Geography series with Dr Iain Stewart. 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. Life on earth only prospers because it is provided with right amount of heat from the sun.

Originally released by BBC Worldwide Learning, 2007. Streaming video file encoded with permission for digital streaming by Infobase on November 21, 2019.