Seen from space, the Earth is blue. The Earth has been blue for over 4 billion years because of the liquid water on its surface. How has the Earth managed to sustain liquid water on its surface for such a long time? There is only one known planet with permanent bodies of liquid water at its surface: ours. Earth sciences allow us to explain why the Earth has almost always been blue: it’s neither too warm nor too cold. If the Earth was first red and black, it has been blue for more than 4 billion years, with rare exceptions when it got too cold and turned into a white snowball.

Source: The Conversation