Australia finds "Habitable Zone" extrasolar planet
Australia has discovered three new extrasolar planets, including one in the "Habitable Zone" of its sun-like star.

The planet makes an almost circular orbit around epsilon Reticulum at approximately the same distance as Earth is from the sun. Its year is equal to 426 days.

The "Habitable Zone" is where liquid water - considered necessary for life - could exist. However this planet is probably a gas giant, weighing in at 1.26 Jupiter masses.

The three new extrasolar planets were found by a team of scientists from the Anglo-Australian Observatory and nine institutions in the UK and USA. They were using a new high-precision system on the four-metre Anglo-Australian Telescope near Coonabarabran in New South Wales.

All three are the size of our solar system's largest planet - Jupiter - as with most of the other extrasolar planets that have been discovered. Instrumentation does not allow small rocky planets like Earth to be detected as yet. "As a result searches are picking up all the weird giant planets first," said team leader Dr Chris Tinney of the AAT.

The smallest of the three is in a class of extrasolar planets known as 'hot Jupiters' because they are extremely close to their star and orbit it at an incredible pace. This latest one has a mass of at least 84% of that of Jupiter and lies far closer to its sun, HD179949 in Sagittarius, than our innermost planet, Mercury. Its year is just three days long.

The last of the three is another probable gas giant of at least 1.86 Jupiter masses. Its orbit extends a little further than Mars does from the sun and takes 743 days to make an elliptical orbit about its star, mu Ara.

Since 1998 the AAT search has looked at 200 nearby stars in the southern sky. There are probably more planets in the pipeline. "In three years you can catch only the short-period planets," said Dr Tinney.

The technique depends on detecting the pull a planet exerts on its star as it makes its orbit, causing the star to 'wobble'.

Seeing wobbling stars directly is the next step in planet hunting. It will need sizeable instruments such as the Very Large Telescope Interferometer now being built in Chile and space misions like NASA's planned Space Interferometry Mission due to launch in 2009.

But do we need to find more planets? Absolutely says Dr Paul Butler, a member of the team. "It will be at least five years before we find enough planets to even begin making sensible guesses about the whole population out there."

The three new planets - the first to be discovered from Australia - have brought the total number of extrasolar planets found since 1995 to 49.