Modelling the non-LTE spectra of exoplanets

In just over two decades since the first extrasolar planet was discovered, astronomers have found that exoplanets are ubiquitous, with nearly every star supporting a planetary system. These planets are generally quite different from those in our own solar system, and astronomers have taken the first steps in characterizing their atmospheres through spectroscopy. James Webb Space Telescope (JWST) has already demonstrated its ground breaking potential for studying exoplanetary atmospheres among other targets. The need to perform spectroscopic studies of exoplanet atmospheres is driving the development of next-generation space missions, such as ESA's Ariel M4 mission (set to launch in 2029), which will observe about 1000 exoplanets ranging from Jupiter- and Neptune-size down to super-Earth size at visible and infrared wavelengths.

We are on the cusp of atmospheric characterisation of complex properties of atmospheres: 3D effects, climates, weather, chemical non-equilibrium and non-LTE effects. While much of the study into exoplanet atmospheres assumes species to be in local thermodynamic equilibrium (LTE), effects arising from non-local thermodynamic equilibrium (non-LTE) are known to be present and have been detected in Earth's atmosphere, in the atmospheres of other solar system planets, gas giants, comets and the ISM. In LTE a single temperature describes the state populations via the Boltzmann distribution. For many (interstellar, circumstellar, or top atmospheric) media, the density is too low to attain LTE. The non-LTE effects can be caused by a variety of factors such as radiative processes, spontaneous emission to space, absorption of radiation from stellar and atmospheric sources, non-thermal collisional processes, chemical recombination, and photochemical reactions.

The non-LTE analysis of the atmospheres of exoplanets is an emerging topic. Despite the importance of the non-LTE on the molecular spectra, its modelling is still a challenging field of research. There are a very few non-LTE studies of exoplanetary atmospheres, mostly constraining on the atomic non-LTE features. Existing non-LTE models are not always well-suited to exoplanetary retrievals, due to their high temperature conditions, low resolution of spectra, and, rather notably, due to the lack of the molecular data. While spectroscopic and other data needs for studies of Earth-clone exoplanets are well met by databases constructed for studying our own atmosphere, this is not the case for those exoplanets which will be the first exoplanets to be characterised. These planets are hot and most are subject to strong bombardment by starlight (and presumably stellar winds). Such studies require data on hot molecules which are not generally available and which are in many cases not easily amenable to laboratory experiments. This means that the traditional methods of non-LTE modelling cannot work and new retrieval strategies need to be developed. This proposal aims to provide spectroscopic data and modelling tools for characterisations of molecular non-LTE effects in atmospheric spectra of exoplanets. By providing these tools and data, the project will significantly advance our understanding of exoplanetary atmospheres and aid in the characterization of these complex and unique systems.