An exoplanet is a planet which orbits a star outside our solar system. The discovery of Gliese 581c caused excitement because the exoplanet's surface is the right temperature to support alien life. In 1995 astronomers were amazed to see over 3,000 galaxies when they pointed the Hubble Space Telescope to a dark corner of Ursa Major. If an average galaxy contains 100 billion stars, and there is a one per cent chance of a star being orbited by an Earth-like habitable planet, that's a pretty good chance of extra-terrestrial life.
The search for another Earth takes a huge step forward after the Kepler space mission discovers 11 new planets in the habitable zone of distant stars, with Kepler-452b dubbed our 'bigger, older cousin.'