Yet more evidence suggests that a moon like ours is rare. Oh, moons are found aplenty around planets, but they tend to be rather small in comparison to the planet they orbit, and most of them are captured. Our Moon, however, was not captured, and is very large in comparison to the Earth. The ratio of the difference in size is so small that some astronomers propose calling the Earth-Moon system a double planet system.
Around 4.5 billion years ago, a large Mars-sized planet collided with the Earth in such away that part of the Earth's mantle was thrown into space. The Mars-sized impactor was absorbed by the Earth, it's iron core joining the Earth's. It also increased the Earth's spin. Most of the section of mantle that was thrown into space coalesced quickly into the Moon (may have taken as little as a year). The rest of the debris that was "kicked up" by the collision, and which did not coalesced, continued to circle the sun for millions of years before gravity and solar radiation cleaned it up.
Again, why is this important information, and included in this blog?
Well, as reported before in this blog (Astronomers Say Moons Like Ours Are Uncommon - How Important is That?), some scientists feel that the Moon was essential to the origin of life on the Earth due to the tidal mixing which helped to mix, mainly from erosion caused by the tides, chemicals from the soil with the oceans, creating the chemical soup from which life arose. This is especially true for the young Earth when the Moon was closer and had a much more dramatic effect on the tides. Only 1/3 of the today's tidal effects is attributed to the Sun, the rest to our Moon.
In the original report, it was stated that maybe only 5 to 10% of planets would a moon like Earth's. However, a new report by Nadya Gorlova of the University of Florida in the November 2007 issue of The Astrophysical Journal, states that the percentage may even be smaller: 1 in 400. As reported by Physics News:
Using the cryogenically-cooled infrared orbiting Spitzer Space Telescope, Gorlova and her colleagues surveyed the 30-million-year old star cluster NGC 2547. They selected this cluster because of its age. The planetary building process usually ends by approximately 50 million years, making the odds of a giant impact unlikely to occur outside this window. The other advantage of NGC 2547 is that it is old enough for the material left out from the original cloud ofwhich solar systems formed to dissipate (this takes about 3-10 million years). By focusing on radiation at a wavelength of about 8 microns, they could detect the heat they would expect from dust at a distance of about one astronomical unit (1 AU) from a solar-type star. The NGC 2547 cluster was previously surveyed spectroscopically, so they could cross-check to make sure that the emission they detected was not due to gas (which would be evident by spectral emission lines). Out of about 400 stars in the NGC 2547 cluster, they found only one that showed evidence of dust from a massive impact.