Aliens

But Neil definitely wishes it was.

Astronomers have found the first Earth-sized exoplanet within a star's habitable zone. The planet is the closest thing yet to the coveted 'Goldilocks' orb that scientists have long sought — a world roughly the size of Earth orbiting a star at a distance that is just right for liquid water to exist.

"We definitely think it's one step closer to finding a true Sun–Earth analogue," says study co-author Elisa Quintana, an astronomer at the SETI Institute in Mountain View, California, and at the nearby NASA Ames Research Center in Moffett Field. But because the star that the exoplanet orbits is a cool, dim one unlike the Sun, Quintana and her colleagues consider the planet more of a cousin to Earth than a twin.

Astronomers believe the rough, icy surface of Jupiter's moon, Europa, is the most likely place in the solar system to harbour alien life.

Now Nasa has set aside £14.6 million ($25 million) to design probes that could reveal whether Europa is, in fact, habitable.

The agency yesterday asked scientists to come forward with potential experiments for a Europa probe that could be launched in the 2020s and arrive at the icy satellite within three years of take-off.