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Dr Ryan MacDonald @distantworlds.space · 01/07/2026

Our paper, 'Aerosols and hydrocarbons in the atmosphere of a white dwarf planet', is published today in Nature and is freely available: www.nature.com/articles/s41...

Screenshot of the paper 'Aerosols and hydrocarbons in the
atmosphere of a white dwarf planet', available in the journal Nature (https://www.nature.com/articles/s41586-026-10514-7).

Authors: Ryan J. MacDonald, Christopher E. O’Connor, Victoria A. Boehm, E. M. May, David K. Sing, Elijah Mullens, L. C. Mayorga, Trevor O. Foote, Simon Blouin, Logan A. Pearce, Nikole K. Lewis, Jeff Valenti, Natasha E. Batalha, Maura Lally, Joshua D. Lothringer, Mark S. Marley, Ishan Mishra & Susan E. Mullally

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Dr Ryan MacDonald @distantworlds.space · 01/07/2026

Perhaps our biggest surprise though is that the planet appears to be significantly warmer than we expected (effective temprature around 400 K). Using planet cooling models, we extrapolated back in time to find that planet must have been reheated billions of years after the star died.

Top: backwards extrapolation of WD 1856b's temperature using planetary cooling models. Bottom: the inferred time of reheating occured billions of years after the star became a white dwarf, pointing towards planetary migration long after the death of the star.

Plot from 'Aerosols and hydrocarbons in the atmosphere of a white dwarf planet', MacDonald et al. 2026, Nature 655, 76 (https://www.nature.com/articles/s41586-026-10514-7).

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Dr Ryan MacDonald @distantworlds.space · 01/07/2026

Our spectrum analysis finds that hydrocarbons (most likely methane) explains the absorption features (with an odds ratio of at least 167:1). The planet's atmosphere is also shrouded in aerosols (odds ratio at least 200,000:1). We also find the mass of the planet is about 7 Jupiter masses.

$Left: spectral contribution plot showing how different components of the atmospheric model for WD 1856b explain its JWST transit spectrum. The downwards slope above 3.5 microns is explained by thermal emission from the side of the planet facing the observer, while the absorption features at shorter wavelengths are explained by methane absorption and a scattering haze. Right: measured constraints on atmospheric properties for WD 1856b's atmosphere. Top row: the fraction of the atmosphere made of CH4, C2H6, and PH3. Bottom row: the planet's mass, cloud top pressure, and haze power-law slope. Plot from 'Aerosols and hydrocarbons in the atmosphere of a white dwarf planet', MacDonald et al. 2026, Nature 655, 76 (https://www.nature.com/articles/s41586-026-10514-7).$

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Dr Ryan MacDonald @distantworlds.space · 01/07/2026

The transit spectrum we measure shows multiple 'bumps' that are best explained by methane (CH4) in the planet's atmosphere, alongside a slope at short wavelengths caused by a haze in the upper atmosphere. This is the first time we've seen the atmosphere of a close-in planet orbiting a white dwarf!

NASA’s James Webb Space Telescope measured the constituents of exoplanet WD 1856 b as it passed in front of its star, finding signs of methane. WD 1856 b orbits a white dwarf star the size of Earth. As a result, the planet blocks more than half of the star’s light. The red bands indicate where bumps in the spectrum show that this planet’s atmosphere contains methane.

Credit:
NASA, ESA, CSA, Joseph Olmsted (STScI)

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Dr Ryan MacDonald @distantworlds.space · 01/07/2026

This planet is 7 times larger than its white dwarf star, so it blocks a record-breaking over 50% of the light from its white dwarf. One of our big surprises is that the planet appears to block *less* light at longer wavelength infrared light, which we explained by planetary thermal emission.

Transit light curve of the white dwarf planet WD 1856b as measured by the James Webb Space Telescope. The plant blocks 56% of the star's light at short wavelengths of light (blue curve), but only 52% of the light at longer infrared wavelengths (red curve).

Plot from 'Aerosols and hydrocarbons in the atmosphere of a white dwarf planet', MacDonald et al. 2026, Nature 655, 76 (https://www.nature.com/articles/s41586-026-10514-7).

