water

Water (H₂O) is a compound with molecules each consisting of one oxygen and two hydrogen atoms. Within astronomy, a major concern regarding water is its contribution (in its gaseous form, i.e., water vapor) to opacity in the atmosphere, preventing observation from the Earth's surface of a substantial portion of infrared, leaving only a number of atmospheric window. Water has many absorption lines within that range. Microwave and radio are also affected, the mechanism by which microwave ovens heat water. A window of comparatively less atmospheric absorption within radio is termed the water hole.

Water occurs in astronomical objects such as molecular clouds, and cool stars such as red dwarfs and brown dwarfs (it dissociates at about 2800 K), and it exists on some planets. Masers can form in clouds that include water, on planets and comets, and in molecular clouds. Some solar system bodies have water, but Earth has an especially large abundance on its surface. The spectral properties of water vary slightly if the molecules include deuterium (e.g., heavy water, with two deuterium atoms) or tritium and their relative abundances can sometimes be determined spectroscopically from a distance.

A large factor in the habitability of a planet is a temperature range that allows water in its liquid state, and the common definition of habitable zone is the distances from a particular star in which the equilibrium temperature of a planet would fall in this range. Water vapor can be detected in an extra-solar planet atmosphere through transmission spectroscopy. Evidence of water on a world constitutes a sign of habitability. Within the solar system, liquid water or water-ice has potential to aid human visits because carrying all the necessary water adds to the challenge.

Earth's atmosphere has characteristics that allow it to retain its water: the atmosphere's cold trap keeps water vapor below the penetration of ultraviolet that would cause photodissociation of water, and the freed hydrogen would gradually escape. The source of Earth's water is of research interest: how and from where it acquired the water to produce its extreme abundance compared to the other solar system rocky planets. Theories include receiving it from asteroids and/or comets. Evidence regarding such theories includes isotope abundances of such bodies and of meteorites.