Astronomers Say Moons Like Ours Are Uncommon

PureInsight | November 25, 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."

How Many Moons Like Ours Are Out There? Our Earth-moon system,
photographed here by NASA's Galileo spacecraft in 1992, might be
somewhat uncommon in the universe. New evidence from NASA's Spitzer
Space Telescope suggests that moons that formed like ours -- out of
colossal collisions between rocky bodies -- might arise in, at most, 5
to 10 percent of planetary systems.
Credit: NASA/JPL-Caltech

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.

Planetary Demolition Derby: This artist's animation shows bodies as big
as mountain ranges smashing together. Such collisions form the basis of
the planet-building process. An even bigger collision between Earth and
a body the size of Mars is thought to have created our moon
NASA/JPL-Caltech/T. Pyle (SSC-Caltech)

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.


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