
The gaseous phase of water, detected in the exoplanet's atmosphere.Water vapor is the gaseous phase of water, present whenever liquid water or ice evaporates or sublimates. In exoplanetary science, its detection in an atmosphere is a major event: water is a key ingredient for life as we know it, and its presence tells astronomers about the planet's formation history, temperature structure, and atmospheric dynamics. The articles in this corpus show water vapor in a variety of exotic settings — from scorching hot Jupiters like WASP-121b, where it coexists with mineral clouds and escaping atmospheres, to warm, salty ocean worlds where it might be the dominant atmospheric constituent.
The first definitive detection of water vapor in an exoplanet atmosphere was reported in 2007 by a team led by Carl Grillmair, using the Spitzer Space Telescope to observe HD 189733b. Earlier claims existed but were less secure.
Water molecules absorb and emit infrared radiation at specific wavelengths, creating a unique spectral fingerprint. When a planet passes in front of its star (transit), starlight filters through the atmosphere, and water vapor imprints its signature on the transmitted light. By analyzing this spectrum, astronomers can infer the abundance of water vapor and even its vertical distribution. In hotter planets, water vapor can be dissociated by ultraviolet radiation or lost entirely through hydrodynamic escape, so its presence also constrains the planet's thermal and chemical evolution.
Water vapor detection is a primary tool for characterizing exoplanet atmospheres, guiding the selection of targets for the James Webb Space Telescope and future missions searching for habitable worlds.
On WASP-121b, water vapor is literally torn apart by the planet's extreme heat, and the hydrogen escapes into space, leaving the planet with a comet-like tail.






















