We live at the edge of the Milky Way Galaxy, which is home to between 100 billion and 400 billion stars, each potentially orbited by planets. There are probably at least two trillion other galaxies like ours in the observable universe, each one populated by trillions of planets orbiting hundreds of billions of stars. So, even if planets that are capable of supporting life are rare, and just 0.1 percent of them exist (around a million planets), there should be intelligent life somewhere in the universe. This conundrum prompted Nobel Prize-winning physicist Enrico Fermi to ask: "Where are they?" and has become known as the Fermi paradox. One answer to the question is the "Great Filter" hypothesis, which posits that before intelligent life has the opportunity to escape the bounds of its original planet, it hits some sort of wall. Some scientists believe climate change could be that wall for us, eventually leading to our extinction before contact with alien lifeforms can be made. To find out more go to The reason some scientists think we've never found aliens implies humans could go extinct sooner than we realise.