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Are we alone?
Planet Earth as seen by the Apollo 17 astronauts
Planet Earth as seen by the Apollo 17 astronauts

Telltale signs

Earth is the only place we know that supports life. What makes it so special? One key factor is the presence of liquid water. Life on Earth seems to have begun in pools or seas. No life on our planet can survive without water.

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.
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