The discovery of the planet, with an estimated radius about twice that of Earth, was announced on Monday at the National Science Foundation.

"This is the smallest extrasolar planet yet detected and the first of a new class of rocky terrestrial planets," Paul Butler of the Carnegie Institution in Washington said. "It's like Earth's bigger cousin."

Geoffrey Marcy, professor of astronomy at the University of California, Berkeley, added: "Over 2000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other Earth-like planets.

"Now, for the first time, we have evidence for a rocky planet around a normal star."

Rocky surface?

Although the researchers have no direct proof that the new planet is rocky, its mass means it is not a giant gas planet like Jupiter, they said.

"Over 2000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other Earth-like planets. Now, for the first time, we have evidence for a rocky planet around a normal star"

Geoffrey Marcy,
Professor of astronomy at the University of California, Berkeley

They estimate the planet's mass as 5.9 to 7.5 times that of Earth.

It is orbiting a star called Gliese 876, 15 light years from Earth, with an orbit time of just 1.94 Earth days.

They estimate the surface temperature on the new planet at between 204.44 and 398.89 degrees celsius.

Gliese 876 is a small, red star with about one-third the mass of the sun.

The researchers said that is the smallest star around which planets have been discovered.

Gas planets

In addition to the newly found planet the star has two large gas planets around it.

Butler said the researchers thought that the most probable composition of the planet was similar to inner planets of this solar system - a nickel/iron rock.

"This is the smallest extrasolar planet yet detected and the first of a new class of rocky terrestrial planets. It's like Earth's bigger cousin"

Paul Butler,
The Carnegie Institution

Gregory Laughlin of the Lick Observatory at the University of California, Santa Cruz, said a planet of this mass could have enough gravity to hold onto an atmosphere.

"It would still be considered a rocky planet, probably with an iron core and a silicon mantle. It could even have a dense steamy water layer."

Three other extrasolar planets believed to be of rocky composition have been reported, but they orbit a pulsar - the flashing corpse of an exploded star - rather than a normal type of star.