An international team, including astrophysicists from the University of Exeter, is taking lessons and techniques learned from Earth climate science to pave the way for robust modeling of the atmospheres of planets orbiting distant stars, aiding in the search for potentially habitable exoplanets.
Crucially, the team believes that this research can also advance our basic understanding and expectations for the future climate on the earth.
The recently launched James Webb Space Telescope (JWST) and upcoming telescopes such as the European Very Large Telescope (E-ELT), the Thirty Meter Telescope (TMT) or the Giant Magellan Telescope (GMT) may soon be able to discern ocean atmospheres. Rocky exoplanets orbit nearby red dwarfs (stars that are cooler and smaller than our Sun). However, without robust models to explain and guide these observations, we will not be able to unlock the full potential of these observatories.
One way is to use 3D general circulation models (GCMs) – similar to those used to predict Earth’s climate, to simulate features of the atmosphere as planets orbit their host stars. However, there are fundamental differences within these complex general circulation models that lead to contradictory climate predictions—and thus our interpretation of exoplanet observations.
In recent years, scientists have refined general circulation models in an effort to reproduce and understand the current warming trend associated with anthropogenic climate change on Earth. The main approach is to model the climate using multiple GCM models and compare them across typical Intercomparison Models, or MIPs, which have been fundamental to our knowledge of Earth’s climate.
The team led by three early career researchers — Thomas Fuchs (NASA GSFC, American University, US), Denis Sergeev (University of Exeter, UK) and Martin Turbet (LMD, France) — used this expertise and recent model upgrades to make a comprehensive comparison of several The world’s leading global rotation models are applied to the study of exoplanets.
Dr Sergeev, a postdoctoral researcher at the University of Exeter said, “Multi-model comparisons are a pillar of modern climate science and a success story for international collaboration. They are useful in our understanding of past, present and future climate processes. By bringing these comparisons to exoplanet research, we can ultimately improving our ability to interpret telescope observations.”
The pivotal new project, called THAI (TRAPPIST-1 Habitable Atmosphere Intercomparison), focuses on a confirmed Earth-size exoplanet named TRAPPIST-1e. It is the fourth planet from its host star, a red dwarf TRAPPIST-1 located about 40 light-years from Earth. Crucially, since the planet’s orbit lies within the habitable zone of TRAPPIST-1, it may have a temperate climate suitable for the presence of liquid water on its surface.
The projects combine four widely used models – ExoCAM (based on the US National Center for Atmospheric Research model), LMD-G (developed by Laboratoire de Meterologie Dynamique in Paris), ROCKE-3D (based on the NASA GISS model) and UM (Developed at the UK Met Office and adapted to exoplanets by researchers at the University of Exeter) – to consider four different scenarios for TRAPPIST-1e’s atmosphere.
This consisted of two surface scenarios (completely dry, one covered by a global ocean providing moisture to the atmosphere) and two atmospheric formation scenarios (a nitrogen-rich atmosphere with recent Earth carbon dioxide levels).2or CO like Mars2atmosphere dominated).
Clouds are one of the biggest sources of differences between GCMs: their optical properties, height, thickness, and coverage have been shown to differ significantly between models due to differences in cloud parameters. “Representing wet physics at a small scale in general circulation models is known to be difficult,” said Dr. Sergeev. “It is one of the main avenues of atmospheric research for both exoplanet science and Earth’s climate.”
Dr. Fuchs, who leads the THAI project, said, “THAI has benefited from valuable experience from similar efforts in the Earth science community to study anthropogenic global warming. However, it has also been able to impart knowledge again, through improvements in the underlying model that was developed as part of of exoplanet applications.
The results of these analyzes, which include a presentation, for the first time, of how GCM use may affect future data interpretation and future planning of monitoring campaigns, are presented in three fully open-access articles. The full results were published on September 15, 2022 in a special issue of Planetary Science Journal (PSJ).
However, the team believes that THAI will not only pave the way for robust modeling of distant, potentially habitable worlds, but also connect our efforts to find extraterrestrial life with studies of our changing climate.
Dr. Sergeev added: “Our work on TRAPPIST-1e, with a completely different Earth orbital configuration, revealed several improvements, for example, in manipulating stellar heating of the atmosphere, which are now being implemented at UM and applied to Earth.”
THAI paves the way for a larger model comparison project, Climate using the Interactive Interval Suite for Exoplanet Studies (CUISINES) that would include a broader diversity of extrasolar planet Objectives and models to be compared systematically and thus validated.
Martin Turbet et al., Comparison of the Habitable Atmosphere TRAPPIST-1 (THAI). I. Dry Cases – GCMs Fellowship, Planetary Science Journal (2022). DOI: 10.3847 / PSJ / ac6cf0
Denis E. Sergeev et al., TRAPPIST-1 Habitable Atmosphere Inter-comparison (THAI). II. Wet states – the two water worlds, Planetary Science Journal (2022). DOI: 10.3847 / PSJ / ac6cf2.0
Thomas J. Fushes et al., Inter-comparison of the TRAPPIST-1 Habitable Atmosphere (THAI). Third. Simulated Watchers – Spectrum Return, Planetary Science Journal (2022). DOI: 10.3847 / PSJ / ac6cf1
University of Exeter
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