Moons Like Earth's Moon are Rare in the Universe
by Rich Deem

Earth's Moon

The earth has a huge moon orbiting around it, which scientists now know 1) did not bulge off due to the earth's high rotational speed and 2) could not have been captured by the earth's gravity, due to the moon's large mass.1 Therefore, scientists hypothesize that the Moon was formed by a large impact between the earth and a large planetesimal early during the formation of the Solar System. A recent study examines the probability of such events occurring in other stellar systems.

Rich Deem


Proponents of intelligent design claim that the universe, Milky Way Galaxy, Solar System, and earth are specifically designed by an intelligent agent for habitation by human beings. Others say that such claims amount to a God of the gaps argument, since scientists do not yet have answers for the apparent design of the universe. If the skeptics of intelligent design are correct, then science should be finding that we do not occupy any special place in the universe. However, a new study disputes the idea that earth's moon is just an ordinary moon.

Formation of the Moon

Scientists now know that the Moon was formed by the collision of the small planet with the earth, which resulted in the ejection of 5 billion cubic miles of the earth's crust and mantle into orbit around the earth. This ring of material, the theory states, would eventually coalesce to form the moon. In addition, the moon is moving away from the earth (currently at 2 inches per year), as is predicted by the theory. If we calculate backwards, we discover that the moon must have formed just outside the Roche limit, the point at which an object would be torn apart by the earth's gravity (7,300 miles above the earth's surface). A collision which would have ejected material less than the Roche limit would have formed only rings around the earth. Computer models show that a collision of a small planet with the earth must have been very precise in order for any moon to have been formed at all.

The new study

When a moon is formed by the kind of collision that formed Earth's Moon, dust would have been blasted throughout the solar system. Astronomer Nadya Gorlova of the University of Florida, Gainesville, decided to study newly forming stars to determine what percentage might have a moon that formed through one of these massive collision events.2 In examining 400 newly formed stars (30 million years old), only one shared characteristics that would suggest that such a large collision had occurred, which might form a moon. Of course, the probability that the newly formed moon would orbit an earth like planet within the habitable zone would be much less likely.

Conclusion Top of page

The Priviledged PlanetSeveral earth-like planets have been detected outside our solar system. However, it is  unlikely that such earths would have a moon the size of Earth's moon, since such a pair could not coalesce together, nor could such a moon be captured by a small planet like the earth. Therefore, the only means by which such a moon could form would be through a collision event early in the history of stellar formation. In the first study to examine the probability of such a formation, a new study shows that only one in 400 new stars might have formed such a moon through a collision event. Why is it important how the moon was formed? Without such a large moon, earth's rotation axis would be unstable, swinging through 90° or more, resulting in extremely variable climate over its history. In addition, the collision event itself blew off earth's early heavy greenhouse atmosphere, which allowed the planet to retain its water for billions of years. The removal of much of the earth's crust also allowed the earth to retain its tectonic activity for billions of years, which allowed for the existence of both land and ocean. Without such tectonic activity, earth would have become and remained a waterworld. For more information, see The Incredible Design of the Earth and Our Solar System. Astronomy and cosmology continue to provide evidence that the earth is not just an ordinary planet in an ordinary solar system.

Rare EarthRare Earth: Why Complex Life is Uncommon in the Universe by Peter D. Ward and Donald Brownlee

A secular book (2000) that recognizes the improbable design of the earth. Paleontologist Peter D. Ward and astrobiologist Donald Brownlee examine the unusual characteristics of our galaxy, solar system, star, and Earth and conclude that ET may have no home to go to. Surprisingly, the authors conclude that the amazing "coincidences" are the result of good luck and chance.

The Creator and the CosmosThe Creator and the Cosmos by Dr. Hugh Ross

A classic book by astronomer Hugh Ross on modern Christian apologetics and science, (updated June, 2001). Dr. Ross presents the latest scientific evidence for intelligent design of our world and an easy to understand introduction to modern cosmology. This is a great book to give agnostics, who have an interest in cosmology and astronomy.

References Top of page

  1. Taylor, G. Jeffrey. July, 1994. "The scientific legacy of Apollo" Scientific American 271 (1): 40-47.
    Giant Impact Theory For Moon Formation Boosted (from
  2. Gorlova, N., Z. Balog, G. H. Rieke, J. Muzerolle, K. Y. L. Su, V. D. Ivanov, and E. T. Young. 2007. Debris Disks in NGC 2547. The Astrophysical Journal 670: 516-535.

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Last updated November 23, 2007


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