Earth-Like Planets May Be More Common Than Once Thought (Forwarded)

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University of Colorado-Boulder
Boulder, Colorado

Contact:
Sean Raymond, (303) 735-3729
Jim Scott, (303) 492-3114

Sept. 7, 2006

Earth-Like Planets May Be More Common Than Once Thought, Says
CU-Boulder-Penn State Study

More than one-third of the giant planet systems recently detected outside
Earth's solar system may harbor Earth-like planets, many covered in deep
oceans with potential for life, according to a new study led by the
University of Colorado at Boulder and Pennsylvania State University.

The study focuses on a type of planetary system unlike our solar system that
contains gas giants known as "Hot Jupiters" orbiting extremely close to
their parent stars -- even closer than Mercury to our sun, said CU-Boulder
researcher Sean Raymond. Such gas giants are believed to have migrated
inward toward their parent stars as the planetary systems were forming,
disrupting the space environment and triggering the formation of
ocean-covered, Earth-like planets in a "habitable zone" conducive to the
evolution of life, according to the new study.

"Exotic Earths: Forming Habitable Worlds with Giant Planet Migration" was
published in the Sept. 8 issue of Science and authored by Raymond, Avi
Mandell of both Penn State and Goddard Space Flight Center in Greenbelt,
Md., and Steinn Sigurdsson of Penn State.

The study indicates Hot Jupiters push and pull proto-planetary disk material
during their journeys, flinging rocky debris outward where it is likely to
coalesce into Earth-like planets, said Raymond. At the same time, turbulent
forces from the dense surrounding gas slow down the orbits of small, icy
bodies in the outer reaches of the disk, causing them to spiral inward and
deliver water to the fledgling planets. Such planets may eventually host
oceans several miles deep, according to the study.

"These gas giants cause quite a ruckus," said Raymond of CU-Boulder's
Laboratory for Atmospheric and Space Physics. "We now think there is a new
class of ocean-covered, and possibly habitable, planets in solar systems
unlike our own."

Scientists had previously assumed that as Hot Jupiters plowed through
proto-planetary material on their inward migrations toward parent stars, all
the surrounding material would be "vacuumed up" or ejected from the system,
he said. "The new models indicate these early ideas were probably wrong,"
said Raymond.

The research team ran exhaustive simulations lasting more than eight months
each on more than a dozen desktop computers, starting with proto-planetary
disks containing more than 1,000 moon-sized, rocky and icy bodies. The
initial conditions for each computer model were based on current theories of
how planets form in our own solar system and simulated about 200 million
years of planetary evolution.

The team concluded that about one of every three known planetary systems
could have evolved as-yet-undetected Earth-like planets in so-called
habitable zones like the one Earth is in, he said. A whopping 40 percent of
the 200 or so known planets around other stars are Hot Jupiters, although
the percentage probably will decrease as more distant planets are
discovered, said Raymond.

In addition to Earth-like planets that form in habitable zones outside Hot
Jupiters, the simulations showed some rocky planets known as "Hot Earths"
often form inside the orbits of Hot Jupiters, said Raymond. A Hot Earth,
with a radius twice that of our own Earth, was discovered in 2005 in a
nearby star system orbiting just 2 million miles from its parent star by a
team led by University of California, Berkeley, planetary scientist Geoffrey
Marcy.

The new simulations showed both Hot Earths and Earth-like planets in
habitable zones formed with large amounts of water, up to 100 times the
water present on Earth today, he said. The models indicate such water-rich
planets would probably contain a lower percentage of iron -- which may be
important for the evolution and possible oxygenation of evolving atmospheres
-- than Earth, he said.

According to the team's simulations, Hot Earths can form astoundingly fast,
in just 100,000 years or so. Earth-like planets in habitable zones form much
more slowly, taking up to 200 million years, said Raymond. Geologists
believe Earth took about 30 million years to 50 million years to fully form.

"I think there are definitely habitable planets out there," said Raymond.
"But any life on these planets could be very different from ours. There are
a lot of evolutionary steps in between the formation of such planets in
other systems and the presence of life forms looking back at us."

The new research effort may allow planet hunters to determine "rough limits"
indicating where to search for habitable planets in known systems of giant
planets, according to the team, whose research was funded by NASA's
Astrobiology Institute headquartered at the NASA Ames Research Center in
Moffett Field, Calif.

"Upcoming space missions such as NASA's Kepler and Terrestrial Planet finder
and ESA's COROT and Darwin will discover and eventually characterize
Earth-like planets around other stars," wrote the authors in Science. "We
predict that a significant fraction of systems with close-in giant planets
will be found to have a Hot Earth or potentially habitable, water-rich
planets on stable orbits in the Habitable Zone."

IMAGE CAPTION:
[http://www.nasa.gov/centers/goddard/...em_diff_lg.jpg
(29KB)]
Shown is a schematic view (not to scale) of a habitable planetary system
with a "hot Jupiter" (bottom row), compared with Earth's solar system (top
row). Earth's solar system has four terrestrial planets: Mercury Venus,
Earth and Mars, all between Jupiter and the sun. Earth is in the habitable
zone, where water can exist on its surface. The "hot Jupiter" system
contains a "hot Earth" a few times larger than our own Earth but very close
to the parent star, and an ocean-covered planet (in blue) in the habitable
zone just outside of the ringed giant. Three icy planets are shown in the
outer reaches of the "hot Jupiter" system. Credit and copyright: Sean
Raymond, 2006