Computational spectroscopy of exoplanets

Computational spectroscopy of exoplanets

Since the first discovery of a planet orbiting a distant star 25 years ago (Nobel Prize in Physics 2019) over 4000 of exoplanets have been discovered. Now we want to know what these exoplanets are made of, what weather is like there, and whether they are habitable. These questions can be answered via characterisation of their atmospheres, which requires detailed models of the planetary atmosphere and large quantities of laboratory data on how possible atmospheric species absorb light.

This artist’s impression shows the planet K2-18b, it’s host star and an accompanying planet in this system. K2-18b is now the only super-Earth exoplanet known to host both water and temperatures that could support life. UCL researchers used archive data from 2016 and 2017 captured by the NASA/ESA Hubble Space Telescope and developed open-source algorithms to analyse the starlight filtered through K2-18b’s atmosphere. The results revealed the molecular signature of water vapour, also indicating the presence of hydrogen and helium in the planet’s atmosphere.

In 2019, a UCL team comprising Dr Angelos Tsiaras, Dr Ingo Waldmann, Prof Giovanna Tinetti, Prof Jonathan Tennyson, and Prof Sergey Yurchenko made the first detection of water in the atmosphere of an exoplanet, called K2-18b orbiting within the star’s habitable zone. This publication in Nature Astronomy, 3, 1086 (2019) was the highest ranked astronomy paper of 2019 with some 4000 worldwide media reports.

An atmospheric retrieval requires significant input on the spectral properties of the individual atoms and molecules which comprise this atmosphere. The ExoMol group led by Profs. Jonathan Tennyson and Sergey Yurchenko has pioneered quantum mechanical-based theoretical procedures which allow them to compute the appropriate lists of absorption lines. The ExoMol database provides comprehensive and accurate opacities for the key molecules which are likely to exist in the atmospheres of exoplanets. ExoMol line lists are used by exoplanetary models world-wide, to model the behaviour of molecules in the atmospheres of exoplanets. Because the atmospheres of most observable exoplanets are rather hot (often well over 1000 K) the molecular line list become huge. The calculation of such big data can be only accomplished on modern HPC facilities and DiRAC has been a reliable partner for ExoMol in providing these facilities since 2012.

ExoMol database contains extensive line lists for 80 molecules, 190 isotopologues. Most its 700 billion transitions have been produced using DiRAC resources, with 10 line lists resulted from the new thematic project “Spectroscopy of hot exoplanets”. One of these data is the line lists for SiO2 consisting of over 32 billion transitions (Owens et al., MNRAS, 495, (2020)). SiO2 is believed to be an important absorber in atmospheres of hot “lava planets” which orbit so close their host star that their rocky surface must be molten. Until our calculations, there existed no laboratory data on the absorption properties of this key molecule.

Owing to the success of ExoMol and the demand for these data ERC have just funded a follow-up project, ExoMolHD, led by Prof Tennyson, on precision spectroscopic data for studies of exoplanets and other hot atmospheres which will run from 2020 to 2025.