WASHINGTON (Reuters) - Planet-seekers who have spotted 28 new planets orbiting other stars in the past year say Earth’s solar system is far from unique and there could be billions of habitable planets.

An artist's rendering of the planet orbiting the star, Gliese 436 is seen May 28, 2007. Planet-seekers who have spotted 28 new planets orbiting other stars in the past year say Earth's solar system is far from unique and there could be billions of habitable planets. REUTERS/ Handout/Lynette Cook

The most recent planet discoveries bring the number of known exoplanets -- planets outside our solar system -- to 236, the researchers told a meeting of the American Astronomical Society in Honolulu on Monday.

“We are beginning to see that our home is not a rarity in the universe,” said Geoffrey Marcy, a professor of astronomy at the University of California Berkeley, who led the team.

“We are easily able to detect giant planets like Jupiter and Saturn around other stars. Most orbit far from the star like our own Jupiter and Saturn orbit from the sun,” Marcy said in a telephone interview.

“It’s a common structure among planetary systems.”

New techniques allow astronomers to detect planets that are not enormous although Earth-sized objects cannot yet be seen, said the researchers, who have posted details of their findings on the Internet at exoplanets.org.

Four of the systems also have multiple planets, like Earth’s own with its sun, eight planets (Pluto was demoted from planet status) and smaller orbiting objects.

“We are finding that most stars have not just one planet but when we find one there is a second or a third or a fourth,” Marcy said.

“The ... attribute which really has us the most excited is this new planet which we found three years ago,” Marcy said. The Neptune-like planet orbiting the star Gliese 436 has intrigued scientists because it appears to be covered with water -- albeit rock-hard, hot water in a most un-Earthlike chemical state because of the intense pressures on the planet.

“JUMPING OUT OF OUR CLOTHES”

Earlier this month, Swiss and Belgian researchers imaged the star as this planet crossed between it and the Earth. The tiny change in the star’s light gave them the planet’s diameter and density.

“From the density of two grams per cubic centimeter -- twice that of water -- it must be 50 percent rock and about 50 percent water, with perhaps small amounts of hydrogen and helium,” Marcy said.

“Now we are very sure it has a rocky core and this giant thick envelope of water,” he added.

“This is why we are jumping out of our clothes. It is the first time we have determined the structure of one of these extrasolar planets. It is rocky like Earth but it has a lot of water which is the essential ingredient for life.”

This is almost certainly happening over and over again, Marcy said. Scientists had theorized this for decades but now the hard evidence is starting to pour in.

“Our Milky Way galaxy has 200 billion stars. I would estimate that 10 percent of them, perhaps, have planets that are habitable,” Marcy said.

“There are hundreds of billions of galaxies, all of which are more or less like our Milky Way Galaxy, which is tens of billions of planets like our own.”

There is one unusual property to our solar system: the nearly circular orbits of the planets, which gives a consistent dose of radiation from the Sun.

Other solar systems seen so far are not usually like this. “Most of the planets are not in circular orbits around the host star but in elongated ones called elliptical orbits,” Marcy said.

“We enjoy nearly constant temperatures throughout the year,” he added. “If the Earth got too close to the Sun, the Earth would heat up, the water would boil off and that would be bad.” Too far, and it would freeze.

“An elongated orbit could not sustain life,” Marcy said.