The new planet is much bigger than Earth, but is a similar distance away from its sun, a star known as 55 Cancri, the astronomers said on Tuesday.

Four planets had already been seen around the star, but the discovery marks the first time as many as five planets have been found orbiting a solar system outside our own with its eight planets, said Debra Fischer, an astronomer at San Francisco State University.

Life could conceivably live on the surface of a moon that might be orbiting the new planet, but such a moon would be far too small to detect using current methods, the astronomers said.

'The star is very much like our own sun. It has about the same mass and is about the same age as our sun,' Mr Fischer told reporters.

'It's a system that appears to be packed with planets.' It took the researchers 18 years of careful, painstaking study to find the five planets, which they found by measuring tiny wobbles in the star's orbit. The first planet discovered took 14 years to make one orbit.

They said 55 Cancri is 41 light-years away in the constellation Cancer, a light-year being the distance light travels in one year - about 9.4 trillion km.

The newly discovered planet has a mass about 45 times that of Earth and may resemble Saturn, the astronomers said.

Harbouring life?,br> It is the fourth planet out from the star and completes one orbit every 260 days - a similar orbit to that of Venus.

'It would be a little bit warmer than the Earth but not very much,' said Jonathan Lunine, a planetary scientist at the University of Arizona.

The planet is 117 million km from its star - closer than the Earth's 149.7 million miles, but the star is a little cooler than our own sun.

'If there were a moon around this new planet ... it would have a rocky surface, so water on it in principle could puddle into lakes and oceans,' said Geoff Marcy, an astronomer at the University of California, Berkeley.

But the moon would have to carry a lot of mass to hold the water, he said. Water is, of course, key to life.

'This discovery of the first-ever quintuple planetary system has me jumping out of my socks,' Marcy added. 'We now know that our sun and its family of planets is not unusual.' Mr Marcy and other astronomers strongly believe that many stars are hosts to solar systems similar to our own. But small objects such as planets are very hard to detect.

Technology that would allow scientists to detect planets as small as Earth is decades away, the scientists agreed.

The researchers have been looking at 2,000 nearby stars using the Lick Observatory near San Jose, California, and the W.M. Keck Observatory in Mauna Kea, Hawaii.

The inner four planets of 55 Cancri are all closer to the star than Earth is to the sun. The closest, about the mass of Uranus, zips around the star in just under three days at a distance of 5.6 million km. -- REUTERS