Billions of 'habitable planets'
2007-05-29 07:29
Washington - 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.
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.
"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 http://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 centimetre -
twice that of water - it must be 50% rock and about 50% 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 theorised 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% 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.