The James Webb Space Telescope (JWST) has had its first look at a hotly anticipated set of targets — the atmospheres of some of the seven Earth-sized planets circling the star TRAPPIST-1, just 12 parsecs (39 light years) from Earth. All seven lie in or near their star’s habitable zone, where liquid water could exist, and astronomers consider them the best known laboratory for studying what might make planets beyond the Solar System suitable for life.

Results so far are preliminary and don’t yet indicate what sorts of atmospheres these planets might actually have. But if they have dense atmospheres containing intriguing molecules such as carbon dioxide or methane, the US$10-billion telescope will be able to detect them in the coming months and years. No other observatory has been powerful enough to spot these atmospheres.

“We’re in business,” said Björn Benneke, an astronomer at the University of Montreal in Canada, during a symposium on first results from JWST in Baltimore, Maryland, on 13 December.

Prized planets

The TRAPPIST-1 planetary system, mapped out in 2017, offers astronomers multiple chances of understanding the formation and evolution of Earth-sized worlds orbiting a single star. The star is relatively faint and cool, and the seven planets are nestled closer to it than Mercury is to the Sun.

JWST is observing all of the planets in its first year of science operations, which began in June. Many of those observations have already been made, but none had been shown publicly until this week’s symposium, which took place at the Space Telescope Science Institute, the JWST operations centre.

Diagram of the TRAPPIST-1 system relative to the inner Solar System.

All of the planets in the TRAPPIST-1 system are closer to their star than Mercury is to the Sun.Credit: NASA/JPL-Caltech

The TRAPPIST-1 planets are designated b to h, with b being closest to the star and h farthest.

Benneke presented the first JWST studies of TRAPPIST-1g. So far, the telescope has been able to make out that the planet probably doesn’t have a hydrogen-rich atmosphere — something the Hubble Space Telescope had previously shown. Such an atmosphere would be physically large owing to its low density, so it would be relatively easy to spot. That could mean that the planet has a denser atmosphere, made of heavier molecules such as carbon dioxide, or no atmosphere at all.

JWST studies planetary atmospheres mainly by watching how they filter starlight as the planets pass in front of the star: particular molecules absorb the starlight in characteristic ways. Which molecules make up the atmosphere can indicate how a planet evolved and whether it might have life on its surface. It will take more observations and analysis time for researchers to discover whether TRAPPIST-1g has an atmosphere and, if so, what it is made of.

Constructing a ‘family portrait’

The TRAPPIST-1 data are much harder to analyse than those gathered from larger exoplanets, including WASP-39b, a planet closer to the size of Jupiter that JWST has studied in detail. TRAPPIST-1’s planets are much smaller, and the signal from their atmospheres is more difficult to tease out. Magnetic disturbances in the star can also induce signals that confound interpretations of the data.

“We needed this first look to know what we’re dealing with,” says Knicole Colón, an astronomer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Benneke declined to speak to reporters about the TRAPPIST-1g results, saying that he is working on a paper for a scientific journal.

In a poster presentation at the conference, Olivia Lim, an astronomer at the University of Montreal, described two JWST observations of the innermost planet in the system, TRAPPIST-1b. Her team, too, has been unable to tease out a signal indicating the composition of the planet’s atmosphere. But preliminary studies suggest that, like planet 1g, it probably doesn’t have a puffy, hydrogen-rich atmosphere.

Lim’s team has several observations of other TRAPPIST-1 planets already in hand, including one set of results gathered last week that she hasn’t had time to look at in the crush of JWST results. “It’s hectic,” she says.

But more results on the extraordinary planetary system are on the way, Colón says: “Within the next year, we’ll have a family portrait.”