Astronomers Say Moons Like Ours Are Uncommon

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

NEWS RELEASE: 2007-132 Nov. 20, 2007

Astronomers Say Moons Like Ours Are Uncommon

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.spitzer.caltech.edu/spitzer

and http://www.nasa.gov/spitzer .

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