  Life is a complex, dynamic chemical system that requires a dense fluid
solvent in which to take place. A common assumption is that the most likely
solvent for life is liquid water, and some researchers argue that water is the
only plausible solvent. However a persistent theme in astrobiological research
postulates that other liquids might be cosmically common, and could be solvents
for the chemistry of life. In this paper we present a new framework for the
analysis of candidate solvents for life, and deploy this framework to review
substances that have been suggested as solvent candidates. Our approach
addresses all the requirements for a solvent, not just single chemical
properties, and does so semi-quantitatively. Only the protonating solvents
fulfil all the chemical requirements to be a solvent for life, and of those
only water and concentrated sulfuric acid are also likely to be abundant in a
rocky planetary context. Among the non-protonating solvents liquid CO2 stands
out as a planetary solvent, and its potential as a solvent for life should be
explored. We conclude with a discussion of whether it is possible for a
biochemistry to change solvents, as an adaptation to radical changes in a
planet's environment. Our analysis provides the basis for prioritizing future
experimental work exploring potential complex chemistry on other planets.
