The discovery stands out because giant planets are usually found at temperature extremes. In our solar system, Jupiter and Saturn are extremely cold because they orbit far from the sun. Many giant exoplanets discovered beyond our solar system are "hot Jupiters," worlds that circle very close to their stars and reach blistering temperatures of thousands of degrees.

TOI-199b falls somewhere in between. It is one of only a small number of known temperate giant planets, and researchers say this is the first time the atmosphere of one has been studied in such detail.

The findings could help scientists improve models of how planets and atmospheres form and evolve. Researchers also say the work may eventually provide new insight into Earth's own atmosphere.

The study, 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."

A Giant Planet With Surprisingly Mild Temperatures

TOI-199b orbits a star located more than 330 light years from Earth and completes one orbit roughly every 100 days.

The planet's temperature is estimated to be around 175 degrees Fahrenheit. While still extremely hot by everyday standards, it is much cooler than most giant exoplanets studied so far. For comparison, temperatures inside parked cars sitting in direct sunlight can easily climb to similar levels on Earth.

That makes TOI-199b far milder than hot Jupiters, while also being dramatically warmer than the frozen gas giants in our own solar system, where temperatures can plunge hundreds of degrees below zero.

How JWST Studied the Planet's Atmosphere

To investigate the atmosphere of TOI-199b, researchers used a method called transmission spectroscopy. This technique allows astronomers to study starlight that passes through a planet's atmosphere while the planet moves across the face of its star from the telescope's point of view.

JWST separates the star's light into different wavelengths, similar to how 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 collected about 20 continuous hours of observations to establish a baseline measurement of the star's light. The planetary transit itself lasted about seven hours, much longer than the transits typically seen for hot Jupiters, which can last an hour or less.

Scientists then compared the light spectrum recorded during the transit with the baseline measurements. The differences revealed which wavelengths were absorbed by the planet's atmosphere, allowing the team to identify the gases present.

Methane Detected in TOI-199b's Atmosphere

"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."

In addition to methane, the observations also hinted at the presence of ammonia and carbon dioxide.

"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."

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 research through a grant from the Space Telescope Science Institute.