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Found: Second smallest planet outside solar system

world Updated: Jan 14, 2010 15:11 IST

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Astronomers have detected an extrasolar planet with a mass just four times that of Earth.

The planet, which orbits its parent star HD156668 about once every four days, is the second-smallest world among the more than 400 exoplanets (planets located outside our solar system) that have been found to date.

It is located approximately 80 light-years from Earth in the direction of the constellation Hercules.

The planet was found using the highly sensitive 10-metre Keck I telescope atop Hawaii's Mauna Kea and made possible through NASA's Eta-Earth Survey for Low-Mass Planets.

It was announced at the 215th American Astronomical Society meeting in January in Washington.

Dubbed HD 156668b, the planet - a so-called "super Earth" that would glow with blast furnace-like temperatures - offers a tantalising hint of discoveries yet to come.

Astronomers hope those discoveries will include Earth-size planets located in the "habitable zone", the area roughly the distance from the earth to the sun and thus potentially favourable to life.

The discovery of low-mass planets like HD 156668b has become possible due to the development of techniques to watch stars wobble with increasing clarity and of software that can pluck the signals of increasingly smaller planets from amid the 'noise' made by their pulsating, wobbling parent stars.

The discovery of a planet that is comparable in size to Earth and found within the habitable zone, however, "will require a great deal of work", says John A. Johnson, assistant professor of astronomy, California Institute of Technology (Caltech), who is co-discoverer of the planet.

"If we could build the best possible radial-velocity instrument tomorrow, we might have answers in three years, and a solid census of Earthlike planets within a decade. We'll need gigantic leaps in sensitivity to get there, and we're hot on the trail," said Johnson, according to a Caltech release.