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The Climates and Habitability of Small Exoplanets Around Red Stars

Periodic Reporting for period 3 - EXOKLEIN (The Climates and Habitability of Small Exoplanets Around Red Stars)

Reporting period: 2021-02-01 to 2022-07-31

The ERC Consolidator Project EXOKLEIN (771620) is an interdisciplinary project that combines knowledge from astrophysics and astronomy, climate science and geochemistry. It 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. The project is divided into three major subprojects, each led by a long-term (4 years) postdoctoral researcher, which aim 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. 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.
Research Theme 1 aims to construct a virtual laboratory of an exoplanetary atmosphere and is led by Dr. Russell Deitrick (Ph.D University of Washington at Seattle), who has training in astrophysics and astrobiology. Dr. Deitrick spent the first two years of the project leading a major upgrade on our in-house general circulation model (GCM) named THOR, which is a state-of-the-art, three-dimensional climate simulator. Our findings are reported in Deitrick et al. (2020, Astrophysical Journal Supplements, 248, 30; 35 pages), which includes a battery of tests that demonstrate the reproducibility of scientific results computed by THOR. This upgrade further establishes THOR as a publicly available resource for the exoplanet community. We expect to shortly report on a further upgrade of this GCM to include a state-of-the-art, multi-wavelength radiative transfer capability (Heng et al. in preparation for 2020/2021). Dr. Deitrick is now using THOR to study atmospheric collapse.

Research Theme 2 aims to generalise Earth-centric understanding of geochemical cycles and is led by Dr. Kaustubh Hakim (Ph.D University of Amsterdam), who has training in astrophysics and experimental geology. Dr. Hakim spent the first year of the project studying the effects of varying lithology (rock composition) on the weathering process of the carbon cycle and the manuscript (30+ pages) has been accepted for publication in the Planetary Science Journal.

Research Theme 3 aims to understand the long-term stability of biosignature gases and is led by Dr. Pierre Auclair-Desrotour (Ph.D Paris Observatory), who has training in astrophysics. Dr. Auclair-Desrotour spent the first year of the project establishing a theoretical framework to study atmospheric collapse, which is the phenomenon that greenhouse gases may suffer runaway condensation on the permanent nightside of a tidally locked exoplanet. The framework allows for the wide exploration of atmospheric conditions (i.e. parameter scanning) and identifies interesting case studies for GCMs to focus on. Our findings are reported in Auclair-Desrotour & Heng (2020, Astronomy & Astrophysics, 638, A77).
The EXOKLEIN project has evolved positively and constructively since its conception. Originally, it was proposed to study the effects of lithology (rock composition) on the long-term carbon cycle, as well as how the solubility of carbonates depend on the acidity of ocean water. With the publication of our first results and regular collaboration with geoscientists, we have realised that it is necessary to understand ocean chemistry, develop a theory of outgassing of intermediate complexity and possibly examine how the carbon cycle interacts, and is influenced by, other geochemical cycles. We also realised that atmospheric collapse is a rich and multi-faceted problem that deserves more attention than was originally proposed and have expanded the scope of our focus on this problem.

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. These formulae are the first new ones to be discovered since the seminal work of Hapke (1981) and Lambert (18th century).
Schematic of long-term carbon cycle (carbonate-silicate cycle).