Thousands of exoplanets have been discovered in the recent years, most of them are gas giant and hundreds appear to be rocky. Many of these exoplanets have very short orbital period, hence hot atmospheres. The hot temperatures are expected to turn these atmospheres into a high pressure steam, thus having very different spectroscopic signatures than cooler objects, which will influence interpretation of the atmospheric observations. At present interpretation is severely impacted by the lack of corresponding spectroscopic data. The massive number of absorbers in the atmospheres of exoplanets also affects the cooling and therefore the evolution of the young hot objects; comprehensive molecular data is also crucial to model these processes.

 Figure 1: A giant planet that orbits close to its sun (Illustration: NASA/ESA/G. Bacon (STScI)).

Our group ExoMol specialises in and leads the field of producing theoretical spectral data (so-called line lists) for molecules of atmospheric, astrophysical, and industrial importance applicable for high temperature applications. The ExoMol database already contains over 300 billion transitions for over than 50 molecular species. In order to accomplish this data intensive task, we use some of the UK’s most advanced supercomputers, provided by the Distributed Research utilising Advanced Computing (DiRAC) project and run by the University of Cambridge. Our calculations require tens millions CPU hours, hundreds thousands GPU hours, and up to 6 TB of RAM, the processing power only accessible to us through the DiRAC project. We are in a unique position to provide line lists for more complex molecules (such as large hydrocarbons), comprising up to 1012 lines.

Methane is a major absorber in hot Jupiter exoplanets, brown dwarfs and cool carbon stars. There is a huge demand for a reliable hot methane line list and there are several groups working towards this objective. As part of our project we have generated a new 34 billion methane line list (known as 34to10), which extends our original 10to10 line list to higher temperatures (up to 2000 K). Even larger line lists are now being completed (for silane, ethylene, acetylene, propene, methyl chloride etc).

Billions of transitions make their direct (line-by-line usage) application in radiative transfer calculations impractical. To mitigate this big data issue we have developed a new, hybrid line list format, based on the idea of temperature-dependent absorption continuum. The new hybrid scheme has being now applied to larger hydrocarbon’s with line lists containing hundreds billions of transitions.