Dr Ryan MacDonald
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distantworlds.space
Dr Ryan MacDonald
@distantworlds.space
2K followers 500 following 540 posts
Lecturer in Extrasolar Planets 🪐 🔭 at the University of St Andrews 🏴󠁧󠁢󠁳󠁣󠁴󠁿
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distantworlds.space
Trotta (2008)
arxiv.org/abs/0803.4089
October 1, 2025 at 6:53 PM
distantworlds.space
Many congratulations, Dr Boldt-Christmas! 🎉

Love the front cover transiting planet atmosphere graphic on your thesis!
September 26, 2025 at 11:15 PM
distantworlds.space
Afraid not. Microlensing relies on a chance alignment between two distant stars, so you see the planet once and then it's gone forever.
September 17, 2025 at 8:13 PM
distantworlds.space
The telescope that discovered this planetary system was named after the beer 😅
September 13, 2025 at 5:41 AM
distantworlds.space
Finally, it's important to highlight that none of this would have been possible without the leadership of Nikole Lewis, who is the PI of this initial TRAPPIST-1e reconnaissance program.

I was fortunate enough to be a postdoc at Cornell with Nikole, and she is a truly *fantastic* advisor and mentor!
September 9, 2025 at 11:14 PM
distantworlds.space
We have follow-up observations of TRAPPIST-1e ongoing (led by Néstor Espinoza and Natalie Allen), which will provide 15 (!) more transits of TRAPPIST-1e.

So if TRAPPIST-1e does indeed have an atmosphere, we will soon have the data to settle the enigma of this world.
Artist's impression of TRAPPIST-1e, showing a rocky world covered in scattered lakes and clouds. Credit: NASA/JPL-Caltech.
September 9, 2025 at 11:14 PM
distantworlds.space
Our constraints on potential atmospheres with molecules heavier than H2 and He (secondary atmospheres) are presented in our second TRAPPIST-1e paper, led by @ana-glidden.bsky.social at MIT. Be sure to check out the paper!

iopscience.iop.org/article/10.3...

So what comes next?
Screenshot of the title page of 'JWST-TST DREAMS: Secondary Atmosphere Constraints for the Habitable Zone Planet TRAPPIST-1 e'.
September 9, 2025 at 11:14 PM
distantworlds.space
Technical point: retrievals of flat transmission spectra for rocky planets usually result in corner plots resembling the prior.

For TRAPPIST-1e, we don't see this behaviour, with the CH4 posterior pushing to include this molecule.

We haven't detected CH4, but future observations can assess this.
Posterior probability plots for the CH4 and CO2 abundances in TRAPPIST-1e's atmosphere. The CH4 abundance shows a spike near high atmospheric abundances (>~ 0.1-100 %), compatible with the CH4 abundance of Venus, Earth, or Titan. The CO2 abundance plot offers few constraints on the abundance of this molecule, though the authors note that the region allowing for 100% CO2 corresponds to an unphysically low temperature (~ 100 K) where CO2 would condense, and hence high-CO2 atmospheres like Venus or Mars are disfavoured. Figure from Glidden et al. (2025).
September 9, 2025 at 11:14 PM
distantworlds.space
Statistically, our current four-transit spectrum of TRAPPIST-1e can also be fit by a flat line (i.e. a featureless spectrum). So we can't rule out a bare rock with these data.

There's also the important caveat that an incomplete stellar contamination correction could also imprint spectral features.
September 9, 2025 at 11:14 PM
distantworlds.space
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.
Spectral fits to TRAPPIST-1 e’s stellar-contamination-corrected transmission spectrum. Top: best-fitting forward models for three different partial pressures of N2 and CH4 (solid, dotted, and dashed colored lines) compared to a flat line (dotted black line). Bottom: GP+atmosphere retrievals for the CLR (blue) and log-uniform priors with a “ghost” background gas (gray) compared to a flat line (dotted black line). All models are plotted binned to the same spectral resolution as the data. The wavelengths of potential CH4 absorption bands are annotated. The corresponding corner plot is in Appendix E. The best-fitting forward models and both retrieval approaches independently identify spectral features tentatively attributed to CH4 features in a potentially N2-rich atmosphere. Figure from Glidden et al. (2025).
September 9, 2025 at 11:14 PM
distantworlds.space
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.
Rejection significance for atmospheric forward models compared to TRAPPIST-1 e’s JWST transmission spectra. Each subplot represents a background gas (N2 or H2) together with an absorber (CO2 or CH4) over a range of surface partial pressures shown on the x- and y-axes. Gases are shown above each subplot. Grid boxes are labeled and colored with the significance of the difference between the data and the forward model. Black colored boxes represent “infinite” σ, meaning that the models are firmly inconsistent with the data and can be ruled out. The four boxes on the left side of the figure are for the combined visits 1 and 2, which were naturally less impacted by stellar contamination, while the four boxes on the right side are for the GP stellar-contamination-corrected spectrum from N. Espinoza et al. (2025), which includes all four visits. In both cases, our data are consistent across a range of N2 atmospheres, but we are able to place additional constraints on H2-rich atmospheres. In particular, we can rule out H2-rich atmospheres with a strong absorber until increasing the amount of the heavier absorber flattens out the spectrum so that any possible features are buried in the uncertainty. When all four transits are combined and stellar contamination is (partially) mitigated, we are able to place moderately tighter constraints on atmospheres with CH4 than we could with just visits 1 and 2 combined. Figure from Glidden et al. (2025).
September 9, 2025 at 11:14 PM
distantworlds.space
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.
Screenshot of the paper 'JWST-TST DREAMS: NIRSpec/PRISM Transmission Spectroscopy of the Habitable Zone Planet TRAPPIST-1 e'
September 9, 2025 at 11:14 PM
distantworlds.space
Our first result was a firm rejection of any significant amount of hydrogen in TRAPPIST-1e's atmosphere.

