Somewhere between the icy depths of hot Jupiter and gas giants like Saturn lies a planet that astronomers have spent years searching for and studying: a temperate giant large enough to qualify as a gas world but with a temperature range that is almost unremarkable by the standards of the universe. That planet now has a name, a confirmed atmosphere and a chemical fingerprint. TOI-199b, a Saturn-sized world orbiting a star more than 330 light-years from Earth, has become the first temperate giant exoplanet to have its atmosphere examined in detail, and what is its role? NASAThe discovery by the James Webb Space Telescope both confirms existing theories and opens up entirely new questions. The planet’s atmosphere is rich in methane and also contains small amounts of ammonia and carbon dioxide, and its surface temperature hovers around 175 degrees Fahrenheit, still hot from a human perspective but closer to Earth’s conditions than anything else previously studied in this category. The research, led by a team from Pennsylvania State University and NASA’s Jet Propulsion Laboratory, was published May 20 in astronomical magazineit marks the beginning of what astronomers hope will be a larger effort to understand a class of planets that, until now, have been almost entirely invisible to science.
Why TOI-199b is different from almost every other exoplanet scientists have studied before
Unlike the gas giants Jupiter and Saturn in Earth’s solar system, which are farther from the sun and therefore extremely cold, as well as the so-called “hot Jupiters” (giant planets outside our solar system that are scorching due to their proximity to the stars they orbit), TOI-199b is one of the few known temperate giants and the first to have its atmosphere analyzed. The importance of this distinction cannot be overstated. The vast majority of exoplanets studied in atmospheric detail fall into one of two extreme categories: a planet so hot that its chemical composition is driven by temperatures of thousands of degrees, or a planet so cold that meaningful comparisons with Earth or its neighbors are difficult. TOI-199b occupies a middle ground that has been theorized by planetary science but rarely directly tested. Renyu Hu, associate professor of astronomy and astrophysics at Pennsylvania State University and leader of the research team, said: “Since the first exoplanet was discovered in 1992 by a team such as Aleksander Wolszczan of Pennsylvania State University, astronomers have discovered thousands of exoplanets. But there are only a few known giant temperate exoplanets, and this is the first time we have been able to study the atmosphere of one of these exoplanets in detail.”
How the James Webb Space Telescope reads the chemical fingerprints of distant planets’ atmospheres
To characterize an exoplanet’s atmosphere, astronomers use a technique called transmission spectroscopy to analyze the light from the star that passes through the planet’s atmosphere. Instruments on the James Webb Space Telescope separate a star’s light into its component wavelengths, much like how a prism separates normal white light into the colors of the rainbow. This process requires the planet’s orbit to be aligned so that it passes directly between its host star and the telescope, and requires a significant amount of observation time to produce reliable data. The spectra during the transit were compared to baseline measurements of the star’s light established by JWST from approximately 20 consecutive hours of observations. The transit itself lasts about seven hours, which is much longer than the transits of hot Jupiters, which can be an hour or less. The extended transit window is one of the practical advantages of studying temperate giants. Its slower orbit gives scientists a longer window to collect data because its atmosphere filters the starlight that reaches the telescopes.
TOI-199b What Methane, Ammonia and Carbon Dioxide in the Atmosphere Actually Tell Us
“When we compared the spectra during the transit to the baseline, we found that the atmosphere blocks the wavelengths of starlight absorbed by methane,” said Aaron Bello-Arufe, a postdoctoral researcher at JPL and first author of the paper. “Compositional models of temperate gas giant exoplanets predict that they will contain methane, so it’s nice to confirm that our theory is accurate.“This detection is not only an independent result, but a validation of the atmospheric models that scientists have built over many years. In addition to methane, the team’s observations also hint that the atmosphere contains ammonia and carbon dioxide. Each of these molecules carries information about how the planet formed, how its chemical composition evolved, and how it compares to the gas giants in our solar system. The relative abundance of these gases, which will require additional observations to be determined precisely, may ultimately help explain how the atmospheres of planets, including Earth, develop and change over time.
TOI-199b’s temperature range makes it the closest object to the planetary middle zone
TOI-199b’s temperature is about 175 degrees Fahrenheit, still hot from a human perspective, but not much hotter than the highest temperature recorded on Earth (about 134 degrees Fahrenheit), and easily reached, for example, on the dashboard of a car parked in direct sunlight. It is much milder than the hot Jupiters, with temperatures reaching thousands of degrees, and the cold solar system gas giants, with temperatures hundreds of degrees below zero. It’s this hot middle ground that makes this planet so valuable as a research target. Under the extreme conditions that define most studied exoplanets, atmospheric chemistry behaves in ways that are of limited relevance to the planets and moons of our solar system. The range in which TOI-199b falls makes comparisons more meaningful, and the chemicals that control its atmosphere could have real implications for understanding how the gaseous world under milder conditions has behaved over geological time.
What this discovery means for future research on temperate giant exoplanets
“By conducting more observations of the planet, we can determine the relative abundance of these different gases in its atmosphere,” Hu said. “This more complete picture of the atmospheres of temperate gas giants can be used to improve our models and potentially provide a better understanding of how planets and their atmospheres formed and evolved, including Earth. The success of the first study of the atmospheres of temperate giant planets also gives us the confidence to devote more resources and observation time to studying other similar planets. We can then see if this planet is unique, or if there are common characteristics among planets of this type. “The research was funded by NASA through a grant from the Space Telescope Science Institute and involved scientists including pennsylvania state universityJet Propulsion Laboratory, Arizona State University, Johns Hopkins University, Carnegie Institution for Science, Caltech, and University of California, Santa Cruz. Whether TOI-199b is a bizarre anomaly or the first confirmed member of a broader population of temperate giant planets is a question to be answered in the next phase of the observation program.
