James Webb Space Telescope Detects Methane in TOI-199b’s Atmosphere

Most exoplanet atmospheres examined by NASA’s James Webb Space Telescope belong to worlds hot enough for iron to vaporise.

Hot Jupiters - giant planets that orbit their stars in just a few days - have gradually been mapped in considerable detail, and their overall chemistry is broadly understood.

By contrast, giant planets on more distant, calmer orbits lasting hundreds of days remain almost completely unexplored.

Models had made predictions about this uncharted group. One forecast that a look into their atmospheres would reveal methane.

Whether those models were correct, however, remained uncertain - until now.

A planet between extremes

The planet at the centre of the study is TOI-199b. Roughly Saturn-sized but with only a fraction of Saturn’s mass, it circles a Sun-like star more than 330 light-years away.

It completes each orbit in approximately 100 days.

An earlier paper had already measured and described TOI-199b’s fundamental characteristics.

The new research was led by Renyu Hu, an associate professor at Penn State (PSU).

With a temperature of about 80 °C (175 °F), TOI-199b is far cooler than the worlds exceeding 1,000 degrees that dominate exoplanet research.

Only a small number of such temperate planets are known, and TOI-199b is the first whose atmosphere has been analysed in detail.

Reading starlight through an atmosphere

Determining the composition of an exoplanet’s atmosphere requires patience.

Astronomers must wait until the planet moves across the face of its star, then observe the starlight passing through its outer atmosphere.

Gases along that path absorb particular wavelengths, creating a fingerprint within the light that reaches the telescope.

The method has a formal term - transmission spectroscopy - although its principle is straightforward.

Webb’s instruments divide incoming starlight into its individual colours, examining one wavelength at a time. Small drops in the spectrum indicate the molecules that are present.

A lengthy observation of TOI-199b

The transit lasted around seven hours, much longer than the rapid crossings of hot Jupiters, which can be finished in less than an hour.

Webb also remained focused on the star for nearly 20 uninterrupted hours, establishing a clear reference for the starlight when the planet was not in front of it.

By comparing these two sets of observations, the researchers could identify the colours absorbed by the planet.

Aaron Bello-Arufe, a postdoctoral researcher at NASA’s Jet Propulsion Laboratory (JPL), was the study’s lead author.

What the spectrum revealed

After the two records were matched, one absorption signature stood out: methane.

The atmosphere absorbed the precise wavelengths associated with methane, producing a fingerprint that no other common molecule reproduces at those colours.

“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.

For temperate gas giants, models had predicted this result for a long time.

Before this research, nobody had directly observed such agreement in a planet of this type. The finding gives modellers a real-world test at temperatures they had not previously sampled.

Chemical clues beyond methane

The observations also contained a weaker and more uncertain feature elsewhere in the spectrum.

According to the team’s models, it could point to ammonia or another compound containing nitrogen.

Planetary chemists are interested in both possibilities because their relative abundances may show how much circulation takes place between the deep interior and the cooler upper atmosphere.

More data will be needed to establish that link. The observations also contained faint indications of carbon dioxide.

None of these secondary features is as significant as the methane detection, and resolving them will require further Webb observing time.

An echo of home

What researchers found around TOI-199b bears a distant resemblance to the atmospheres of Jupiter and Saturn, where methane and ammonia are well-known constituents.

Webb had previously detected methane on the smaller, cooler planet K2-18b, in a study that attracted considerable attention because the planet lies within its star’s habitable zone.

This pattern indicates that methane occurs consistently in temperate worlds with light, hydrogen-rich atmospheres.

Researchers had suspected this for years, but could not verify it without instruments of this sensitivity.

Further observations are needed

The observation had limitations. A pointing misalignment made the spectrum less precise than the team had intended.

Nevertheless, the methane signal remained strong.

The researchers also investigated whether the atmosphere could contain hazes - suspended particles capable of softening certain absorption features - using models based on the chemistry of Titan, Saturn’s moon.

The models only weakly favoured haze over a clear atmosphere. A longer observation will be needed to determine whether TOI-199b truly has clouds or hazes.

Future research directions

For decades, the chemistry of giant planets that are neither intensely hot nor as cold as those in the Solar System has remained a blank area on the map.

Hu’s team has now added one data point to that gap.

Methane is present in the atmosphere of a temperate gas giant in approximately the quantities predicted by models.

This indicates that the underlying chemistry used by researchers remains valid when tested against a real planet within this temperature range.

With additional Webb time, the team could determine the relative amounts of methane, ammonia and carbon dioxide on TOI-199b, then compare them with other temperate giants.

A clearer understanding should then emerge of how Earth’s own atmosphere fits into the wider story of planetary chemistry.

Image credit: NASA/JPL-Caltech