JWST detects methane on TOI-199b

Penn State astronomers and researchers at NASA’s Jet Propulsion Laboratory reported on May 20 that the James Webb Space Telescope detected methane in the atmosphere of TOI-199b, a Saturn-mass exoplanet more than 330 light-years from Earth. (eurekalert.org) The result comes from a transmission-spectrum study taken during a single transit with Webb’s NIRSpec G395M instrument, according to the team’s paper in *The Astronomical Journal*. (arxiv.org) ScienceDaily and EurekAlert summarized the finding on May 21 and May 20, respectively, based on the Penn State release. TOI-199b stands out because it is neither a frozen outer-system giant like Saturn nor a close-in “hot Jupiter” with temperatures in the thousands of degrees. NASA’s exoplanet catalog lists the planet at 0.17 Jupiter masses, 0.81 Jupiter radii and an orbital period of 104.9 days. The paper gives its equilibrium temperature as 350 kelvin, placing it in the relatively sparse category of temperate gas giants. ### Why are astronomers paying attention to one more methane detection? Renyu Hu of Penn State said the value of exoplanet studies is that they let astronomers examine kinds of planets not represented in the solar system. (eurekalert.org) Hu said only a few temperate giant exoplanets are known and that this is the first time researchers have studied the atmosphere of one of them in such detail. The arXiv version of the paper says temperate gas giants with equilibrium temperatures below 400 kelvin are still an “unexplored frontier” in atmospheric spectroscopy. (science.nasa.gov) The authors said TOI-199b is one of the most favorable low-temperature gas giants for atmospheric study because it transits its star roughly every 100 days. ### What exactly did Webb see in the atmosphere? The paper reports that Bayesian retrievals of the Webb spectrum revealed methane, or CH4, in a cloudy-atmosphere model with a Bayes factor of about 700. The authors said the same analysis did not find detectable carbon monoxide or carbon dioxide, which they said disfavors very high atmospheric metallicities above roughly 50 times solar. (eurekalert.org) A second feature in the spectrum appears near 3 microns. The team said its self-consistent models point to ammonia, NH3, or less likely hydrogen cyanide, HCN, as the source of that signal. (arxiv.org) ### How “Earth-like” is this planet, really? Penn State’s release said TOI-199b has a temperature of about 175 degrees Fahrenheit. That is still hotter than Earth’s average surface conditions, but much cooler than the giant exoplanets that dominate many Webb atmosphere studies. ScienceDaily described the planet as having “surprisingly Earth-like temperatures,” but the comparison is relative, not literal. (arxiv.org) TOI-199b is a gas giant about Saturn’s size, and NASA classifies it as a planet orbiting a K-type star at 0.4254 astronomical units. ### What does the paper say about the broader planetary system? The TOI-199 system contains more than one giant planet. The paper says strong transit timing variations in TOI-199b are caused by an outer, non-transiting giant planet, TOI-199c. (eurekalert.org) The authors said their transit-timing analysis reduced the mass uncertainty for planet c by 50% and favored a slightly longer orbital period that remains within the system’s conservative habitable zone. They also said TOI-199b can serve as a first data point for studying clouds and hazes in temperate gas giants. (sciencedaily.com) ### What comes next for TOI-199b? The paper says follow-up observations will be needed to distinguish whether the 3-micron feature is ammonia or hydrogen cyanide. That distinction, the authors wrote, would help determine the planet’s vertical mixing regime. (arxiv.org) The published study in *The Astronomical Journal* and the preprint “Methane on the temperate exo-Saturn TOI-199b” are now the main reference points for the next round of analysis by teams at Penn State, JPL and other exoplanet groups. (eurekalert.org) (arxiv.org)