Tiniest extrasolar planet discovered

Astronomers have sighted the smallest extrasolar planet yet orbiting a normal star - a distant world just three times the size of our own.

Discovering a planet with a similar mass to that of Earth is considered the "holy grail" of research into planets that lie outside our Solar System.

It is vital because researchers want to find other worlds that could host life.

The planet orbits a star which is itself of such low mass it may in fact be a "failed star", or brown dwarf.

The planet, called MOA-2007-BLG-192Lb, is about 3.3 times the size of Earth. Some researchers have suggested the planet could have a thick atmosphere and have even speculated there could be a liquid ocean on its surface.

Nasa's planned James Webb Space Telescope, due to launch in 2013, could search for signatures of life on Earth-mass planets orbiting low-mass stars in the vicinity of the Sun.