Atmospheres and Climates of Exoplanets

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In recent years, the study of exoplanets has shifted from their detection to the characterization of their physical properties, in particular their atmospheres. The work on exoplanets proposed here consists of using some of the basic tools and principles of atmospheric science, originally developed for Earth, to deepen our understanding of the diversity of atmospheric behaviors expected on exoplanets and to clarify how those might shape astronomical observables. **1) Hot Exoplanets**Hot Jupiters/Saturns/Neptunes form the best characterized class of exoplanets and the next generation of astronomical observatories and surveys, such as TESS, JWST and ECho, will yield a wealth of new data on these planets over the next decade. Recently, it became clear that new physics in the form of magnetic induction plays an important role in the weakly-ionized atmospheres of many such planets, when temperatures exceed about 1300 K, leading to magnetic drag on the atmospheric winds and ohmic dissipation at depth (which results in radius inflation for these planets).**Using a combination of analytical considerations and numerical models, we will focus our research program for this class of exoplanets on three specific topics designed to further test and consolidate the magnetic induction scenario. We will clarify expected trends for magnetic drag and ohmic dissipation with planet temperature for the ensemble population of known planets, which should be manifest given the exponential dependence of atmospheric ionization with temperature. For specific, well characterized planets, we will also explore the interplay between several planetary observables which are directly impacted by magnetic induction, such as dragged winds vs. the degree of radius inflation. Finally, we will study the possible role of an MHD runaway convergence of electric currents on the hot dayside to locally increase heating in those regions, a process which could be at the origin of the thermal inversions observationally inferred on several hot Jupiters.**2) Habitable worlds around M-dwarfs**Terrestrial exoplanets orbiting in the habitable zone of low-mass M-dwarf stars are the focus of astronomical searches because several biases favor their detection and characterization. They may thus offer the first targets for atmospheric characterization of nearby habitable worlds in the next decade. Studying the rich climate dynamics of such worlds is important, to guide observational efforts and to provide an interpretation of the data when it becomes available.**The use of adapted climate models to evaluate surface conditions on such remote worlds with permanent day and night sides is our preferred approach, given the inherent complexity of the climate system. Our work uses an advanced, yet versatile, Earth-system climate simulator, with a full hydrological cycle, adjustable land/ocean fractions and diagnostic cloud prescriptions, to study the climates of habitable worlds around M-dwarfs. **A focus of our work on this class of planets will be to understand their unusual hydrological cycle, with the likely trapping of the majority of their surface water in the form of nightside ice (water-trapped climate configuration). The amount of residual water left on the dayside of such planets will be an important factor determining the habitability of these worlds. More generally, our work will explore a variety of expected orbital configurations for such planets (spin rate, eccentricity, obliquity, asynchronism) and their consequences for the climate. Throughout this exploration, we will strive to quantify to what extent surface conditions and habitability can be inferred from the set of astronomical measurements that will be available to characterize such planets.