Three planets around distant stars have been found by scientists using a new high-precision system on the Anglo-Australian Telescope (AAT).

The new planets were found around nearby stars within 150 light-years of Earth. Forty-six other extrasolar planets have been found since 1995, with the most recent three being the first found by a telescope "down under."

Most planet searches have detected 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 Chris Tinney of the Anglo-Australian Observatory.

The smallest of the new trio is a kind planet hunters call a "hot Jupiter." It has a mass at least 84 percent that of Jupiter's but lies scorchingly close to its parent star, far closer than Mercury does to the Sun. Its "year," or the time it takes to make a single revolution around its star, is a mere three Earth days.

The middleweight planet lies in an Earth-like orbit inside the "habitable zone" where liquid water could exist. 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 also a 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. 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 Tinney.

"In three years you can catch only the short-period planets," he said. "To pick up ones with longer orbits you have observe for a few more years."

The AAT searchers also found a single brown dwarf, a small "failed-star star," in orbit around HD164427.

The AAT search complements searches of the northern sky being done by veteran planet hunters Geoffrey Marcy, Paul Butler and Michel Mayor.

Both these and the AAT search use the "wobble" technique. As an unseen planet 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 Alan Penny of Rutherford Appleton Laboratory in the United Kingdom.

The precision comes from simple glass tube containing specks of iodine, and "a bunch of clever software" written by Paul Butler, says 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," Butler said.

Along with Butler, of the Carnegie Institution of Washington, and Marcy, of UC Berkeley, Tinney worked to find the three planets with researchers from Liverpool John Moores University, Rutherford Appleton Laboratory, University of Sussex, University of Colorado, University of California Santa Cruz and Tennessee State University.

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. Present-day searches will provide target lists for SIM and the VLTI.

Is it worth finding more planets? Absolutely, says 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 said.

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