Southern sky survey finds distant worlds

The "wobble technique" is biased towards large planets

By BBC News Online science editor Dr David Whitehouse

Three new planets orbiting Sun-like stars close to our Solar System have been discovered by an international team of astronomers.

The expectation is that we will eventually find lower-mass planets

Dr Alan Penny, Rutherford Appleton Lab

One of the planets, estimated to be about the size of Jupiter, lies in the so-called "habitable region" around its parent star where water could exist as a liquid.

The astronomers stress that this not an Earth-like object and is unlikely to host any sort of life - but if the planet has any rocky moons then they could have conditions that are more favourable.

The new exoplanets are the first to be found by the Anglo-Australian Telescope (AAT) in Australia and were picked up as part of a multinational effort to scan for such bodies among nearby stars in the southern sky.

Parent star

The discovery brings the total number of known exoplanets to about 50. The three new objects are all in the class of our Solar System's largest planet - Jupiter - as are most of the other far-off worlds that have been found so far.

Theories suggest large planets like Jupiter form far out from stars

Astronomers are able to detect the planets' gravitational influence on the motion of their parent stars - they wobble as the worlds orbit about them.

The nature of the technique and the technology available means it is not presently possible to detect Earth-sized planets.

"As a result, searches are picking up all the weird giant planets first," said Dr Chris Tinney of the AAT.

Hot Jupiter

The exoplanets were found around nearby stars within 150 light-years of Earth.

The smallest of the three is an object called a "hot Jupiter" because it sits just six million kilometres from its parent star, HD179949 in the constellation Sagittarius.

The discovery is a first for the Anglo-Australian Telescope

The planet, which has a mass that is 84% of our Jupiter, orbits the star in only three days.

Another of the new planets, which is almost twice as massive as Jupiter, circles a star, mu Ara, in the constellation Altar. The planet occupies an orbit a bit further than Mars is from our Sun.

But it is the third discovery that is really exciting astronomers: a gas giant slightly larger than Jupiter circling in an Earth-like orbit around epsilon Reticulum in the constellation of the Net. This exoplanet has a year that lasts 426 Earth days.

Planetary formation

Dr Alan Penny, from the Rutherford Appleton Laboratory, UK, is part of the AAT team. He explained the significance of the discoveries to BBC News Online: "Two of these planets are Earth- and Mars-distances from their stars, so we are now working our way out as the survey techniques get more sensitive.

"Present theories of planetary formation suggest that large planets form far out and, in a small number of cases, are driven in close to their suns - a process that didn't happen in our Solar System.

"We find these very close Jupiters in about 3% of stars and another few percent have Jupiters at Mars and Earth distances.

"Theories suggest nearly all stars should have planets about them, and we are finding planets where we look for them - so the expectation is that we will eventually find lower-mass planets and Jupiter-like planets at Jupiter-like distances. But it will take more time."