Irrespective of the cloud-surface pressure, we find a H2 abundance limit of < 80% (to 3σ). This is a significant improvement over what was possible with Hubble data.
H2 abundance constraints for TRAPPIST-1 e from HST and JWST as a function of surface pressure. The posterior distribution showcases the improvement on constraints on possible H2-dominated atmospheres on TRAPPIST-1 e between HST (left in gray; obtained by applying our GP retrieval methodology to the HST/WFC3 data in Z. Zhang et al. 2018) and JWST (right in blue; obtained by applying it to the four NIRSpec/PRISM transits presented in this work). The distribution for HST mainly follows the centered log-ratio prior allowing the H2-dominated solution at virtually all pressures ≳1 bar; the JWST one disfavors the H2-dominated solution. Figure from Espinoza et al. (2025).
September 9, 2025 at 11:14 PM
distantworlds.space
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.
This graphic compares data collected by Webb’s NIRSpec (Near-Infrared Spectrograph) with computer models of exoplanet TRAPPIST-1 e with (blue) and without (orange) an atmosphere. Narrow colored bands show the most likely locations of data points for each model. Illustration: NASA, ESA, CSA, STScI, Joseph Olmsted (STScI)
September 9, 2025 at 11:14 PM
distantworlds.space
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.
The transmission spectra of TRAPPIST-1 e interpreted with GPs and atmospheric/atmosphereless models. (Top) Transmission spectra on our four visits (black points with error bars) modeled with a GP times either an atmospheric model (blue) or a flat-line spectrum (i.e., with no atmosphere or with a high-altitude cloud deck; orange); a GP (offset; dashed lines) acts multiplicatively to distort those signals. Bands represent the 1σ and 3σ credibility bands. (Bottom) Visit-combined transmission spectrum by (weighted) averaging the four visits after correcting for the modeled GP component (using the flat-line model-derived GP; black points with error bars). The atmospheric model and the flat-line model are indistinguishable according to the Bayesian evidence—more data are needed to distinguish between those. Bands represent the 1σ and 3σ credibility bands. Note how, within the error bars, an Earth-like model (gray; with the locations of the main active spectroscopic features) is still consistent with our data. Also note that the blue and orange models are shared but fitted to each individual visit. Figure from Espinoza et al. (2025).
September 9, 2025 at 11:14 PM
distantworlds.space
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...
Plots showing that standard stellar contamination models cannot fit the data from the third and fourth JWST transmission spectra of TRAPPIST-1e. Figure from Espinoza et al. (2025).
September 9, 2025 at 11:14 PM
distantworlds.space
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 ✴️
JWST transmission spectra from four observations of the habitable zone rocky exoplanet TRAPPIST-1e. Each visit shows significant wavelength-dependent bumps and wiggles caused by stellar contamination from active regions on the system's red dwarf star. The different structures in each visit demonstrate that the stellar contamination is time-dependent. Figure from Espinoza et al. (2025).
September 9, 2025 at 11:14 PM
distantworlds.space
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.
This artist’s concept shows the volatile red dwarf star TRAPPIST-1 and its four most closely orbiting planets, all of which have been observed by NASA’s James Webb Space Telescope. Webb has found no definitive signs of an atmosphere around any of these worlds yet. Artwork: NASA, ESA, CSA, STScI, Joseph Olmsted (STScI)
September 9, 2025 at 11:14 PM
distantworlds.space
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.
Artist concept of the TRAPPIST-1 planetary system (credit: NASA/JPL-Caltech) annotated with crosses 'X' over planets b, c, and d. Underneath them is the writing "No atmospheres found (yet)".
September 9, 2025 at 11:14 PM
distantworlds.space
CH4 is the best explanation we found for the potential bumps on the spectrum. N2 doesn't have any notable absorption of its own, but we need a background gas heavier than CH4 (~ 16 proton masses) to match the data, and N2 (~ 28 proton masses) does a good job.
September 9, 2025 at 6:56 AM
distantworlds.space
This was TRAPPIST-1d, I'll make a thread on our TRAPPIST-1e results later today 🪐
September 9, 2025 at 6:48 AM
distantworlds.space
I do think it could be worth doing a deep dive to get an eclipse of TRAPPIST-1e, but that's a big ask for JWST time.
September 9, 2025 at 6:47 AM