[meteorite-list] Astronomers Say Moons Like Ours Are Uncommon

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 20 Nov 2007 14:10:01 -0800 (PST)
Message-ID: <200711202210.OAA05409_at_zagami.jpl.nasa.gov>


Astronomers Say Moons Like Ours Are Uncommon
Jet Propulsion Laboraboty
November 20, 2007

The next time you take a moonlit stroll, or admire a full, bright-white
moon looming in the night sky, you might count yourself lucky. New
observations from NASA's Spitzer Space Telescope suggest that moons like
Earth's - that formed out of tremendous collisions - are uncommon in the
universe, arising at most in only 5 to 10 percent of planetary systems.

"When a moon forms from a violent collision, dust should be blasted
everywhere," said Nadya Gorlova of the University of Florida,
Gainesville, lead author of a new study appearing Nov. 20 in the
Astrophysical Journal. "If there were lots of moons forming, we would
have seen dust around lots of stars - but we didn't."

It's hard to imagine Earth without a moon. Our familiar white orb has
long been the subject of art, myth and poetry. Wolves howl at it, and
humans have left footprints in its soil. Life itself might have evolved
from the ocean to land thanks to tides induced by the moon's gravity.

Scientists believe the moon arose about 30 to 50 million years after our
sun was born, and after our rocky planets had begun to take shape. A
body as big as Mars is thought to have smacked into our infant Earth,
breaking off a piece of its mantle. Some of the resulting debris fell
into orbit around Earth, eventually coalescing into the moon we see
today. The other moons in our solar system either formed simultaneously
with their planet or were captured by their planet's gravity.

Gorlova and her colleagues looked for the dusty signs of similar
smash-ups around 400 stars that are all about 30 million years old -
roughly the age of our sun when Earth's moon formed. They found that
only 1 out of the 400 stars is immersed in the telltale dust. Taking
into consideration the amount of time the dust should stick around, and
the age range at which moon-forming collisions can occur, the scientists
then calculated the probability of a solar system making a moon like
Earth's to be at most 5 to 10 percent.

"We don't know that the collision we witnessed around the one star is
definitely going to produce a moon, so moon-forming events could be much
less frequent than our calculation suggests," said George Rieke of the
University of Arizona, Tucson, a co-author of the study.

In addition, the observations tell astronomers that the planet-building
process itself winds down by 30 million years after a star is born. Like
our moon, rocky planets are built up through messy collisions that spray
dust all around. Current thinking holds that this process lasts from
about 10 to 50 million years after a star forms. The fact that Gorlova
and her team found only 1 star out of 400 with collision-generated dust
indicates that the 30-million-year-old stars in the study have, for the
most part, finished making their planets.

"Astronomers have observed young stars with dust swirling around them
for more than 20 years now," said Gorlova. "But those stars are usually
so young that their dust could be left over from the planet-formation
process. The star we have found is older, at the same age our sun was
when it had finished making planets and the Earth-moon system had just
formed in a collision."

For moon lovers, the news isn't all bad. For one thing, moons can form
in different ways. And, even though the majority of rocky planets in the
universe might not have moons like Earth's, astronomers believe there
are billions of rocky planets out there. Five to 10 percent of billions
is still a lot of moons.

Other authors of the paper include: Zoltan Balog, James Muzerolle, Kate
Y. L. Su and Erick T. Young of the University of Arizona, and Valentin
D. Ivanov of the European Southern Observatory, Chile.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer
Space Telescope mission for NASA's Science Mission Directorate,
Washington. Science operations are conducted at the Spitzer Science
Center at the California Institute of Technology, also in Pasadena.
Caltech manages JPL for NASA.

For more information about Spitzer, visit http://www.nasa.gov/spitzer
and http://www.spitzer.caltech.edu/spitzer .


Media contact: Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.

Received on Tue 20 Nov 2007 05:10:01 PM PST

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