Astronomers scanning distant stars have detected six more massive planets, five of which orbit their suns at just the right distance to support liquid water and--theoretically--life, scientists announced today.

The discoveries brought to 28 the total number of "extrasolar" planets found over the past five years as astronomers survey hundreds of stars similar to Earth's sun for signs they may have planets in tow.

Each set of new discoveries helps to hone scientific theories about the development of our own solar system and the possibility that Earth-like planets may be found.

"Planet hunting is a lot like making wine," said Steven Vogt of the University of California at Santa Cruz, one of the team of astronomers collecting and analyzing data provided by Hawaii's Keck I Telescope, the sharpest optical telescope in the world.

"You have got to plant the grapes, you have got to be patient, and at some point they ripen and are ready for harvest," Vogt said. "We have a lot of star systems that we have been looking at, and now are ready for harvest."

None of the new planets unveiled today are anything like Earth. Instead, they are gaseous giants ranging in size from slightly smaller to several times larger than Jupiter, the largest planet in our own solar system. And, like Jupiter, they were seen as extremely inhospitable to life themselves--made up of swirling masses of helium and hydrogen gases.

But five out of the six are in what astronomers call the "habitable zone," which could allow the existence of liquid water, a prerequisite for life. This makes them different from most of the extrasolar planets found before this, which have been either too hot or too cold.

Hopes for life in these star systems would focus on possible moons of the giant gas planets, Vogt said. Like Jupiter and Saturn, which have their own rocky satellites, moons of these planets could conceivably harbor liquid water and therefore life, he said.

Besides Vogt, the discovery team also included Geoffrey Marcy of the University of California at Berkeley, Paul Butler of the Carnegie Institution of Washington, D.C., and Kevin Apps of the University of Sussex in England. Their findings will be published in the Astrophysical Journal.

Planet-hunting astronomers do not actually see new planets, but rather detect their presence by watching for a telltale wobble in the stars they orbit; the wobble is caused by the gravitational pull the planets exert on the star.