S1 • E5 Dec 17, 2007 60m
8.7
Our planet is unique within the solar system. Four-and-a-half billion years ago it had a 'twin' named Theia which was absorbed into the Earth, increasing its gravity and allowing it to form an atmosphere. Iain travels to Meteor Crater in Arizona to explore the atmosphere's role in protecting us from bombardment by meteorites, and finds that life on earth only prospers because it is provided with right amount of heat from the sun.
