Meet the Fellows Research Refining models of sub-Neptune exoplanets to construct more accurate mock observations, aiding interpretations of JWST’s most detailed glimpses of distant worlds. Host University Arizona State University Year Awarded 2025 Website Professional Page Matthew Nixon, Ph.D. Ph.D., Astrophysics, University of Cambridge Research Refining models of sub-Neptune exoplanets to construct more accurate mock observations, aiding interpretations of JWST’s most detailed glimpses of distant worlds. Host University Arizona State University Year Awarded 2025 Website Professional Page “What excites me about exoplanet science is how quickly we’ve found worlds unlike anything in our own solar system. There are so many fundamental questions we still need to answer: What are these objects made of? How do they form?” Matthew Nixon, Ph.D. Matthew Nixon, Ph.D., was waiting for a train home when he stumbled upon a scientific journal headline that stopped him in his tracks: The Hubble Space Telescope had detected water vapor in the atmosphere of the exoplanet K2-18 b. Considered to be a sub-Neptune, K2-18 b is among the planet type most abundant in the galaxy but missing from our own solar system. The surprising findings suggested that beneath its fluffy hydrogen atmosphere, K2-18 b might host a liquid water ocean. However, later studies found that the observations could also be explained by violent interactions between the hydrogen and a molten rocky surface. To Dr. Nixon, this was a puzzle worth solving. Dr. Nixon develops models to help decode the enigmatic atmospheres and interiors of sub-Neptunes. During his Ph.D., he built an open-source model that explores the diverse structures of water worlds—some with deep oceans sandwiched between layers of ice, others holding water at extreme pressures and temperatures under a steamy atmosphere. In another project, he created Aura-3D, a powerful tool that more accurately accounts for the three-dimensional nature of exoplanet atmospheres, reducing errors that could otherwise steer toward misleading conclusions. In his current research, he applies machine learning techniques to rapidly match new JWST data to the most appropriate planetary models. An artist’s impression compares present-day Earth to its molten past, highlighting how magma oceans on sub-Neptunes may interact with their atmospheres to shape chemical composition. Credit: Tobias Stierli As a 51 Pegasi b Fellow, Dr. Nixon will expand on his tools to explore how magma oceans may influence atmospheric composition, and he will identify observable signatures that could distinguish varied types of sub-Neptune exoplanets. By generating synthetic JWST observations informed by real data, he will determine which signals are most promising and where biases may be hiding. In the coming years, Dr. Nixon’s research could rewrite our knowledge of sub-Neptune composition, test long-standing formation theories, and bring us closer to answering whether these strange worlds can harbor life. Dr. Nixon received his Ph.D. in astrophysics from the University of Cambridge in Summer 2023.