The image compares the orbits of the four new planets - each discovered around its own star - with the orbits of the inner Solar System planets. Like the planets of our own solar system, epsilon Reticulum and HD179949 lie on nearly circular orbits. In comparison the brown dwarf HD164427 and mu Ara lie on very elongated orbits. If mu Ara lay in our own Solar system it would swing between the orbits of the Earth and Mars once every year. Image credit: C.Tinney, AAO.
Forty-six other extrasolar planets have been found since 1995. None are believed to be capable of supporting life.
Most planet searches have been able to find only planets more massive than Jupiter, the largest planet in our Solar System. "As a result searches are picking up all the weird giant planets first," says team leader Dr Chris Tinney of the Anglo-Australian Observatory.
The new planets were found around nearby stars within 150 light-years of Earth.
The smallest is a kind planet hunters call a 'hot Jupiter'. It has a mass at least 84% that of Jupiter's but lies scorchingly close to its parent star, far closer than Mercury does to the Sun. Its 'year' is a mere three Earth days.
The middle-weight planet lies in an Earth-like orbit inside the 'habitable zone' where liquid water could exist. But the planet itself is not Earth-like: weighing at least 1.26 Jupiter masses it is almost certainly a Jupiter-like gas giant. It takes a leisurely 426 days to complete the voyage around its star, epsilon Reticulum in the constellation of the Net.
The third planet is another gas giant, of at least 1.86 Jupiter masses. Its orbit extends just a bit further from its star than Mars does from the Sun and it takes 743 days to crawl around its star, mu Ara, in the constellation of the Altar.
Since 1998 the AAT search has looked at 200 nearby stars in the southern sky. There are probably more planets in the pipeline, says Dr Tinney.
"In three years you can catch only the short-period planets," he explains. "To pick up ones with longer orbits you have observe for a few more years."
The AAT search complements searches of the northern sky being done by veteran planet hunters Drs Geoffrey Marcy, Paul Butler and Michel Mayor.
Both these and the AAT search use the 'Doppler wobble' technique. As an unseen planets orbits a distant star it tugs on it, causing the star to move back and forth in space. That wobble can be detected by the 'Doppler shift' it causes in the star's light.
"The AAT search is the most sensitive search in the Southern Hemisphere," says team member Dr Alan Penny of Rutherford Appleton Laboratory in the UK. "It can detect planets moving at only 10 ms**-1 -- the speed of a world-class sprinter."
The precision comes from simple glass tube containing specks of iodine, and "a bunch of clever software" written by Dr Paul Butler, says Dr Tinney.
Heating the glass cell turns the iodine to a purple gas. Starlight passing through the gas has its spectrum modified. This 'reference' spectrum is then compared with unmodified starlight. "This helps us get much of the junk out of the spectrum," Dr Butler explains.
Seeing wobbling stars directly is the next step in planet hunting. That job will fall first off to the Very Large Telescope Inteferometer (VLTI) now being built in Chile and NASA's Space Interferometry Mission (SIM), due to launch in 2009. SIM will spend five years probing nearby stars for Earth-sized planets. "The AAT will provide target lists for the VLTI and SIM," says Dr Tinney.
Is it worth finding more planets? Absolutely, says Dr Butler. "It will be at least five years before we find enough planets to even begin making sensible guesses about the whole population out there."
But the planets found to date are so different from those in the Solar System that theories of planet formation have been "turned on their head," he adds.
The members of the AAT planet search team are: from Australia, Dr Chris Tinney (Anglo-Australian Observatory); from the UK, Drs Hugh R. A. Jones (Liverpool John Moores University), Alan J. Penny (Rutherford Appleton Laboratory) and Mr Kevin Apps (University of Sussex); and from the US, Drs R. Paul Butler (Carnegie Institution of Washington), Geoffrey W. Marcy (University of California Berkeley), Steven S. Vogt, (University of Colorado and University of California Santa Cruz) and Gregory W. Henry (Tennessee State University).