Water

The Earth formed about 4.5 billion years ago, too close to the Sun and too hot for water vapour to condense. Water ice could form only further out, so most of Earth’s water is thought to be extraplanetary, delivered by asteroids. Some carbonaceous chondrite meteorites hold water in their minerals, with a deuterium-to-hydrogen ratio close to that of seawater. Earth kept its water because its gravity holds water vapour and an atmosphere, and it is cool enough for vapour to condense into oceans. Smaller Mars lost most of its atmosphere and surface water.

All known life needs liquid water as a solvent, so the search for life elsewhere begins with water. The Goldilocks (habitable) zone is the range of distances from a star where a planet’s surface could hold liquid water. Astronomers detect water in the atmospheres of distant planets by transit spectroscopy. Being in the zone does not guarantee life: a planet also needs enough mass, an atmosphere and time. Moons such as Europa may hold oceans under ice even outside the zone.