Is the Earth unique, and if so, why? To find an answer, scientists have had to explore the Solar System, searching for clues about our planet's birth. Uniquely amongst the terrestrial planets, the Earth has retained liquid water on its surface for over 4 billion years, despite a steady increase in the Sun's heat output.This water has had a profound influence on the planet's geological activity, as well as being a breeding ground for life. But living organisms may have played a crucial role in ensuring that liquid water exists on Earth, linking the planet's geology and biology tightly together.

• Aubrey Manning