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

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

Hot Jupiters – enormous planets that complete an orbit around their stars in only a few days – have been mapped extensively over the years, and their overall chemistry is broadly understood.

By contrast, giant planets at greater distances, travelling around their stars on roughly 100-day orbits that are cooler and less turbulent, are still almost wholly unexplored.

Models had made predictions about this uncharted class. One forecast was that an examination of their atmospheres would reveal methane.

Until now, however, it was unclear whether those models were correct.

TOI-199b, a planet between extremes

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

It takes about 100 days to complete a single orbit.

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

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

At about 80°C, TOI-199b is substantially cooler than the 1,000-degree worlds that feature in most exoplanet research.

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

How astronomers read light through an atmosphere

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

Astronomers wait for a planet to move across the face of its star, then study the starlight passing through the planet’s outer atmospheric layers.

Gases in the atmosphere absorb particular wavelengths, leaving a signature in the light that arrives at the telescope.

The method has a technical name – transmission spectroscopy – although its principle is straightforward.

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

A long observation of TOI-199b

The transit lasted around seven hours, considerably longer than the rapid transits of hot Jupiters, which can finish in less than an hour.

Before and around the transit, Webb observed the star for almost 20 uninterrupted hours, establishing a clear reference for the star’s light when the planet was not in front of it.

By comparing these two sets of observations, the researchers could identify the colours quietly 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 had been matched, one absorption feature stood out: methane.

The atmosphere absorbed precisely the wavelengths known to be taken up by this gas, 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.

This is what models had predicted for temperate gas giants for many years.

Before this study, that prediction had never been directly observed in a planet of this type. The finding gives modellers a real-world test within a temperature range they had not previously sampled.

Chemical evidence beyond methane

The observations also contained a weaker and less certain 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 relationship. The researchers also found faint indications of carbon dioxide.

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

A resemblance to the Solar System

The atmospheric chemistry identified around TOI-199b has a distant similarity to that of Saturn and Jupiter, where methane and ammonia are familiar components.

Webb also detected methane in the smaller, cooler planet K2-18b in an earlier study that attracted broad attention, as that world lies within its star’s habitable zone.

Together, these results suggest that methane occurs consistently in the atmospheres of temperate planets with light, hydrogen-rich air.

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

Further observations are required

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 include hazes – airborne particles that might obscure some absorption features – through models based on the chemistry of Titan, Saturn’s moon.

The models showed only a weak preference for haze over a clear atmosphere. A longer observation will be needed to determine whether TOI-199b genuinely has clouds or hazes.

Future research on temperate giant planets

For decades, the chemistry of giant planets that are neither extremely 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 point of data to that gap.

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

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

With further Webb observing time, the team will be able to determine the relative quantities of methane, ammonia and carbon dioxide on TOI-199b, and compare them with other temperate giants.

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

Image credit: NASA/JPL-Caltech