Two U.S. astronomers have discovered a new planet outside our solar system, orbiting a star about 50 light years from earth, one of the researchers said on Wednesday.

Geoffrey Marcy, professor of physics and astronomy at San Francisco State University, said the unnamed planet, located in the last few weeks, was the fifth planet outside our solar system discovered in recent months. Three of those have been discovered by Marcy and Paul Butler, also of San Francisco State University. Marcy said the team found the latest planet using the Lick Observatory near San Jose, California.

The planet is orbiting a star, known as HR3522, that is about 50 light years away from the earth. "We're seeing light from the star as it was 50 years ago," Marcy told Reuters.

Marcy and Butler caused a stir in January when they announced they had discovered two new planets whose environments might be able to support life. But Marcy said the latest planet would be "certainly inhospitable to life as we know it. "It's much too hot, 500 degrees Centigrade on the surface, scorching hot ... any water would be in vapour," he said. The planet has a mass about 80 percent as large as that of Jupiter and is about nine million miles (14.48 million km) from its star, Marcy said.

The researchers discovered the planet as part of a 10-year-old research project "to make the initial reconnaissance of planets around other normal stars," Marcy said. He said that, until recently, astronomers did not know for sure of any planets orbiting nearby stars. Last year, a team at the Geneva Observatory in Switzerland discovered the first planet orbiting a normal star outside our solar system and then the California researchers found two more, Marcy said.

A fourth planet was located by a team at Harvard University, he said. Marcy said the researchers do not actually see the planets with their eyes or with the telescope. "A planet orbiting a nearby star would be about one billion times fainter than a star, so planets are just simply too dim," he said. Instead, the astronomers have developed a new technique to measure what he called "the reflex wobble of a star as it is tugged gravitationally by its attendant planet." "We watch for the star wobbling in space ... The light from a star changes its frequencies when the star is wobbling towards us or away from us, so in effect we are detecting or measuring a change in the pitch of light from a star," he said.

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