Back in May, astronomers pointed NASA’s James Webb Space Telescope at a world that behaves like no planet in our own backyard — and read its air. The target, TOI-199b, is a giant roughly the size of Saturn sitting more than 330 light-years away, yet its temperature is surprisingly Earth-like, and its atmosphere turned out to be packed with methane. Here’s what it means: for the first time, researchers have studied the atmosphere of a temperate gas giant in real detail.
That is rare territory. Giant planets tend to cluster at the extremes — think Jupiter and Saturn, frozen far from the sun, or the “hot Jupiters” found circling close to their stars at temperatures in the thousands of degrees. TOI-199b, which loops around its star about once every 100 days at an estimated 175 degrees Fahrenheit, sits comfortably in between. In plain terms: still extremely hot by everyday standards, but comparable to what builds up inside a car parked in direct sunlight.
How You Read An Atmosphere From 330 Light-Years Away
The team, led by Penn State and NASA’s Jet Propulsion Laboratory and published May 20 in the Astronomical Journal, used transmission spectroscopy. Think of it like this: the telescope watches the planet cross the face of its star, and some starlight filters through the planet’s atmosphere on the way. Webb then splits that light into wavelengths, the way a prism spreads white light into a rainbow.
“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,” explained Aaron Bello-Arufe, a JPL postdoctoral researcher and the paper’s first author. “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 observation took patience: roughly 20 continuous hours to establish a baseline reading of the star’s own light, plus a transit that ran about seven hours — far longer than the hour-or-less crossings typical of hot Jupiters. Comparing transit spectra against the baseline showed which wavelengths the planet’s air had absorbed.
Methane, Right Where Theory Said
“When we compared the spectra during the transit to the baseline, we saw that the atmosphere blocked the wavelengths of starlight absorbed by methane,” Bello-Arufe said. “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 data also hinted at ammonia and carbon dioxide.
Team leader Renyu Hu of Penn State noted the longer arc of the work. “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.”
The practical upshot: follow-up observations could pin down the relative amounts of these gases, sharpening the models scientists use for planetary and atmospheric formation and evolution — including, eventually, our own. As Hu put it, the success “gives us confidence to dedicate more resources and observation time to study other similar planets.” Scientists from Arizona State, Johns Hopkins, the Carnegie Institution, Caltech and UC Santa Cruz joined the effort, funded by NASA through a Space Telescope Science Institute grant.
Sources
- James Webb discovers a rare giant planet with surprisingly Earth-like temperatures | ScienceDaily
- JWST Transmission Spectroscopy Of TOI-3235 b: Challenges In Constraining Giant Planet Atmospheres Around M Dwarfs Amid Stellar Contamination – Astrobiology
