Sorry, E.T.: The Aliens May All Be Dead

"The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," lead author Aditya Chopra, of the Australian National University (ANU) in Canberra, said in a statement. In the first 500 million years or so of a wet, rocky planet's life, for example, it will be too hot and heavily bombarded to support life. There's a good chance that the planet will end up shifting from habitable to uninhabitable, as Venus and Mars apparently did, by roughly 1