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Dr Ryan MacDonald @distantworlds.space · 01/07/2026

We used the NASA/ESA/CSA James Webb Space Telescope to measure the spectrum of WD 1856b during a transit event. Our goal was to measure the atmosphere of this ancient planet (that formed around 10 billion years ago) to learn what planets like Jupiter may be like after the death of the Sun.

Artist concept of the James Webb Space Telescope floating in space, with a field of stars behind it.

Credit: NASA

Illustration of transmission spectroscopy. Light from a star passes through the atmosphere of a planet, with a PRISM splitting the light into its constitutent colours. Colours with less light indicate absorption from the planet's atmosphere.

Credit: Christine Daniloff/MIT, Julien de Wit

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Dr Ryan MacDonald @distantworlds.space · 01/07/2026

WD 1856b orbits a very old white dwarf, the leftover core of a star like our own Sun that died about 5.4 billion years ago. It was discovered in 2020 (Vanderburg+2020). The planet is a similar size to Jupiter, but orbits at 0.02 AU. A big mystery is how the planet survived the death of its star.

The white dwarf planet WD 1856b orbits 50x closer to its white dwarf than the Earth orbits the Sun.

Credit: NASA

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Dr Ryan MacDonald @distantworlds.space · 01/07/2026

🚨 New research alert 🚨 We have found an atmosphere on a planet orbiting a dead star 🔭✴️💀🪐 Our James Webb Space Telescope (JWST) observations of the white dwarf planet WD 1856b give us a preview into the fate of planets like Jupiter after the death of the Sun. 🧵⬇️ (1/10)

Artist concept of the white dwarf planet WD 1856b

Caption:
Exoplanet WD 1856 b, shown in this artist’s concept, is a gas giant that survived the death of its star. It now orbits a white dwarf at a distance 50 times closer than Earth orbits the Sun. Observations by NASA’s James Webb Space Telescope not only determined the planet’s temperature but also detected molecules in its atmosphere. The former measurement provides evidence that WD 1856 b migrated to its current location billions of years after its star became a white dwarf.

Credits:
NASA, ESA, CSA, Ralf Crawford (STScI)

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Dr Ryan MacDonald @distantworlds.space · 09/06/2026

Kicking off #BPSC26 with a workshop on exoplanet atmosphere spectroscopy for planetary scientists. Be sure to follow @bpsc2026.bsky.social for updates over this 3 day conference! 🪐 🔭

Slide show 'Measuring Exoplanet Atmospheres with the James Webb Space Telescope' with Dr Ryan MacDonald (St Andrews) standing next to the screen.

Part of a Jupiter notebook for the BPSC 2026 Exoplanet Workshop showing a transiting exoplanet.

Part of a Jupiter notebook for the BPSC 2026 Exoplanet Workshop, showing a James Webb Space Telescope spectrum of the giant planet WASP-39b

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Dr Ryan MacDonald @distantworlds.space · 04/03/2026

Quite the crowd today in St Andrews for Max Klymenko filming the Career Ladder. I had a blast discussing our search for life of #exoplanets 🪐 🔭 Max asked many @uniofstandrews.bsky.social staff and students about their jobs - keep an eye out for the episode!

Max Klymenko and Ryan MacDonald standing on a ladder surrounded by a crowd of people.

In the background are the old stone buildings of St Salvator's quad in St Andrews, Scotland.

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Dr Ryan MacDonald @distantworlds.space · 01/10/2025

Trotta (2008) arxiv.org/abs/0803.4089

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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.

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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'.

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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).

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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).

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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).

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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'

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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).

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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)

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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).

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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).

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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).

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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)

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Dr Ryan MacDonald @distantworlds.space · 10/09/2025

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)".

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Dr Ryan MacDonald @distantworlds.space · 09/09/2025

A lingering worry is whether the residual deviations from a flat line (that our retrievals and forward models explain by CH4 absorption) are due to incomplete correction of the stellar contamination. The picture will be much clearer once we have the full 19-transit dataset (soon!)

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Dr Ryan MacDonald @distantworlds.space · 09/09/2025

One of the reasons this analysis was so challenging is that stellar contamination models couldn't fit the transmission spectra across the entire wl range (see below for visit #4). So we *know* our stellar models aren't sufficient for TRAPPIST-1, and had to turn to GPs for the stellar contamination

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Dr Ryan MacDonald @distantworlds.space · 13/08/2025

The outer planets, TRAPPIST-1e,f,g,h, may still retain atmospheres (even if TRAPPIST-1d is a bare rock). JWST has already observed these planets, so stay tuned for the results!

