A planet that is about the size of Saturn, but with a temperature more like Earth’s, has an atmosphere rich in methane, according to a new study using NASA’s James Webb Space Telescope (JWST). Unlike the gas giant planets — Jupiter and Saturn — in Earth’s solar system, which are distant from the sun and therefore extremely cold, and so-called “hot Jupiters” — giant planets beyond the solar system that are scorching hot due to their proximity to the stars they orbit — the planet is one of only a handful of known temperate, giant planets and the first to have its atmosphere analyzed. The new details about the composition of the planet’s atmosphere will inform models of planetary formation and evolution and could improve astronomer’s understanding of how Earth’s atmosphere works, according to the research team.

A paper describing the study, by a team of researchers led by astronomers at Penn State and NASA’s Jet Propulsion Laboratory (JPL) at the California Institute of Technology, published today (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.”

The planet, called TOI-199b, orbits a star that is more than 330 light-years from Earth about every hundred days. Its temperature is about 175 degrees Fahrenheit, which is still hot from a human perspective but not too much hotter than the highest recoded temperatures on Earth at around134 degrees and is easily reached, for example, on the dashboards of cars parked in directly sunlight. It’s significantly more temperate than the hot Jupiters that can reach 1000s of degrees and the cold solar-system gas giants that are hundreds of degrees below zero. 

To characterize an exoplanet’s atmosphere, astronomers use a technique called transmission spectroscopy to analyze light from the star that passes through the planet’s atmosphere. To work, the planet’s orbit must align such that it passes between its star and the telescope. Instruments on the JWST separate the star’s light into its component wavelengths, like how a prism can separate normal white light into the colors of the 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.”

The spectrum during the transit is compared to baseline measurements of the star’s light established through about 20 consecutive hours of observations by JWST. The transit itself lasts about seven hours, which is much longer than the transits for hot Jupiters, which can be an hour or less. The differences between the baseline and transit spectra show which wavelengths of light are being absorbed by the planet’s atmosphere and are used to identify the elements and molecules that comprise the atmosphere, the researchers explained.

“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 team’s observations gave hints that the atmosphere also contained 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.”

IMAGE CREDIT: NASA/JPL-Caltech.