Dr Ryan MacDonald

@distantworlds.space

1993 Followers 527 Following

Lecturer in Extrasolar Planets šŸŖ šŸ”­ at the University of St Andrews šŸ“󠁧󠁢󠁳󠁣󠁓ó æ

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. Oā€™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
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).
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).
NASAā€™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 starā€™s light. The red bands indicate where bumps in the spectrum show that this planetā€™s atmosphere contains methane. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)
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).
Artist concept of the James Webb Space Telescope floating in space, with a field of stars behind it. Credit: NASA
The white dwarf planet WD 1856b orbits 50x closer to its white dwarf than the Earth orbits the Sun. Credit: NASA
Artist concept of the white dwarf planet WD 1856b Caption: Exoplanet WD 1856 b, shown in this artistā€™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 NASAā€™s James Webb Space Telescope not only determined the planetā€™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)
Slide show 'Measuring Exoplanet Atmospheres with the James Webb Space Telescope' with Dr Ryan MacDonald (St Andrews) standing next to the screen.
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.
Artist's impression of TRAPPIST-1e, showing a rocky world covered in scattered lakes and clouds. Credit: NASA/JPL-Caltech.
Screenshot of the title page of 'JWST-TST DREAMS: Secondary Atmosphere Constraints for the Habitable Zone Planet TRAPPIST-1 e'.
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).
Spectral fits to TRAPPIST-1 eā€™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 ā€œghostā€ 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).
Rejection significance for atmospheric forward models compared to TRAPPIST-1 eā€™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 ā€œinfiniteā€ Ļƒ, 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).
Screenshot of the paper 'JWST-TST DREAMS: NIRSpec/PRISM Transmission Spectroscopy of the Habitable Zone Planet TRAPPIST-1 e'
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 ā‰³1 bar; the JWST one disfavors the H2-dominated solution. Figure from Espinoza et al. (2025).
This graphic compares data collected by Webbā€™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)
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 1Ļƒ and 3Ļƒ 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 evidenceā€”more data are needed to distinguish between those. Bands represent the 1Ļƒ and 3Ļƒ 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).
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).
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).
This artistā€™s concept shows the volatile red dwarf star TRAPPIST-1 and its four most closely orbiting planets, all of which have been observed by NASAā€™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)
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)".
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)".
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.
Screenshot of the title page of the paper 'Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d'.