JWST finds TOI-199b methane atmosphere

Penn State and NASA’s Jet Propulsion Laboratory said on May 20 that the James Webb Space Telescope detected methane in the atmosphere of TOI-199b, a gas giant about the size of Saturn orbiting more than 330 light-years from Earth. The result was reported in a paper published that day in the Astronomical Journal and described in a Penn State release carried by EurekAlert and ScienceDaily. The planet is unusual because it is neither a frigid outer-system giant like Jupiter and Saturn nor an ultra-hot “hot Jupiter” hugging its star. Researchers said TOI-199b is one of only a small number of known temperate giant planets and the first of that class to have its atmosphere studied in detail. ### Why are astronomers focused on this one planet? TOI-199b completes an orbit about every 100 days around a G-type star, according to the paper and Penn State’s release. The team said its equilibrium temperature is about 350 kelvin, and the release described that as about 175 degrees Fahrenheit — warm by human standards but far cooler than the thousands of degrees often measured on hot Jupiters. That puts the planet in a middle category that astronomers have had few chances to study with atmospheric data. (eurekalert.org) Renyu Hu, an associate professor of astronomy and astrophysics at Penn State and leader of the research team, said the value of exoplanet studies is that they let astronomers examine kinds of worlds not present in the solar system. 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 of them in detail. (eurekalert.org) ### How did Webb detect methane there? The paper said the team used transmission spectroscopy from a single transit observed with JWST’s NIRSpec G395M mode. In that method, astronomers measure starlight that passes through a planet’s atmosphere as the planet crosses in front of its star, looking for wavelength-specific signatures from gases. The authors said Bayesian retrievals showed methane, or CH4, in a cloudy-atmosphere model with a Bayes factor of about 700. (eurekalert.org) The same analysis did not find detectable carbon monoxide or carbon dioxide at levels expected in very high-metallicity models, the paper said. The authors reported that the methane result held up across independent reductions and retrieval approaches described in the study summary and related coverage. ### What else did the spectrum show besides methane? The paper reported an increase in transit depth near 3 microns. (arxiv.org) The authors said their self-consistent models attribute that feature either to ammonia, NH3, or, less likely, hydrogen cyanide, HCN. They also tested several haze prescriptions, including Titan-like tholin, soot and water-rich tholin, but said the preference for haze models over a clear case was weak. Penn State said the finding gives astronomers a first data point for studying clouds and hazes in temperate gas giants. The paper said TOI-199b could help define how low-molecular-weight atmospheres at moderate temperatures appear in transmission spectra. ### Does this make TOI-199b Earth-like? TOI-199b is a gas giant, not a rocky planet, and the comparison to Earth applies to temperature range rather than surface conditions. (arxiv.org) The Penn State release said the world is about the size of Saturn, while the paper described it as a Saturn-mass exoplanet. Researchers said the significance is that its atmosphere sits in a temperature regime between the cold giants of our solar system and the much hotter giant exoplanets more commonly observed. (eurekalert.org) ### What happens next in this research? The authors said follow-up observations will be needed to distinguish whether the 3-micron feature is ammonia or hydrogen cyanide. The paper also noted that the TOI-199 system shows strong transit timing variations caused by an outer non-transiting giant planet, and that the team’s analysis reduced the mass uncertainty for TOI-199 c by 50%. The next step, according to the study, is more Webb or related follow-up work aimed at refining the chemistry and vertical mixing in TOI-199b’s atmosphere. (eurekalert.org) (arxiv.org)