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27/11/2025

Uur van de waarheid voor planeet Trappist-1e

Dit artikel verscheen in het NRC van 27 november 2025

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The moment of truth for the planet that might resemble Earth: does TRAPPIST-1e have an atmosphere?

The moment of truth for the planet that might resemble Earth: does TRAPPIST-1e have an atmosphere?

Exoplanet TRAPPIST-1e currently offers the best chance of finding out whether Earth-like worlds around small stars can be habitable. New observations from the James Webb Space Telescope may provide the answer.

“There are two possible outcomes, and both are enormous,” says astronomer Natalie Allen in a video call from Johns Hopkins University in Baltimore. Next to her, Nestor Espinoza, her supervisor and co-leader of their research team, nods in agreement. They are working on one of the most ambitious projects of the James Webb Space Telescope (JWST): the search for an atmosphere around the rocky planet TRAPPIST-1e. The first four observations have now been taken and two papers have been published, but the biggest question — does this planet have an atmosphere? — remains unanswered for now. Just a little longer, not much. Fifteen new Webb observations scheduled for the coming year should settle the matter.
Exoplanets (planets orbiting stars other than the Sun) are found around nearly every star; thousands have been discovered in recent years. Among them, the TRAPPIST-1 system stands out as one of the most evocative discoveries. It is a miniature version of our own Solar System: seven planets, all roughly Earth-sized, orbiting a small red dwarf star. In our Solar System, the entire system would fit inside Mercury’s orbit. It was discovered in 2017 with TRAPPIST, a pair of telescopes developed in Belgium (hence the name; the successor is called SPECULOOS). The world reacted euphorically. If a second Earth existed anywhere in the cosmos, surely it had to be here.


Crown jewel

Within this compact system, TRAPPIST-1e is considered the crown jewel: slightly smaller than Earth and located in the “habitable zone,” the orbital belt where liquid water can exist. The term does not guarantee habitability; that depends on the presence of an atmosphere, which determines how much heat a planet retains and what its climate is like. Our own Solar System illustrates this vividly. Venus, Earth, and Mars all lie in the habitable zone, yet could hardly be more different. Venus has a thick atmosphere, and its surface is scorching hot. Mars has lost most of its air and is a barren desert. And Earth? Without an atmosphere, the average temperature here would be about twenty degrees below zero. Only with a stable, protective atmosphere does a planet become a potentially habitable world.
TRAPPIST-1e therefore carries a particular burden. Not because our hopes of finding extraterrestrial life depend on it, but because this world is a test case: can small rocky planets around red dwarf stars hold on to their atmospheres? Co-lead researcher Espinoza puts it concisely: “This is it. This is the key system with which we can test this question.”
TRAPPIST-1 is a red dwarf star, the most common type of star in the Milky Way. Red dwarfs are small (TRAPPIST-1 is about the size of Jupiter), cool, and ten times more numerous than Sun-like stars. That makes them appealing targets for planet hunters: around a small star, even a thin atmosphere casts a noticeable shadow.
Of the nearly four hundred known planets around red dwarfs, a large share lies relatively close to Earth. The closer a star is, the more of its light our telescopes receive — and the more precisely we can measure tiny variations. TRAPPIST-1 sits at a “comfortable” forty light-years: close enough that, in principle, the atmosphere of a rocky planet could be detected.
But red dwarfs have a downside: they are active. They eject bright flares and UV and X-ray radiation that can strip away a fragile atmosphere. In addition, planets orbit so close that they become tidally locked: one side in eternal day, the other in eternal night. Only a stable atmosphere can moderate such extremes.


Litmus test

For that reason, TRAPPIST-1e is a true litmus test. If this planet, around this star, at this distance, turns out not to have an atmosphere — what would that mean for all the other Earth-like candidates around red dwarf stars?
The existence of an atmosphere around a distant planet can be established using transmission spectroscopy: when a planet passes in front of its star, its atmosphere filters out a tiny fraction of the starlight. That missing light — sometimes only a few thousandths of a percent — contains the fingerprints of molecules such as CO₂, water, or methane. To distinguish these wisps of signal, the telescope’s detection limit is pushed to the extreme. Espinoza: “We write our own scripts to process the Webb data, because the standard software isn’t designed for signals this small.”
The first four transits of TRAPPIST-1e produced a clear non-detection: the characteristic “bump” that CO₂ would create in the spectrum is absent. A Venus-like, CO₂-rich atmosphere therefore seems very unlikely. A thin, Mars-like atmosphere also fits poorly with the data. Ana Glidden of the Massachusetts Institute of Technology, lead author of the atmospheric-interpretation paper, says: “The observations do not rule out nearly as much as one might think. For example, it is still possible that there is a global ocean on the surface. But we can confidently say that a thick or thin CO₂ atmosphere is not likely.” Other scenarios remain viable: a bare rocky planet, a nitrogen-rich atmosphere like Earth’s, or a hazy smog atmosphere like that of Saturn’s moon Titan. “A habitable planet is still among the possibilities,” Glidden adds.


