Atmospheres — do Earth-like alien planets have them or not? This is the killer question currently facing astronomers. In this context, “Earth-like” means terrestrial (i.e. rocky worlds) planets close enough to Earth’s mass and other conditions that we might 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 while finding signatures of biospheres (i.e. biosignatures) is going to be hard even for the James Webb Space Telescope (JWST), finding evidence for terrestrial exoplanet atmospheres is definitely something it can do. That’s why everyone got so excited last year when JWST started dropping new data on astronomers’ favorite Earth-like exoplanets.
We were excited, but, to be truthful, also a little bummed out. Today’s story covers 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 (aka an “M-dwarf star,” or a “red dwarf star”). It has a full family of seven orbiting planets. Best of all, the orbital distances for these worlds span the entire range of possibilities, from too-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 to exist on the surface. Using the JWST, scientists have been marching outward looking for 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 big bummer because it was so close to the star that most astronomers thought its blazing hot surface could never retain a blanket of molecules. Hopes were higher for TRAPPIST-1 c, however. Some folks were hoping it might be a kind of Venus twin. TRAPPIST-1 c has a mass that’s slightly higher than Venus and gets about the same dose of stellar energy. Like Venus, TRAPPIST-1 c is not in its star’s Goldilocks zone of habitable orbits. But considering that Venus’ atmosphere is heavy in carbon dioxide, astronomers were hoping that maybe TRAPPIST-1 c had its own thick layer of gas.
Sadly, the JWST results ruled out a Venus-like CO2 atmosphere. Worse, they seemed to point to either bare rock (i.e. no atmosphere at all) or a wimpy thin layer of gas. Given how important finding any kind of atmosphere on terrestrial exoplanets is to astrobiologists, the results for TRAPPIST-1 c were a bit sad.
But science marches forward by constantly refining the understanding of its own findings. After the initial JWST results for TRAPPIST-1 c were published, Andrew Lincowski, Vikki Meadows, and collaborators went back and reexplored exactly what kind of atmospheres could be consistent with what the JWST saw. Their results showed there could be more to TRAPPIST-1 c than we first expected.
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