:INFO
The best rocky world we can actually check
Of every planet outside the solar system, this is the one best suited to the question people actually care about: does it have air? It is rocky, it sits in its star's habitable zone, and it crosses a very small, very dim star, which is what makes an atmosphere detectable at all. A planet passing a large bright star hides its thin shell of air in the glare. TRAPPIST-1 is barely bigger than Jupiter and cooler than a candle flame, so the contrast works in our favour. That combination is rare enough that JWST has spent significant time here.

:STATS
[icon:TARGET] Radius | 0.920 Earth radii
[icon:CHART] Mass and density | 0.692 Earth masses at 4.90 g/cm3
[icon:CLOCK] One year | 6.101 Earth days
[icon:STAR] Host star | TRAPPIST-1 at 2566 K and 12% of the Sun's radius
[icon:FIRE] Orbit radius | 0.029 AU or about one thirteenth of Mercury's
[icon:CHECK] Habitable zone | inside it, fourth of the seven planets

:CHECKLIST
What four transits of JWST settled
[ ] Cloudy hydrogen-dominated atmosphere above 80 percent by volume. Ruled out at better than three sigma.
[ ] A Venus-like atmosphere. Disfavoured by the data.
[ ] A Mars-like atmosphere. Disfavoured by the data.
[ ] A nitrogen-rich atmosphere with traces of methane and carbon dioxide. Still permitted.
[ ] No atmosphere at all. Still permitted.

:TIMELINE
🔭🪐📡
2016-05-01 | Three transiting Earth-sized planets found around an obscure red dwarf, using a 60 cm telescope in Chile.
2017-02-01 | Four more announced. Seven rocky worlds, three of them in the habitable zone, all orbiting closer than Mercury.
2023-01-01 | JWST watches four transits of planet e with NIRSpec PRISM, hunting for starlight filtered through an atmosphere.
2025-09-01 | Results published. A thick hydrogen atmosphere is ruled out. A nitrogen one is still allowed. No detection either way.

:NOTE.half
The hardest problem is not the planet, it is the star. Red dwarfs are covered in spots and faculae, and starlight crossing those regions imprints a signal that looks very like an atmosphere. The published spectra show that contamination across 3 to 5 microns, which is exactly where the interesting molecules live.

:NOTE.half
The archive lists no measured equilibrium temperature for this planet. Derived from the star's output and distance, assuming a third of the light is reflected, it lands near 228 K, about minus 45 C. That is an estimate from two numbers, not a measurement, and the real surface depends on the air question above.

:THREAD

:LINK
https://iopscience.iop.org/article/10.3847/2041-8213/adf42e
JWST-TST DREAMS: NIRSpec PRISM transmission spectroscopy of TRAPPIST-1 e