Could human engineering stabilise the Earth's climate and chemistry in the long term? Tim Lenton of Exeter University explains why the Gaia hypothesis is the key to understanding the future of life on Earth. But what about life beyond Earth? Justin Rowlatt speaks to astronomer Lisa Kaltenegger - a hunter and explorer of planets outside our solar system - and to the science fiction author David Brin. Plus paleobiologist Jan Zalasiewicz describes what might remain of human civilisation in the geological record 100 million years hence.