Research

Interests


My research focuses on planetary habitability. 'Habitability' is a rather broad term, but can be summarized as the ability of a planet to support life. What factors control the ability of life to survive and thrive in some places (in many places on Earth, for example), but not in other places (seemingly everywhere else)? 

Answering this question requires some: 

Published Research

Atmospheric Circulation on Dry Planets in the TRAPPIST-1 system

Variations in the reflective properties of the bulk material that comprises the surface of land-dominated planets will affect the planetary energy balance by interacting differently with incident radiation from the host star. We investigated this effect by studying the climates of 3 land-covered terrestrial planets in the TRAPPIST-1 system with a 3D GCM (CESM2). We found that lower albedo surfaces, such as granite, result in warmer average surface temperatures while simultaneously increasing equator-pole temperature contrast as well as day-night hemisphere temperature contrast. Crucially, our simulations suggested that the composition of the land surface of land-covered planets affects the rotational regime of the planet’s atmosphere, with implications for observations and climate assessments.

Rushby, A.J., Shields, A.L., Wolf, E. T., Laguë, M. & Burgasser, A. (2020). The Effect of Land Albedo on the Climate of Land-dominated Planets in the TRAPPIST-1 System. The Astrophysical Journal 904 (2). [arXiv: https://arxiv.org/abs/2011.03621 ]

Ice-albedo feedbacks on M-dwarf planets

Planets dominated by land reflect more starlight and have lower surface temperatures than do ocean-covered worlds, but the longer wavelength of starlight emitted by small, cool stars (M-dwarfs) means that water ice on their planets absorbs more energy than it would on Earth as water ice is absorptive in the near-infrared region. This keeps the planet warmer than it would be if it was orbiting a Sun-like star, and prevents ice growing all the way to the equator (also known as the ice-albedo feedback) and stabilises their climate. 

Rushby, A.J., Shields, A.L., & Joshi, M. (2019). The Effect of Land Fraction and Host Star Spectral Energy Distribution on the Planetary Albedo of Terrestrial Worlds. The Astrophysical Journal 887 (1). [arXiv: https://arxiv.org/abs/1910.05439 ]

Terrestrial planet size and the carbonate-silicate cycle

We find that likely changes in global topography, tectonic outgassing and uplift, and the hydrological cycle on larger planets results in proportionally greater surface temperatures and atmospheric carbon dioxide (CO2) when accounting for incoming stellar radiation. For planets between 0.5 and 2 Earth radii the effect of these changes results in average global surface temperature deviations of up to 15 K, which suggests that these relationships be considered in future studies of planetary habitability. Setting an upper temperature limit of 343 K, the habitable period of the Earth-sized world around the Sun can be quantified. This limit is approximately 6.3 billion years (Gyr) after planet formation, or 1.81 Gyr from present day. Additionally, atmospheric CO2 falls below the level at which C3 and C4 plants, the majority of land plants, can effectively photosynthesize after ~5.4 Gyr and ~6.1 Gyr respectively, which may initiate a significant reorganization of the biosphere of the planet well before average surface temperatures render it uninhabitable. 

Rushby, A.J., Johnson, M., Mills, B.J.W., Claire, M.W., & Watson, A.J., (2018). Long Term Planetary Habitability and the Carbonate-Silicate Cycle. Astrobiology 18 (5)

Earlier papers


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Updated April 2024

AJR_CV_2024.pdf