How JWST telescope can measure whether life might be possible on a distant exoplanet
1. When exoplanet TRAPPIST-1e passes the TRAPPIST-1 star, its atmosphere absorbs specific colours of the starlight. 
2. Which light is absorbed depends on the molecules in that atmosphere: water molecules absorb different light than carbon dioxide molecules.
3. To filter out the star’s variable light, a new technique is now being used: the transit of planet 1e is measured immediately after a transit of planet 1b.
4. By comparing the two planets, the noise from the star is filtered out. This makes it easier to see which molecules the exoplanet atmosphere contains, and thus whether life could in principle be possible.

The spectrum also shows a small bump near the wavelength where methane would reveal itself. That gas quickly attracts attention because on Earth it is produced partly by life — though there are also entirely natural, non-biological sources. Precisely for that reason, any hint of methane outside our Solar System can trigger a media storm — and demands utmost caution. The team deliberately chose not to highlight the bump in press materials. Glidden: “We absolutely wanted to avoid people thinking we had found methane. It’s more likely to be instrumental noise or a remnant of the stellar spectrum.” She points out that methane on this type of planet is rapidly broken down by UV radiation and cannot persist long enough in such quantities to be detectable. “It is very unlikely that this methane is from the planet,” she concludes.
That caution does not come out of nowhere. In 2023, a weak spectral bump in the planet K2-18b led to worldwide headlines about possible biosignatures — after which fellow astronomers quickly corrected the claim. Among them was Leiden astrophysicist Ignas Snellen, who strongly sounded the alarm at the time. Now Snellen praises the careful approach of the TRAPPIST-1 team. During the interview he only notices the subtle methane bump in the figure after staring at it for a while: “Oh wait, there actually is a small peak.”

According to Snellen, the team’s restraint is entirely justified. The measurements lie at the limits of detectability; atmospheric features of one-thousandth of a percent hide among instrumental noise and the whims of an active dwarf star. “Their approach shows that they really understand where the pitfalls lie,” he says. Exactly the intention, Allen adds: “Even if you see something, it’s on us to investigate first whether it’s trustworthy, before we say anything about it.”

Neutralizing the star’s variability
The next fifteen Webb observations should make the spectrum of TRAPPIST-1e much clearer. To neutralize the star’s variability — the biggest obstacle — the team is using an approach never tried before: they use another planet as a “mask.” The innermost planet, TRAPPIST-1b, which almost certainly has no atmosphere, serves as a reference. By dividing its spectrum by that of TRAPPIST-1e, the researchers hope to filter out the “fingerprint” of the changing star. Allen: “If you measure the two transits right after each other, before the star has time to change, you can in principle remove the starlight from the planet’s signal.”

It is a bold strategy: an investment of 130 hours of Webb time, an astronomically costly resource. But the potential return is large. This project will help determine how far transmission spectroscopy can be pushed and how research on red dwarf systems should proceed.

Analyzing the data is extremely delicate. A single spectrum of a rocky planet can take researchers months of work; here the dataset consists of nineteen transits with two different telescope instruments. Glidden: “We are pushing JWST to its limit to capture every signal coming from the planet.” The smallest fluctuation in the star — a spot, a flare, a hint of instrument noise — can look like an atmospheric signature. Allen adds enthusiastically: “It will be the largest exoplanet dataset we’ve ever worked on. It will take at least a year to get everything out.”
According to Snellen, current technology is being put to the test: “With planets this small, you really run into the limits of what the James Webb can do.” New breakthroughs will come from the next generation of telescopes, such as the European Extremely Large Telescope in Chile, with its giant 39-meter mirror. This will make direct imaging possible: the starlight is suppressed so strongly that the faint light of a planet becomes visible. “With the ELT we can suppress the starlight at the planet’s position by a factor of a thousand or more,” Snellen says. “That is a huge step forward.”
The coming months will be decisive. Once the final fifteen transits are taken, a lengthy analysis begins. But by the end of next year, Allen, Espinoza and Glidden expect, we will probably know whether TRAPPIST-1e has an atmosphere — or whether it remains hidden for now.

Whatever the outcome, it will reshape the field. TRAPPIST-1e, the small world that always faces its star with the same side, has become one of the key testing grounds for a question that has occupied astronomers for decades: how often does a world like ours really occur? Glidden believes it would be naïve to assume that life arose only on Earth. “The Copernican revolution taught us that Earth is not the center of the Universe — and why would it be the only place where life can emerge? There are so many other worlds. Only now are we truly beginning to explore their habitability.”





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