Astronomers have uncovered a Saturn-sized exoplanet with temperatures similar to Earth that harbors an atmosphere rich in methane, offering unprecedented insights into the chemistry of temperate gas giants. The discovery, made using NASA’s James Webb Space Telescope (JWST), marks the first detailed atmospheric study of a planet in this temperature range, according to a new paper published in The Astronomical Journal.
A New Frontier in Exoplanet Exploration
The planet, designated TOI-199b, orbits a distant star more than 330 light-years from Earth with a period of about 100 days. Its surface temperature is roughly 175 degrees Fahrenheit, making it far cooler than scorching “hot Jupiters” yet much warmer than the icy giants of our solar system. This positions TOI-199b in a rare class of temperate, giant exoplanets, a category astronomers have long sought to study in detail. By examining such planets, researchers hope to uncover patterns in planetary formation and evolution that could illuminate the history of Earth-like atmospheres.
Probing Alien Atmospheres With Webb
The team utilized transmission spectroscopy, a technique that measures how starlight is filtered through a planet’s atmosphere during transit. “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, postdoctoral researcher at JPL and first author of the study published in The Astronomical Journal. “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.”
For TOI-199b, the transit lasts about seven hours, allowing JWST to gather detailed spectral data. By comparing these observations with baseline measurements of the star’s light over 20 consecutive hours, the team could identify molecules in the atmosphere with high confidence.
Methane and Other Atmospheric Signatures
Analysis revealed that the planet’s atmosphere contains methane, with possible traces of ammonia and carbon dioxide. “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.” This confirmation validates existing models of giant planet atmospheres and provides a rare glimpse into chemical processes on temperate exoplanets.
Implications for Planetary Science
The findings have far-reaching implications for the study of planetary atmospheres. “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 at Penn State and lead researcher. “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.”
By establishing the composition of TOI-199b’s atmosphere, astronomers can refine models of planet formation and chemical evolution, offering insights that could extend to understanding Earth’s own atmosphere. The team plans to continue observing TOI-199b and other temperate giants to determine whether similar methane-rich atmospheres are common or unique.
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