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Dr Ryan MacDonald @distantworlds.space · 13/08/2025

The JWST observations should have seen gases like CO2, H2O, or CH4 for relatively clear thick atmospheres. So the non-detections rule out many classes of rocky planet atmospheres, including: 🌍 Modern and Archean (early) Earth. ♀️ Venus (unless very cloudy). 🔱 Titan.

Plot from 'Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d' showing the range of atmospheric partial pressures consistent with TRAPPIST-1d's JWST transmission spectra observations.

The exclusion regions include the compositions of the early Earth, the modern Earth, the Archean (early) Earth, early Mars, Titan, and a cloud-free Venus.

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Dr Ryan MacDonald @distantworlds.space · 13/08/2025

The results - led by Caroline Piaulet-Ghorayeb (Université de Montréal / University of Chicago) - are described in: 'Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d' (Piaulet-Ghorayeb et al. 2025): iopscience.iop.org/article/10.3...

Screenshot of the title page of the paper 'Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d'.

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Dr Ryan MacDonald @distantworlds.space · 30/06/2025

Seems I'm a trusted news source for astronomy! 🔭 I'd encourage other professional astronomers to fill in the short official Bluesky form to apply for verification: bsky.social/about/blog/0...

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Cubillos: after Hubble, we have no new UV missions until HWO in the 2040s... A white paper will appear next week for a high-resolution UV to near-IR instrument proposal for HWO. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Cubillos: we can see metal lines from escaping upper atmosphere using near-UV observations (especially from Hubble). #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Cubillos: but JWST primarily operates at infrared wavelengths that probe the lower atmosphere. To explore the upper atmosphere, we need shorter wavelength observations (especially in the UV). #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Cubillos: JWST has observed (or will observe) 181 exoplanets in its first 4 years of operations. #Exoplanets #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Cubillos: exoplanet atmospheres provide a powerful tool to understand planetary physics. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Last talk of the conference! Patricio Ernesto Cubillos (INAF Osservatorio Astrofisico di Torino): Upper-Atmosphere Exoplanet Constraints from Ultraviolet Transmission Observations. #Exoplanets #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Pezzella: focusing on CS, a molecule expected to be important for sulphur chemistry in hot giant exoplanet atmospheres. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Pezzella: molecular collisions matter in non-local thermal equilibrium conditions. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Marco Pezzella (Università G. Annunzio Chieti-Pescar): NRMolCol: Inelastic Scattering Data for (exo)Planetary Atmospheres. #Exoplanets #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Rianco-Silva: 100 new CH4 lines discovered in the visible wavelength spectrum of Titan. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Rianco-Silva: Titan's visible spectrum contains CH4 lines that are not well characterised, due to the low wavelength cutoff for CH4 line lists. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Rianco-Silva: Titan's atmosphere provides a window into probiotic chemistry. Near-infrared observations of Titan have mainly been used; can we use visible wavelength high-resolution spectra to search for molecules like C3? #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Rafael Rianco-Silva (Instituto de Astrofisica e Ciencias do Espaco / UCL): A Study of Very High-Resolution Visible Spectra of Titan: Line Characterisation in Visible CH4 and the Search for C3. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Smola: in trying to fit the potential energy of methanol, we can exploit symmetry to make the problem tractable. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Smola: methanol is a quite complex molecule, with many degrees of freedom. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Smola: methanol is known as a 'molecular weed', due to its forest of lines in the interstellar medium. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Oleksiy Smola (UCL): Towards a Spectroscopic Potential Energy Surface of Methanol. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Hróðmarsson concludes with a summary of challenges for VUV laboratory cross section measurements and a future outlook. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Hróðmarsson: for CH4, the branching ratios into different byproducts of photolysis are crucial to get the UV cross section right. This matters for Titan, and was a big success in understanding hydrocarbon chain formation. #AllTheWavelengths

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Dr Ryan MacDonald @distantworlds.space · 29/06/2025

Hróðmarsson: VUV lab measurements of cross sections can make a huge difference. Highlights SH and NH2, which were off by 2 orders of magnitude in the Leiden database vs experiment. #AllTheWavelengths

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