Earth is the only place in our Solar System with large oceans on its surface. This is because it lies in the 'Goldilocks' zone.
Venus is too near the Sun and too hot. Mars is too far away and too cold. Earth is just the right distance and the right temperature.
Could there be 'Earths' orbiting other stars? How would we recognise them? The first thing to do is to measure how hot or cold a planet is. There should also be evidence that water is plentiful.
Then we must look for gases such as oxygen and methane. These gases are released into the air by living creatures.
Future missions will look for these 'biomarkers' in thousands of star-planet systems. Although they will not detect oxygen directly, they will 'see' ozone, a form of oxygen. They will also see carbon dioxide, water, and, in certain cases, methane.
Last update: 11 November 2010