Lunar Moons Uncommon in the Universe

Posted by Joshua S. Hill

November 22, 2007

For me personally, I have been captivated by the evolution and history behind our own moon. It has for so long been an intrinsic part of the human way of life, that at times it is just gone unnoticed. Wolves have howled at it for millennia, it was Kennedy’s big dream, and it has long been an object of poetry, music and myth.

But only recently have I become aware of its true history. It did not just magically appear out of the big bang, along with our sun and our planet Earth. Granted, there could be an argument made that it did appear out of our planet Earth.

We know now that a Mars sized object, most likely a planet, occasionally named Orpheus or Theia, collided with a very young Earth. Amidst its debris came together what is now our own Moon, while the rest of the debris floated in to the asteroid belt or the sun.

But according to NASA’s Spitzer Space Telescope, there are very few moons alike to our own inhabiting the universe.

Apparently, they are so uncommon that they appear 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 November 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.”

The study looked at around 400 stars that all roughly log in at 30 million years old – approximately the same age our own sun was when Earth’s moon formed. In those 400 stars, only one was surrounded by the telltale dust that would suggest a collision had occurred, large enough to create a Lunar-style moon.

“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.

On top of the lack of Lunar-style moons, the researchers also found that the planet-building process had, by the 30 million year mark, wound down. The current belief is that the messy collisions that formed our own solar systems rocky planets occur between 10 and 50 million years after a star forms. However, that only 1 planet in the 400 showcased the dust from such collisions, it suggests that by 30 million years, the process has slowed to a halt.

“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.”

Luckily, for the moon obsessed, take heart. Five or ten percent of billions, is still a fair few moons for you to gaze at.

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