Understanding the nature and distribution of habitable environments in the Universe is one of the fundamental goals of modern astrophysics. For the life we know, liquid water on the planetary surface is a prerequisite. However, a direct detection of liquid water on exoplanets, and especially on a potentially habitable Earth-size planet, is not yet possible. The existence of water almost certainly implies the presence of atmospheric water vapour which must evaporate under stellar irradiation from a cloud deck or from the surface, together with other related molecules. Therefore, devising sensitive methods to detect hot molecules on exoplanets is of high importance. This proposal develops several exploratory theoretical and observational aspects of precision spectropolarimetry for detecting water vapour and other volatiles on exoplanets and in the inner part of protoplanetary disks. These are new tools for making progress in our understanding which fraction of planets acquires water and how planet formation influences their habitability. As a “double differential” technique, spectropolarimetry has enormous advantages for dynamic range problems, like detection of weak line signals against a large stellar background and exploration at scales beyond the angular resolution of telescopes, which are crucial for both exoplanets and inner disks. Direct detection of polarized spectral lines enables recovering precise orbits of exoplanets (including non-transiting systems) and evaluating their masses as well as potentially their magnetic fields. First applied to hot Jupiters the developed tools will create a firm foundation for future exploration of Earth-like planets with larger telescopes. The same technique applied to planetesimals in the inner disks of young stars yields their orbits, temperature, and chemical composition. These will provide constraints on the formation of a planetary atmosphere in the vicinity of the star and its habitable zone.