NASA’s James Webb Space Telescope has uncovered a rare alien world that sits in a cosmic “sweet spot” between scorching hot Jupiters and frozen gas giants like Saturn.
Astronomers using NASA’s James Webb Space Telescope (JWST) have examined the atmosphere of a rare giant planet with temperatures surprisingly close to those found on Earth. The planet, called TOI-199b, is about the size of Saturn and contains methane in its atmosphere, according to a new study.
The discovery is unusual because giant planets are typically found in extreme environments. In our solar system, Jupiter and Saturn are icy cold because they orbit far from the Sun. Many giant planets discovered around other stars are “hot Jupiters,” which orbit extremely close to their stars and can heat up to thousands of degrees.
TOI-199b is different. Scientists classify it as a temperate giant planet, a category that includes only a small number of known worlds. Researchers say this is the first time the atmosphere of one of these milder giant planets has been studied in detail.
The findings could help researchers improve computer models that explain how planets and atmospheres form and change over time. The work may also provide new clues about atmospheric processes on Earth.
The research, led by scientists from Penn State and NASA’s Jet Propulsion Laboratory (JPL) at the California Institute of Technology, was published May 20 in the Astronomical Journal.
“One of the main advantages of studies of planets beyond our solar system, known as exoplanets, is the ability to study many different types of planets — especially ones that we don’t see in the solar system — to learn about how planetary systems form and evolve,” said Renyu Hu, associate professor of astronomy and astrophysics in the Penn State Eberly College of Science and leader of the research team. “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.”
TOI-199b Has Surprisingly Earth-Like Temperatures
TOI-199b circles a star located more than 330 light-years from Earth and completes an orbit about every 100 days.
Its estimated temperature is around 175 degrees Fahrenheit. While that is still extremely hot by human standards, it is far less intense than the temperatures found on hot Jupiters. Researchers noted that similar temperatures can even occur inside cars parked in direct sunlight on Earth.
That makes TOI-199b much warmer than the frozen gas giants in our solar system, but far cooler than the superheated giant planets commonly studied around other stars.
How Scientists Detected Methane in the Atmosphere
To study the atmosphere of TOI-199b, astronomers used a method known as transmission spectroscopy. This technique measures how starlight changes as it passes through a planet’s atmosphere during a transit, when the planet moves in front of its star from the telescope’s perspective.
JWST separates incoming starlight into different wavelengths, similar to the way a prism splits 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,” said Aaron Bello-Arufe, a postdoctoral researcher at JPL and the first author of the paper. “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.”
Researchers first recorded about 20 straight hours of observations to establish a baseline measurement of the star’s light. The transit itself lasted around seven hours, much longer than the shorter transits commonly seen for hot Jupiters.
Scientists then compared the spectrum collected during the transit with the baseline observations. The differences revealed which wavelengths were absorbed by gases in the atmosphere, helping the team identify its chemical makeup.
Methane, Ammonia, and Carbon Dioxide Hints
“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 observations also suggested that ammonia and carbon dioxide may be present in the atmosphere.
“With additional observations of this planet, we could establish the relative abundance of these various gases in its atmosphere,” Hu said. “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 success of this first study of a temperate giant planet’s atmosphere also gives us confidence to dedicate more resources and observation time to study other similar planets. We can then see if this planet is unique or if there are general shared characteristics for this type of planet.”
Reference: “Methane on the Temperate Exo-Saturn TOI-199 b” by Aaron Bello-Arufe, Renyu Hu, Mantas Zilinskas, Jeehyun Yang, Armen Tokadjian, Luis Welbanks, Guangwei Fu, Michael Greklek-McKeon, Mario Damiano, Jonathan Gomez Barrientos, Heather A. Knutson, David K. Sing and Xi Zhang, 20 May 2026, The Astronomical Journal.
DOI: 10.3847/1538-3881/ae4fba
The research team also included scientists from Arizona State University, Johns Hopkins University, the Carnegie Institution for Science, the California Institute of Technology, and the University of California, Santa Cruz.
NASA funded the study through a grant from the Space Telescope Science Institute.
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