JWST Detects Methane on Temperate Saturn-Sized Exoplanet TOI-199b for the First Time

A First-of-Its-Kind Atmospheric Discovery

Astronomers have for the first time analysed the atmosphere of a temperate gas giant beyond our solar system, detecting methane on a Saturn-sized world with temperatures not far from those found on Earth. The discovery, published on 20 May in the Astronomical Journal, opens a new chapter in the study of how planetary atmospheres form and evolve.

The planet, TOI-199b, orbits a star more than 330 light-years from Earth once every 100 days and has an atmospheric temperature of about 175 degrees Fahrenheit. That places it in a rare category between the scorching 'hot Jupiters' that reach thousands of degrees and the frigid gas giants of our own solar system.

A Rare World Between Extremes

'Since the first exoplanet was discovered in 1992 by a team that included Aleksander Wolszczan at Penn State, astronomers have found thousands of exoplanets,' said Renyu Hu, associate professor of astronomy and astrophysics at Penn State and leader of the research team. '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.'

Using a technique called transmission spectroscopy, the team observed about 20 consecutive hours with NASA's James Webb Space Telescope, capturing a seven-hour transit as the planet passed in front of its star. The resulting spectrum showed clear absorption signatures of methane.

Validating Theoretical Models

'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,' said Aaron Bello-Arufe, a postdoctoral researcher at NASA's Jet Propulsion Laboratory and first author of the paper.

Beyond methane, the observations yielded hints of ammonia and carbon dioxide. The team said additional observations could establish the relative abundance of these gases and provide a more complete picture of how temperate giant planets and their atmospheres evolve, including potential insights relevant to understanding Earth's own atmosphere.

What Comes Next

The research team, which included scientists from Arizona State University, Johns Hopkins University, the Carnegie Institution for Science, Caltech, and UC Santa Cruz, was funded by NASA through a grant from the Space Telescope Science Institute. Hu said the success of this first study gives the community confidence to dedicate more observation time to similar planets, testing whether TOI-199b's atmospheric makeup represents a broader class of temperate giants or is an outlier among known worlds.