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We observed the habitable zone planet TRAPPIST-1e with JWST to search for an atmosphere. You've seen the headlines, now let's dive into the science! 🧪 THREAD (1/N) #Exoplanets 🔭

Previously, on TRAPPIST-1: ➡️ No thick atmospheres on TRAPPIST-1b,c (Greene+2023, Lim+2023, Zieba+2023, Radica+2025, Gillon+2025). ➡️ Earth-like atmospheres ruled out for TRAPPIST-1d (Piaulet-Ghorayeb+2025) - see thread below. Now we turn to a planet more firmly in the habitable zone: TRAPPIST-1e.

TRAPPIST-1e is 92% Earth's size, 69% Earth's mass, and is illuminated by 66% of the integrated light that Earth receives. This means TRAPPIST-1e can potentially have liquid surface water *if* it has an atmosphere with a sufficient greenhouse effect. So TRAPPIST-1e was a priority target for JWST.

We observed TRAPPIST-1e four times with JWST in 2023 to measure how the apparent size of the planet changes with colour (i.e. transmission spectra) - more on why this took 2 years in a moment! Our spectra show *huge* wavelength-dependent features that are caused by active regions on the star ✴️

When we modelled the stellar contamination (similar to previous studies on TRAPPIST-1b,c, d), the models couldn't simultaneously explain the entire wavelength range. Simply put, our stellar models for ultra-cool M-dwarf stars like TRAPPIST-1 don't work 😱 So we had to try something new...

We turned to Gaussian Processes (GPs) to fit the stellar contamination affecting the TRAPPIST-1e spectra. Since: Observed_spectrum_i = contamination_i * planet_spectrum The idea is to extract the time-independent (non-GP) common factor caused by any planetary atmosphere.

Using GPs to account for the stellar contamination, we combined the time-independent spectral information from the four transits to produce the spectrum of TRAPPIST-1e shown in the press release. We then turned to atmospheric models to see if there were any signatures of atmospheric absorption.

The observations, stellar contamination GP magic 🪄, and H2-upper limit we've discussed so far are covered in our first TRAPPIST-1e paper, led by Néstor Espinoza at STScI (not on Bluesky). Be sure to check out the paper! iopscience.iop.org/article/10.3... Next, we looked for secondary atmospheres.

In Paper #2, we ran a grid of atmospheric models considering combinations of strong infrared absorbers (CO2 / CH4) and transparent background gases (N2 / H2). The figure below (from Glidden+2025) shows the range of excluded partial pressures. Big takeaway: large CO2 concentrations are unlikely.

Intriguingly, forward models with N2 + CH4 provided a great fit to TRAPPIST-1e's transmission spectrum 😯 We found the same solution independently through atmospheric retrievals, which latched onto CH4 absorption as a potential explanation. 🔍 But this is not (yet!) an atmospheric detection.