The answer may not please those who believe such planetary systems are required for the existence of extraterrestrial intelligence.

"Our results seem to indicate that whatever happened when our solar system formed was not the norm," elsewhere in the galaxy, said B. Scott Gaudi, a graduate student in astronomy at Ohio State.

Gaudi has developed a new method for zeroing in on the likelihood that extra-solar planets exist. He and his colleagues at the Probing Lensing Anomalies NETwork (PLANET) collaboration have calculated that less than 45 percent of stars could harbor planets in a configuration similar to our solar system.

"While a couple of dozen extra-solar planets have been detected by other collaborations, these systems are very unlike our own, and probably not capable of sustaining advanced life forms," said Andrew Gould, associate professor of astronomy and Gaudi's advisor. "What Scott has shown is that when you look for systems we Earthlings would feel comfortable in, they are not that easy to find."

At the biannual meeting of the American Astronomical Society in Atlanta, Gaudi presented an analysis of two years' worth of PLANET data.

The eight institutions that make up the PLANET collaboration, including Ohio State, watch the skies for signs of gravitational lensing -- what happens when a massive dark object in space such as a dim star crosses in front of a luminous source star in the background. The dark object bends light rays emanating from the source, like a lens or magnifying glass.

Here on Earth, we see the star get brighter as the lens crosses in front of it, and then fade as the lens gets farther away. This is what astronomers call a "lensing event."

Gaudi explained that the collaboration's data closely matches astronomers' theories of what lensing events should look like. Given that, planets orbiting stars should reveal themselves as a warp in the gravitational lens -- a "blip" of extra brightness during a lensing event.

Other planet hunters use different techniques that can only detect planets as close to a star as Mercury is to ours, whereas gravitational lensing enables PLANET to see Jupiter-like planets at Jupiter-like distances from stars.

They haven't seen any blips yet. In its five years of operation, PLANET hasn't detected any planets.

"If you look really hard to find something, and you don't find it, there are two possible reasons why: either you're not looking hard enough, or it's not there," Gaudi said.

"The difference between these two possibilities depends on the efficiency with which gravitational lensing should reveal the existence of planets given their size, distance, and angle of alignment with a star as viewed from earth," he continued.

Without any detections, PLANET cannot point to an exact number of planets that do exist, but can put an upper limit on the number of such planets that can exist. Based on 23 lensing events that occurred in 1998 and 1999, Gaudi calculated that less than 30 percent of stars could have a Jupiter-like planet at distances between Earth's orbit and Jupiter's orbit.

"Radial velocity surveys are finding that about 10 percent of stars have planets close in," said Gaudi. "We can say that most don't have Jupiter-mass planets in the distance between Earth's orbit and Jupiter's. So more often than not there aren't Jupiter-mass planets around these stars at those distances."

With the current data, PLANET cannot put limits on how many identical twins of Jupiter exist. But the fact that PLANET hasn't found any planets three times the size of Jupiter or bigger at Jupiter-like distances suggests that Jupiter-like planets in general are not very common, Gaudi said.

"You might expect that if we have a Jupiter, then other solar systems probably have Jupiters three or even five times bigger," he said. "But the fact that we're not detecting any planets bigger than Jupiter probably indicates that there aren't many Jupiters out there."

"We're going to continue to push that limit down farther," Gaudi said. "Either we'll push it down to 10 percent, or we'll find something."

Other PLANET member institutions include: the Kapteyn Institute, Rijksuniversiteit Groningen, The Netherlands; Space Telescope Science Institute, Baltimore, Maryland; South African Astronomical Observatory, Capetown, South Africa; University of Canterbury, Christchurch, New Zealand; Perth Observatory, Bickley, Western Australia; and Canopus Observatory, Hobart, Tasmania, Australia.

PLANET operates telescopes at many different longitudes around the world, so members can watch the sky at all times, weather permitting. "We like to say the sun never rises on our collaboration," Gaudi smiled.