CORDIS - EU research results

The Climates and Habitability of Small Exoplanets Around Red Stars

Periodic Reporting for period 4 - EXOKLEIN (The Climates and Habitability of Small Exoplanets Around Red Stars)

Reporting period: 2022-08-01 to 2023-01-31

The ERC Consolidator Project EXOKLEIN (771620) seeks to understand the extent to which small, rocky exoplanets (planets found outside our Solar System, orbiting other stars) orbiting red dwarfs are able to retain atmospheres and climates that are habitable. It is an interdisciplinary project that combines knowledge from astrophysics and astronomy, climate science and the geosciences. Three postdoctoral researchers were each hired on long-term contracts (4 years) to take charge of subprojects aiming to construct a virtual laboratory of an atmosphere, generalise Earth-centric knowledge of the long-term carbon cycle and examine the long-term stability of biosignature gases. As the project evolved, several other personnel were hired on short-term contracts. Highlights of the completed project work include: the development of a next-generation general circulation model (GCM) for simulating climates of exoplanets, of which the computer code is publicly available; the development of a hierarchy of models for understanding atmospheric collapse; understanding of the inorganic carbon cycle, ocean chemistry and outgassing that generalises beyond Earth-centric conditions; and the unexpected discovery of mathematical solutions for how light reflects from a celestial body (planet, moon, etc) as it rotates around a larger body (star, planet, etc), which is the approximate solution to a century-old problem in classical astronomy. The "blue sky research" and highly-interdisciplinary nature of the EXOKLEIN project contributes to the development of creative problem solving, "out of the box" thinking and scientific innovative in society.
Dr. Russell Deitrick (Ph.D Washington) led two lengthy peer-reviewed papers on major developments and improvements to the general circulation model (GCM) THOR. In Deitrick et al. (2020, ApJS, 248, 2), we added several physics modules to THOR and subjected it to a battery of tests and benchmarks. In Deitrick et al. (2022, MNRAS, 512, 3759), we coupled the improved two-stream treatment of radiative transfer to THOR for the first time. Furthermore, we worked closely with a professional software engineer to professionalise the computer code, which is publicly available as a community resource. In Hakim et al. (2021, PSJ, 2, 49), Dr. Kaustubh Hakim (Ph.D Amsterdam) led a study generalising the inorganic carbon cycle or carbonate silicate cycle of Earth for application to exoplanets. Specifically, a broad variety of different minerals were examined and their effects on the carbon cycle were quantified. In Hakim et al. (2023, ApJL, 942, L20), we generalise the ocean chemistry of Earth for application to exoplanets. Specifically, we took a close look at carbonate precipitation and the depth of the carbonate compensation depth (CCD). In Tian & Heng (2023, under review), we elucidated a theoretical framework for computing geochemical outgassing that extends beyond Earth-like conditions. Dr. Pierre Auclair-Desrotour (Ph.D Paris) led a pair of papers that provided a physical basis for the understanding of atmospheric collapse: Auclair-Desrotour & Heng (2020, A&A, 638, A77) and Auclair-Desrotour, Deitrick & Heng (2022, A&A, 663, A79). In Heng, Morris & Kitzmann (2021, Nature Astronomy, 5, 1001), we derived ab initio solutions for the shape and amplitude of reflected-light phase curves for an arbitrary scattering phase function, building on decades-old work by Hapke and Chandrasekhar. In Heng & Li (2021, ApJL, 909, L20), we applied these novel mathematical solutions towards interpreting data of Jupiter measured by the Cassini spacecraft.
The pair of papers led by Dr. Russell Deitrick established THOR as a resource and provided the exoplanet community with a state-of-the-art climate simulator coupled with an accurate treatment of scattering. The pair of papers led by Dr. Kaustubh Hakim generalised our understanding of carbon cycles and ocean chemistry beyond Earth-like conditions. The pair of papers by Dr. Pierre Auclair-Desrotour advanced our first-principles understanding of atmospheric collapse and demonstrated how rocky exoplanets orbiting red dwarfs may survive such a fate. This work has inspired broad collaboration with geoscientists. An unanticipated development of the project is the discovery of a novel mathematical solution to the radiative transfer equation and novel mathematical formulae for the albedo and phase curves of exoplanets, which has major implications for the analysis and interpretation of data measured by present and future state-of-the-art space- and ground-based telescopes.
Schematic of long-term carbon cycle (carbonate-silicate cycle).