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.

Astronomers found the new world using a technique called gravitational microlensing. This takes advantage of the fact that light is bent as the rays pass close to a massive object, like a star.

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.

A smaller planet than this one has been found orbiting a pulsar, a spinning neutron star which produces powerful beams of radiation.

Lead author David Bennett, from the University of Notre Dame, commented: "This is leading the way to finding lower mass planets, including Earth-mass planets, by microlensing.

He added: "It also encourages astronomers who search for planets in the habitable zones of very low-mass stars."

Water world?

The planet orbits its host star, or brown dwarf, with an orbital radius similar to that of Venus. But the host is likely to be between 3,000 and one million times fainter than the Sun, so the top of the planet's atmosphere is likely to be colder than Pluto. It would also be extremely dim if one were to stand on its surface.

Nicholas Rattenbury, a co-author from the University of Manchester and Jodrell Bank, told BBC News: "Our best ideas about how planets form suggests the planet could have quite a thick atmosphere. This atmosphere could act like a big blanket, keeping the planet warm.

"So even though there's very little energy coming from its host star, hitting the planet and warming it up that way, internal heat coming from within the planet could be warming up the surface.

"This has led to some speculation that there could, possibly, be a liquid ocean on the surface of this planet. The reason why that's exciting, is one of the properties we'd like to have on a habitable planet is liquid water on the surface."

MOA-2007-BLG-192Lb was found with the new MOA-II telescope at New Zealand's Mount John Observatory.

The technique employed to find the new planet uses the gravitational field of a star like a lens - magnifying the light from a distant background star. This effect occurs only when the two stars are in almost perfect alignment.

Astronomers are able to detect planets orbiting the lens star if the light from the background star is warped by one or more planets.

The team's measurements cannot distinguish whether the planet's host is a brown dwarf or a very low-mass hydrogen burning star called a red dwarf.