Earth's closest relative found by astronomers

WASHINGTON - Earth's bigger, hotter planetary cousin may have been detected orbiting a star in our cosmic neighbourhood, say astronomers.

The most Earth-like of all 155 so-called extra-solar planets found orbiting stars besides our sun, the newly unveiled planet is probably rocky like Earth, rather than big and gassy like Jupiter and most other extra-solar planets discovered in the past decade, the scientists said at a briefing this week.

"It took 150 observations of this star to glean the existence of this Earth-like planet," said planet-hunter Geoffrey Marcy, of the University of California, Berkeley.

"This will definitely be one of our favourite stars from now on."

Jack Lissauer, of Nasa's Ames Research Centre, said: "It's a very unearthly world.

"It's likely to be the first rocky planet orbiting a star like our sun."

Paul Butler, of the Carnegie Institution in Washington, described it as "like Earth's bigger cousin".

Butler is a member of the team who found the planet.

It is the smallest extra-solar planet yet detected, with about 7.5 times the Earth's mass.

Before this, the smallest planets found orbiting stars besides the sun were at least 15 times Earth's mass, making them more like distant, icy Neptune.

By contrast, this planet's surface is far too hot - ranging from 200C to almost 400C - to support life as it is known on Earth.

While Earth orbits the sun at a distance of about 150 million km, this extra-solar planet almost hugs its star, orbiting about 3.2 million km from the star Gliese 876 in the constellation Aquarius, just 15 light-years from Earth.

A light-year is about 10 trillion km, but in astronomical terms, 15 light-years is just round the corner.

The planet races round its star once every two Earth days or so, and has a radius about twice the size of Earth, the astronomers said.

The star it orbits is about one-third the sun's mass.

Like dozens of other extra-solar planets detected over the past 10 years, this one was first discovered by the distinctive wobble its gravity exerts on the star it orbits. As it happened, this is the third planet circling that particular little star.

The two others are gas giants like Jupiter and orbit much further out.

Astronomers have no direct proof the planet is a space rock like Earth, but they say its low mass means it probably is not a giant gas planet like Jupiter.

Planet-hunters have detected three other rocky planets outside our solar system, but all three orbit a pulsar - the remnant of an exploded star.

Marcy said a team of scientists had known about the new planet for three years but wanted to confirm their theory that it was a terrestrial planet before going public.

Planet X

Compared to Earth, the newly discovered planet is:

* Bigger - 7.5 times the Earth's mass

* Hotter - surface temperature 204C to 371C

* Closer to its star - 3.2 million km, vs the 150 million km between Earth and the sun

- REUTERS