Astronomers find a Saturn-size planet with surprisingly Earth-like temperatures

A giant planet more than 330 light-years from Earth has given astronomers something unusual: a close-up look at the atmosphere of a world that is neither a frozen gas giant nor a scorched “hot Jupiter.” Designated TOI-199b, the planet is roughly the size of Saturn but sits at a temperature closer to conditions on Earth than any planet its size previously studied in this much detail. A team using NASA’s James Webb Space Telescope analyzed starlight filtering through the planet’s atmosphere and found it rich in methane, a signature that places TOI-199b among a small handful of known temperate giant planets. The findings give researchers a rare middle data point between the solar system’s cold outer planets and the blistering gas giants found orbiting close to other stars, one that had simply not existed in this much detail before this study.

A Hundred-Day Orbit Around a Distant Star

TOI-199b circles its host star roughly every hundred days, a long period for a transiting planet and one that keeps it far enough from its star to avoid the extreme heating that defines most previously characterized giant exoplanets. The planet was first flagged by NASA’s Transiting Exoplanet Survey Satellite, which watches for the periodic dimming that occurs when a planet crosses in front of its star. That initial detection set up the follow-up observations with Webb that eventually revealed the composition of its atmosphere. Long-period transiting giants like TOI-199b are comparatively rare in survey data, since the odds of catching a planet transiting its star from Earth’s vantage point shrink as the orbital period grows, making repeated, carefully timed observations essential to confirming the signal.

Transmission Spectroscopy Reveals What the Planet Is Made Of

To study the atmosphere, researchers used transmission spectroscopy, a technique that examines starlight as it passes through a planet’s atmosphere during a transit. Different gases absorb specific wavelengths of light, leaving a chemical fingerprint that Webb’s instruments can detect from hundreds of light-years away. Applying that method to TOI-199b, the team identified methane as a dominant component, along with signs of a broader chemical mix that had not previously been measured in a planet of this size and temperature. The technique requires precise timing, since researchers must compare the starlight collected while the planet crosses the star’s disk against light collected just before and after, isolating the thin sliver of signal that passed specifically through the planet’s atmosphere.

A Temperature That Splits the Difference

TOI-199b’s atmosphere sits at roughly 175 degrees Fahrenheit, hot by human standards but nowhere near the extremes recorded on hot Jupiters, some of which reach temperatures in the thousands of degrees. For comparison, the highest temperature ever recorded at Earth’s surface is around 134 degrees Fahrenheit. That places TOI-199b in a narrow, underexplored temperature range for giant planets, far warmer than Jupiter or Saturn, which sit hundreds of degrees below zero, but nowhere close to the scorched planets that orbit within a few days of their stars. Astronomers have cataloged thousands of giant exoplanets since the 1990s, yet only a small fraction fall into this temperate middle range, largely because planets at these distances are harder to detect than either extremely hot, close-orbiting worlds or the handful of directly imaged giants farther from their stars.

Why a Methane-Rich Atmosphere Matters for Planet Formation

Methane-dominated atmospheres are difficult to sustain at higher temperatures, since heat tends to break the molecule down into other carbon compounds. Finding methane as a leading component of TOI-199b’s atmosphere gives researchers a data point for testing models of atmospheric chemistry at intermediate temperatures, conditions that are common around other stars but rarely observed in this much detail. The composition also offers clues about how far from its star the planet may have originally formed before settling into its current orbit, since planets are thought to inherit much of their initial chemistry from the location in a protoplanetary disk where their ices and gases first accumulated.

Filling a Gap Between Cold Giants and Hot Jupiters

Researchers involved in the study, affiliated with Penn State’s Eberly College of Science, describe TOI-199b as one of only a few known temperate giant planets and the first of its kind to have its atmosphere analyzed in this level of detail. The result adds a reference point that planetary scientists have lacked: a giant world warm enough to study easily with current telescopes but cool enough to preserve chemistry closer to what theorists expect from planets that formed farther from their stars, a category some astronomers informally call this planet’s “Goldilocks” temperature zone. The team says future observations of similar temperate giants could help determine whether TOI-199b’s chemistry is typical for its class or an outlier worth further scrutiny.

Details of the analysis were reported by ScienceDaily, with the underlying research also summarized by EurekAlert and Penn State’s Eberly College of Science news office.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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