Planet Earth - a water world
Seen from spacecraft or from the Moon, the Earth appears as a blue planet – an oasis in space. This is because about seven tenths of its surface is covered by oceans. When the water is heated by the Sun, it changes to a gas (known as water vapour). If this gas is cooled, it turns back to water droplets, forming clouds and rain.
This water cycle is made possible because Earth lies 150 million km from the Sun, in the 'Goldilocks' zone of the Solar System. As the third planet from the Sun, Earth is neither too hot nor too cold for oceans to exist. The temperature is also just right for life. And it has stayed that way for billions of years.
Last update: 6 December 2004