Earth's Moon is Rare Oddball
A gibbous moon visible in this view of Earth's horizon and airglow, photographed by an astronaut in 2007. Astronomers now estimate moons that form like our own--from a collision event--occur in only about one in 20 planetary systems in the universe.
Credit: NASA

The moon formed after a nasty planetary collision with young Earth, yet it looks odd next to its watery orbital neighbor. Turns out it really is odd: Only about one in every 10 to 20 solar systems may harbor a similar moon.

New observations made by NASA's Spitzer Space Telescope of stellar dust clouds suggest that moons like Earth's are—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, an astronomer at the University of Florida in Gainesville who analyzed the telescope data in a new study. "If there were lots of moons forming, we would have seen dust around lots of stars. But we didn't."

Gorlova and her team detail their findings in today's issue of the Astrophysical Journal.

Violent birth

Shortly after the sun formed about 4.5 billion years ago, scientists think a vagrant planet as big as Mars smacked into infant Earth and ripped off a chunk of our home's smoldering mantle. The rocky, dusty leftovers fell into orbit around our wounded planet, eventually coalescing into the moon we see today.

The scenario is unique among other moons in the solar system, which formed side-by-side with their planet or were captured by its gravity. Gorlova and her colleagues looked for the dusty signs of similar smash-ups around 400 stars, all about 30 million years old—roughly the age of our sun when Earth's moon formed.

Only one of all the stars they studied, however, displayed the telltale dust. Considering the frequency of planetary solar systems, the amount of time the dust should stick around and the window for moon-forming collisions to occur, the scientists were able to peg the frequency of extrasolar bodies that formed like our moon.

The estimate, however, is possibly a generous one.

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

Odd moon out?

Planetary scientists like Gorlova and Rieke think infant solar systems can form moons between 10 and 50 million years after a star forms. That only a single star with collision-generated dust could be found in their latest research, the astronomers said, indicates that the 30 million-year-old stars in the study have finished making their planets.

"Astronomers have observed young stars with dust swirling around them for more than 20 years now," said Gorlova, noting that the dust could be collision-derived or primitive planet-forming material. "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."

While most the our type of moon may be rare, astronomers think there are billions of rocky planets out there with plenty of moons orbiting around them. The upshot for lunar lovers? There could be millions—or billions—of Earth-like moons drifting through the cosmos.

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