The nearby exoplanet TRAPPIST-1 may still have a rich atmosphere

Atmospheres: do Earth-like alien planets have them or not? This is the killer question astronomers are currently facing. In this context, “Earth-like” means terrestrial (that is, rocky worlds) planets that are so close to Earth’s mass and other conditions that we can imagine them hosting the holy grail of astrobiology: an alien biosphere. But you can’t have a biosphere unless you first have an atmosphere for all that life to breathe. And as we find signatures of biospheres (e.g biosignatures) will be difficult even for the James Webb Space Telescope (JWST), finding evidence for the atmospheres of exoplanets on Earth is certainly something it can do. That’s why everyone got so excited last year when JWST started publishing new data on astronomers’ favorite Earth-like exoplanets.

We were excited, but honestly a little disappointed. Today’s story explains why that disappointment may have been a bit premature.


The biggest news of the past year came with observations of the TRAPPIST-1 system. TRAPPIST-1 is a small, cool star (also called an ‘M dwarf star’ or a ‘red dwarf star’). It has an entire family of seven planets in orbit. Best of all, the orbital distances for these worlds cover the whole range of possibilities, from overly hot inner worlds to frozen outer worlds and, most importantly, a few planets in the ‘Goldilocks Zone’, where temperatures are just right for liquid water. exist on the surface. Using the JWST, scientists have marched out in search of atmospheres on successively distant (from the star) TRAPPIST-1 worlds.

No atmosphere was found for the innermost planet, TRAPPIST-1 b. This wasn’t a major drawback, because the planet was so close to the star that most astronomers thought its red-hot surface could never hold a blanket of molecules. However, expectations were higher for TRAPPIST-1 c. Some people hoped it could be some kind of Venus twin. TRAPPIST-1 c has a mass slightly higher than that of Venus and receives about the same dose of stellar energy. Like Venus, TRAPPIST-1 c is not in its star’s Goldilocks zone of habitable orbits. But given that Venus’ atmosphere contains a lot of carbon dioxide, astronomers hoped that TRAPPIST-1 c might have its own thick layer of gas.

Unfortunately, the JWST results ruled out a Venus-like CO2 atmosphere. Worse, they seemed to indicate bare rock (i.e. no atmosphere at all) or a limp thin layer of gas. Considering how important finding any atmosphere on terrestrial exoplanets is for astrobiologists, the results for TRAPPIST-1 c were a bit sad.

But science makes progress by continually refining the understanding of its own findings. After the first JWST results for TRAPPIST-1 c were published, Andrew Lincowski, Vikki Meadows and collaborators went back and re-examined exactly what kind of atmosphere could be consistent with what the JWST saw. Their results showed that TRAPPIST-1 c can deliver more than we initially expected.

Reviving atmospheric hope

The team used computer models of planets with different types of atmospheres and then simulated what JWST would observe from these worlds. These tools went a step beyond what had previously been used to interpret the TRAPPIST-1c results. Three different types of worlds were examined: worlds with a Venus-like thick CO2 atmosphere, an oxygen-dominated atmosphere or a water-rich ‘steam’ atmosphere. Confirming the previous results, Lincowski and company found that TRAPPIST-1 c likely does not have a thick CO2 blanket. But the team also found that fairly thick oxygen atmospheres with pressures of 1 to 10 times Earth’s surface pressure fit well within the margins of error of the JWST measurements. Moreover, steam atmospheres were also possible, in which a runaway greenhouse effect drives liquid water into the atmosphere.

TRAPPIST-1 c with a thick oxygen atmosphere or a relatively thick steam atmosphere would be scientifically great. While we might not expect life to happen in both worlds (but who knows), the point is diversity. The most important task for astrobiology and exoplanet science right now is to simply answer the question: how varied are the worlds in the Milky Way? Are they mainly dead and airless worlds? Are they mainly worlds with atmospheres but with different compositions? While the initial cut to TRAPPIST-1 c may have leaned toward the first possibility, Lincowski’s results tell us that nature may be more fertile and imaginative than we previously thought.

Which view is correct? Ultimately, there’s only one way to find out. Search harder. And that is exactly what we do. Stay tuned!

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