hot giants that dominate current samples. That makes the planet useful as a comparison case for atmospheric models, rather than as a candidate inhabited world. ### What are researchers still trying to pin down? The study reports a separate increase in transit depth near 3 microns that the team says could be explained by ammonia, or less likely hydrogen cyanide. (sciencedaily.com) The authors said follow-up observations should help distinguish between those possibilities and clarify how gases are mixed vertically in the atmosphere. The same paper also notes that the TOI-199 system shows strong transit timing variations caused by an outer non-transiting giant planet. (arxiv.org) The researchers said their timing analysis reduced the mass uncertainty for that outer planet by 50% and favored a slightly longer orbital period that remains within the system’s conservative habitable zone.