Earth is the only planet known to hold liquid water across much of its surface.

The longer version

Liquid water needs a narrow range of temperature and pressure, and Earth happens to sit in the zone where sunlight delivers just enough warmth. Its atmosphere provides pressure that keeps water from boiling away or freezing solid across most of the surface. Too close to the Sun and oceans would vaporize; too far and they would freeze.

A magnetic field helps shield the atmosphere from being stripped, and the carbon cycle stabilizes temperature over long spans. Water is also plentiful because the planet formed where volatile materials could survive. Other worlds show signs of ice or subsurface water, but none matches Earth's surface oceans.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.