By Andy Sullivan

Washington - Scientists said on Thursday they have spotted an unusually large and light planet orbiting a distant star - a discovery that could force them to re-examine theories about how planets are formed.

"We have discovered a very bizarre new object," research astrophysicist Robert Noyes said at a news conference. "The people who do theoretical models are left scratching their heads as to what's going on."

The planet, dubbed HAT-P-1, is the largest planet ever detected, roughly one-third larger in diameter than Jupiter, but only weighs half as much, astronomers with the Smithsonian Astrophysical Observatory at Harvard University said.

Unlike Jupiter, Saturn and other "gas giants", this planet does not have a solid core, they said.

It is extremely close to its parent star, one-seventh of the distance between Mercury and the Sun, and it makes a complete orbit once every 4.5 days.

What is puzzling scientists is that the planet is larger than predicted by current theories.

The large size could be the result of heat coming from the interior of the planet but scientists have yet to determine how that could happen, Noyes said.

Scientists found the planet because light from the star dims when the planet passes in front of it.

The star, one of a double-star system, is about 450 light-years from Earth.

Another group of Smithsonian scientists said they had devised a way of determining if distant planets could harbour life by comparing the main gases in a planet's atmosphere with the historical makeup of the Earth's atmosphere.

An atmosphere with a high level of methane gas, like the Earth 3.5 billion years ago, could signal the presence of anaerobic bacteria that expel methane as waste. A high level of oxygen might indicate more sophisticated life forms.

Researchers have discovered about 200 planets outside the solar system so far, but currently lack the ability to detect smaller planets the size of the Earth. That's likely to change in coming years thanks to improved technology, the Smithsonian scientists said.

"We are at the beginning of a new stage of exploration of these types of objects," said Dimitar Sasselov, director of the Harvard Origins of Life Initiative.