Planet discovery opens up worlds

A revolutionary star-gazing technique pioneered by New Zealand astronomers that may unlock the secrets of the Milky Way's solar systems has discovered a planet.

Astronomers confirmed yesterday that a mass first detected last May was a super-Earth: a low-mass planet with a solid surface, rather than a gaseous giant such as Jupiter or Saturn.

The planet is about 9000 light years from Earth, is 13 times as heavy and, at -193C, is one of the coldest planets discovered outside our solar system.

The discovery is a step closer to finding an Earth-like planet in the Milky Way.

"The new planet is Neptune-sized and icy but unlikely to be covered by a thick layer of gas like Neptune," said astronomer and Auckland University science professor Philip Yock. "Instead it may be more akin to a large, chilly version of our own Earth with a rocky, icy interior ... Ten years ago, such a finding would have been unthinkable."

Dr Yock was part of the group of astronomers based in New Zealand, the United States and Chile that discovered the planet using a phenomenon first thought of by Albert Einstein, then refined and pioneered by New Zealand astronomers Dr Yock, Ian Bond and Nicholas Rattenbury.

Known as gravitational microlensing, when a massive object, such as a star, crosses in front of another star, the object's strong gravity bends the light rays from the more distant star and magnifies them like a lens. The magnification can be up to 1000 times, which increases the chances of discovering a smaller planet similar to Earth.

Dr Yock said the planet's system is among the first of its kind to be discovered because astronomers have never before had the means to find it.

"Revealing Neptune-like planets orbiting these dots in the sky is a step on the road to finding another planet like Earth and counting how many there are out there."

Astronomer Grant Christie, who helped monitor the planet last year from Auckland's Stardome Observatory, said microlensing would reveal how solar systems are arranged.

"Years down the track and looking back, microlensing will be what opened the window into what the typical arrangement of planets around most stars are like."

Four Neptune-like planets have been discovered using microlensing, three in the past year. About one-third of all stars are thought to have super-Earths orbiting them.