Water is the assumed solvent for life — not because biochemistry requires it specifically, but because Earth-based biology never developed an alternative. Ionic liquids and deep eutectic solvents possess vapor pressures orders of magnitude lower than water, remain liquid across temperature ranges that water cannot span, and support a surprising range of biomolecular activity in laboratory studies. On worlds where water cannot persist as a liquid — too hot, too cold, too irradiated — these solvents might fill the gap.
The evidence is not purely speculative. Desiccation-tolerant plants already use deep eutectic mixtures as protective intracellular fluids, replacing water when it becomes unavailable. The chemistry of life-in-non-water isn't alien; it exists on Earth as an emergency measure. Seager and colleagues extend this observation to planetary science, identifying environments — asteroid surfaces, high-temperature exoplanets, comet interiors — where ionic liquids could be thermodynamically stable and potentially prebiotically active.
The structural insight: the question of habitability has been conflated with the question of water stability. Broadening the solvent space doesn't just add candidate worlds — it redefines what “habitable” means. Life's deepest requirement may not be a specific molecule but a liquid phase with sufficient complexity to host reactions. Water is one answer. It may not be the only one.
(arXiv:2603.02776)