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