James Webb found methane on a giant planet that is neither scorching nor frozen

A planet the size of Saturn is sitting at a temperature mild enough to seem almost familiar, around 175 degrees Fahrenheit. The James Webb Space Telescope has now pulled apart the light filtering through its atmosphere and found methane, with fainter signs of ammonia and carbon dioxide. That matters because giant planets are supposed to live at extremes, and this one refuses to.

Every gas giant astronomers know well falls into one of two camps. In our own solar system, Jupiter and Saturn orbit far from the Sun and stay brutally cold. Beyond the solar system, the easiest giants to study are the so-called hot Jupiters, worlds whipping so close to their stars that their atmospheres glow at thousands of degrees. The planet known as TOI-199b belongs to neither group. It circles a star more than 330 light-years away once every hundred days or so, far enough out to stay temperate.

That orbit is the whole point. A world that is neither roasted nor frozen holds on to chemistry in its air that the extreme planets destroy or bury. Methane in particular tends to be ripped apart in the heat of a hot Jupiter. Finding it intact here gives researchers a rare look at what a giant planet’s atmosphere actually contains when its star is not cooking it.

The reading came from watching the planet cross in front of its star. As TOI-199b transits, a sliver of starlight passes through the thin upper layer of its atmosphere, and the gases there stamp their fingerprints onto that light. Webb split the light into its component wavelengths and picked out methane in the pattern with high confidence. Traces of ammonia and carbon dioxide appeared too, though far more faintly.

The result is a first. Only a handful of temperate giant planets are known at all, and none had ever had its atmosphere examined in this kind of detail. The makeup of that air is a record of how and where the planet came together, whether it gathered its gas close to the star or far out and drifted inward over time.

The case is not closed. The methane is solid, but the ammonia and carbon dioxide remain hints rather than confirmed ingredients, and the relative amounts of each gas are still unknown. A single set of transmission measurements samples only the outer skin of an atmosphere, not its depths, and one planet cannot stand in for an entire class. The team itself treats the methane as the firm result and the rest as a reason to keep looking.

The analysis came from a group led by Renyu Hu of Penn State, with co-lead Aaron Bello-Arufe at NASA’s Jet Propulsion Laboratory, and appeared in The Astronomical Journal in late May. The researchers now plan to aim Webb at other temperate giants to learn whether TOI-199b’s chemistry is typical or strange, the start of a small catalogue of worlds our own solar system never built.