JWST Detects Methane in Atmosphere of TOI-199b, the First Temperate Giant Exoplanet to Have Its Chemistry Mapped

Overview

Astronomers using NASA’s James Webb Space Telescope have mapped the atmospheric chemistry of TOI-199b, a Saturn-sized planet orbiting its star more than 330 light-years from Earth, in a study published May 20, 2026 in The Astronomical Journal. The findings mark the first time scientists have studied the atmosphere of a temperate giant exoplanet in detail, according to the research team from Penn State University and NASA’s Jet Propulsion Laboratory.

What We Know

TOI-199b sits at a temperature of approximately 175 degrees Fahrenheit and completes one orbit around its host star roughly every 100 days, placing it in a thermal regime quite unlike the “hot Jupiters” — gas giants that orbit so close to their stars that temperatures reach thousands of degrees — that dominate most exoplanet atmospheric studies so far. According to Penn State, only a handful of such temperate giant planets are known.

The detection relied on transmission spectroscopy, a technique in which JWST records starlight filtering through the planet’s atmosphere during a transit. As lead author Aaron Bello-Arufe, a postdoctoral researcher at JPL, described the method to Phys.org: “As a planet passes in front of its star, some of the star’s light passes through the planet’s atmosphere where it interacts with the elements and molecules in the atmosphere. Specific elements will absorb specific wavelengths of light, creating a fingerprint in the spectrum of light that JWST detects that reflects the atmosphere’s composition.”

The transit of TOI-199b lasts approximately seven hours, considerably longer than the one-hour transits typical of hot Jupiters, and the team gathered approximately 20 consecutive hours of baseline and in-transit observations. When the team compared spectra taken during the transit to the out-of-transit baseline, the signal was unmistakable. Bello-Arufe told Phys.org: “When we compared the spectra during the transit to the baseline, we saw that the atmosphere blocked the wavelengths of starlight absorbed by methane. Models for the composition of temperate, gas-giant exoplanets had predicted that they would contain methane, so it is good to get confirmation that our theories are accurate.”

The paper, published in The Astronomical Journal (Vol. 171, No. 6, Article 354), characterizes the planet as having a radius of 0.810 ± 0.005 Jupiter radii and a mass of 0.17 ± 0.02 Jupiter masses orbiting a G9V host star. The methane detection carries a Bayes factor of approximately 700, corresponding to a carbon-to-hydrogen ratio roughly 13 times solar metallicity. JWST’s NIRSpec G395M instrument captured the critical transit on December 23, 2024, though the paper notes the spectral uncertainties ran 4–5 times larger than predicted due to a pointing misalignment during observations. The team also identified an absorption feature near 3 microns suggesting the possible presence of ammonia in the atmosphere, though additional observations will be needed to distinguish it from other candidate molecules.

Renyu Hu, an associate professor of astronomy and astrophysics at Penn State who led the research team, stressed the broader value of studying such planets. Per Penn State: “Since the first exoplanet was discovered in 1992 by a team that included Aleksander Wolszczan at Penn State, astronomers have found thousands of exoplanets. But only a few giant, temperate exoplanets are known and this is the first time that we have been able to study the atmosphere of one of them in detail.”

What We Don’t Know

The single-transit dataset leaves several questions open. The team has not yet established the precise relative abundances of methane and the other candidate molecules, and the pointing issue that inflated spectral uncertainties means the composition constraints remain preliminary. Additional transits — each lasting seven hours and requiring roughly 20 hours of telescope time — will be needed to sharpen the picture. Hu said, per Phys.org: “With additional observations of this planet, we could establish the relative abundance of these various gases in its atmosphere.”

The paper also notes transit timing variations consistent with the presence of an outer, non-transiting giant planet in the system, which could affect the orbital dynamics of TOI-199b over time, according to the published study.

Analysis

The significance of TOI-199b lies less in what it contains than in what it represents: a first data point in an atmospheric class that has until now existed only in theory and in solar system analogs such as Saturn and Uranus. Hot Jupiters receive the majority of exoplanet atmospheric study time partly because their short transits and strong thermal signals make them easier targets; temperate giants like TOI-199b are rarer, their transits are long, and the atmospheric signals are subtler.

With this detection in hand, researchers now have an empirical foundation to test formation and evolution models that predict how temperate giant planets accumulate and retain volatiles. As Hu noted to Phys.org: “This more complete picture of a temperate gas giant’s atmosphere can then be used to improve our models and potentially better understand how planets and their atmospheres form and evolve, including for Earth.”

The study was funded through NASA via the Space Telescope Science Institute, according to Penn State. Co-authors are drawn from Penn State, JPL, Johns Hopkins University, and Arizona State University, among other institutions.