Dr Ryan MacDonald is an expert in the atmospheres of planets orbiting other stars (exoplanets). He joined the School in 2025, after completing a NASA Sagan Fellowship at the University of Michigan and serving as a Research Associate at Cornell University's Carl Sagan Institute. Dr MacDonald completed his PhD in 2019 at the University of Cambridge, where he developed new Bayesian techniques to characterise exoplanet atmospheres.

Exoplanet science raises some of the most profound questions in modern astronomy. What is the nature of worlds around other stars? Do the rocky planets in our galaxy resemble Earth, or is our blue marble an anomaly? Are we alone in the Universe?

Dr MacDonald's research at St Andrews combines cutting-edge telescope observations—such as the James Webb Space Telescope—with state-of-the-art theoretical modelling to decode exoplanet atmospheres. He is the lead developer of the POSEIDON atmospheric retrieval code, which has enabled numerous breakthroughs in exoplanet science, including:

• The first detection of titanium oxide in an exoplanet atmosphere (Sedaghati et al. 2017)—ESO press release.
• The first detection of mineral clouds in a transiting exoplanet atmospheres (Grant et al., 2023)—NASA press release.
• The first discovery of a 'steam world' (Piaulet-Ghorayeb et al., 2024)—University of Michigan press release.

Dr MacDonald leads several observing programs using JWST to characterise exoplanet atmospheres, whilst also being a core member of many international collaborations with access to ground- and space-based exoplanet spectra.

Students at St Andrews working with Dr MacDonald will therefore be directly involved with research at the frontier of exoplanetary science.

Dr MacDonald's research broadly focuses on exoplanet and substellar atmospheres, including:

• Exoplanet atmosphere modelling.
• Bayesian atmospheric retrieval.
• Spectroscopy and radiative transfer.
• Exoplanet biosignatures.
• Hot Jupiters.
• Sub-Neptunes.
• Terrestrial exoplanets.
• Brown dwarfs.