ExoMol is a database of molecular line lists that can be used for spectral characterisation and simulation, and as input to atmospheric models of exoplanets, brown dwarfs and cool stars, and other models including those for combustion and sunspots.

There is an infinite number of worlds of different sizes: some are larger than ours, some have no sun or moon, others have suns or moons that are bigger than ours. Some have many suns and moons. Worlds are spaced at differing distances from each other; in some parts of the universe there are more worlds, in other parts fewer. In some areas they are growing, in other parts, decreasing. They are destroyed by collision with one another. There are some worlds with no living creatures, plants, or moisture.

—Democritus

The Universe started with a Big Bang synthesis of a few chemical elements that eventually led to self-replicating, competitive structures of molecules we call life. With ever more powerful telescopes, some of the oldest questions in science can now be addressed: Are there worlds beyond our solar system? Are they numerous or rare? How many of them have the right conditions for life? A scientific approach to these questions, however, needs to start with more fundamental questions such as: How do stars and planets form and evolve? and What are they made of? The detection, observation and characterisation of sub stellar objects now plays a key role in the schedules of many ground-based (e.g. the European South Observatory) or space-based (Hubble and Spitzer) telescopes and is a major driver for new telescope developments.

The wealth of observational discoveries, however, is not well matched by theoretical advances in the same field: modelling the atmospheres of planetary and stellar objects, and hence deriving their chemical composition, temperature, etc. is still impeded by the lack of fundamental data, especially in molecular spectroscopy. Extremely expensive, new and forthcoming European-funded telescopes (Herschel, ALMA, ELT, JWST, SPICA) or proposed ones (EChO) will only help to characterise such objects if a commensurate effort is made to secure the tools required to interpret the observations. ExoMol aims to provide the information needed to understand the physics and chemistry of astronomical bodies cool enough to form molecules in their atmospheres such as cool stars, extrasolar planets and planetary disks.