Retired astronaut Chris Hadfield chats about the existence of aliens and extraterrestrial life. Hadfield is the first Canadian astronaut to walk in space and the best-selling author of "An Astronaut's Guide to Life on Earth." Produced by Christine Nguyen and Will Wei. Additional camera by Alex Kuzoian. Follow TI: On Facebook

Many other planets throughout the universe probably hosted intelligent life long before Earth did, a new study suggests. The probability of a civilization developing on a potentially habitable alien planet would have to be less than one in 10 billion trillion - or one part in 10 to the 22nd power - for humanity to be the first technologically advanced species the cosmos has ever known, according to the study. "To me, this implies that other intelligent, technology-producing species very likely have evolved before us," said lead author Adam Frank, a professor of physics and astronomy at the University of Rochester in New York. [13 Ways to Hunt Intelligent Alien Life] "Think of it this way: Before