Benchmarking Photolysis Rates: Species for Earth and Exoplanets
Geoscientific Model Development, 19, 5305 (2026)
Abstract:
Using the Socrates photolysis scheme, we present newly calculated photolysis rates under modern Earth atmospheric conditions for species directly relevant to Earth and species relevant to different atmospheric compositions. We compare to a previous photolysis comparison exercise, namely PhotoComp 2011. Overall, we find good agreement between our results and previous work, with discrepancies usually caused by the implementation of temperature or pressure dependent quantum yields and updated cross-section data. We provide a new set of benchmark photolysis rates for additional species both for Solar irradiance and when irradiated by an M dwarf host star. In general, the higher actinic flux at far-UV and shorter wavelengths of the M dwarf compared to the Sun drives increased photolysis rates for reactions with high threshold energies. This work provides an updated set of benchmark results for further studies of photolysis in the Earth's atmosphere and that of other planets.
Modeling Atmospheric Ion Escape from Kepler-1649 b and c over Time
The Astrophysical Journal Letters, Volume 994, Number 2, L50 (2025)
Abstract:
Rocky planets orbiting M dwarf stars are prime targets for atmospheric characterization, yet their long-term evolution under intense stellar winds and high-energy radiation remains poorly constrained. The Kepler-1649 system, hosting two terrestrial exoplanets orbiting an M5V star, provides a valuable laboratory for studying atmospheric evolution in the extreme environments typical of M dwarf systems. In this Letter, we show that both planets could have retained atmospheres over gigayear timescales. Using a multispecies magnetohydrodynamic model, we simulate atmospheric ion escape driven by stellar winds and extreme-ultraviolet radiation from 0.8 to 4.0 Gyr. The results reveal a clear decline in total ion escape rates with stellar age, as captured by a nonparametric LOWESS regression, with O+ comprising 98.3%–99.9% of the total loss. Escape rates at 4.0 Gyr are 2 to 3 orders of magnitude lower than during early epochs. At 0.8 Gyr, planet b exhibits 3.79× higher O+ escape rates than planet c, whereas by 4.0 Gyr its O+ escape rates becomes 39.5× lower. This reversal arises from a transition to sub-magnetosonic star–planet interactions, where the fast magnetosonic Mach number, Mf, falls below unity. Despite substantial early atmospheric erosion, both planets may have retained significant atmospheres, suggesting potential long-term habitability. These findings offer predictive insight into atmospheric retention in the Kepler-1649 system and inform future JWST observations of similar M dwarf terrestrial exoplanets aimed at refining habitability assessments.
Transformational astrophysics and exoplanet science with Habitable Worlds Observatory's High Resolution Imager
White paper submitted to the UK Space Agency's initiative "UK Space Frontiers 2035
Abstract:
Habitable Worlds Observatory (HWO) will be NASA’s flagship space telescope of the 2040s, designed to search for life on other planets and to transform broad areas of astrophysics. NASA are
seeking international partners, and the UK is well-placed to lead the design and construction of its
imaging camera — which is likely to produce the mission’s most visible public impact. Early participation in the mission would return investment to UK industry, and bring generational leadership for
the UK in space science, space technology, and astrophysics.
seeking international partners, and the UK is well-placed to lead the design and construction of its
imaging camera — which is likely to produce the mission’s most visible public impact. Early participation in the mission would return investment to UK industry, and bring generational leadership for
the UK in space science, space technology, and astrophysics.
Simulating biosignatures from pre-oxygen photosynthesizing life on TRAPPIST-1e
Monthly Notices of the Royal Astronomical Society, Volume 531, Issue 1, pp.468-494 (2024)
Abstract:
In order to assess observational evidence for potential atmospheric biosignatures on exoplanets, it will be essential to test whether spectral fingerprints from multiple gases can be explained by abiotic or biotic-only processes. Here, we develop and apply a coupled 1D atmosphere-ocean-ecosystem model to understand how primitive biospheres, which exploit abiotic sources of H2, CO, and O2, could influence the atmospheric composition of rocky terrestrial exoplanets. We apply this to the Earth at 3.8 Ga and to TRAPPIST-1e. We focus on metabolisms that evolved before the evolution of oxygenic photosynthesis, which consume H2 and CO and produce potentially detectable levels of CH4. O2-consuming metabolisms are also considered for TRAPPIST-1e, as abiotic O2 production is predicted on M-dwarf orbiting planets. We show that these biospheres can lead to high levels of surface O2 (approximately 1–5 per cent) as a result of CO consumption, which could allow high O2 scenarios, by removing the main loss mechanisms of atomic oxygen. Increasing stratospheric temperatures, which increases atmospheric OH can reduce the likelihood of such a state forming. O2-consuming metabolisms could also lower O2 levels to around 10 ppm and support a productive biosphere at low reductant inputs. Using predicted transmission spectral features from CH4, CO, O2/O3, and CO2 across the hypothesis space for tectonic reductant input, we show that biotically produced CH4 may only be detectable at high reductant inputs. CO is also likely to be a dominant feature in transmission spectra for planets orbiting M-dwarfs, which could reduce the confidence in any potential biosignature observations linked to these biospheres.
3D simulations of TRAPPIST-1e with varying CO2, CH4, and haze profiles
Monthly Notices of the Royal Astronomical Society, Volume 530, Issue 3, pp.2933-2933 (2024)
Abstract:
Using a 3D General Circulation Model, the Unified Model, we present results from simulations of a tidally locked TRAPPIST-1e with varying carbon dioxide CO2 and methane CH4 gas concentrations, and their corresponding prescribed spherical haze profiles. Our results show that the presence of CO2 leads to a warmer atmosphere globally due to its greenhouse effect, with the increase of surface temperature on the dayside surface reaching up to ∼14.1 K, and on the nightside up to ∼21.2 K. Increasing presence of CH4 first elevates the surface temperature on the dayside, followed by a decrease due to the balance of tropospheric warming and stratospheric cooling. A thin layer of haze, formed when the partial pressures of CH4 to CO2 (pCH4/pCO2) = 0.1, leads to a dayside warming of ∼4.9 K due to a change in the water vapour H2O distribution. The presence of a haze layer that formed beyond the ratio of 0.1 leads to dayside cooling. The haze reaches an optical threshold thickness when pCH4/pCO2 ∼ 0.4 beyond which the dayside mean surface temperature does not vary much. The planet is more favourable to maintaining liquid water on the surface (mean surface temperature above 273.15 K) when pCO2 is high, pCH4 is low, and the haze layer is thin. The effect of CO2, CH4, and haze on the dayside is similar to that for a rapidly rotating planet. On the contrary, their effect on the nightside depends on the wind structure and the wind speed in the simulation