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Earth's Moon is a Rarity
Smivs writes "Scientists have concluded that moons like the Earth's are actually quite rare. Only 5-10% of planetary systems are likely to contain moons formed by planetary collisions. 'By the time the Earth's moon formed, when the Sun was 30 million years old, the planet formation process in our Solar System should have been approaching its end. In the latest study, Dr Gorlova's team looked at the heat signature of stars using the infrared. This allows astronomers to predict how much of that heat comes from the star itself and how much is re-emitted by dusty material encircling it.'"
Isn't this rather old news? I thought that it was already pretty well known that Luna is rather rare, as shown in the Rare Earth hypothesis?
Earth and Pluto are similar in having a moon which is a decent fraction of their own mass. The two moons of Mars, and the moons of the four gas giant planets are minute in comparision to their primary bodies.
Earth and Pluto are sometimes called binary planets for this reason. And there is no easy way to show how they formed in this way, other than invoking chance impacts shortly after formation.
http://michaelsmith.id.au
While it's true that the moon is currently receding, it will eventually stop and begin approaching the Earth, then pass the Roche Limit, break up, and generally cause everyone to have a bad day. However, the universe might not last long enough for that to happen. As for the proto-earth/moon impactor, it was absorbed into the system, with the lighter materials of both it and proto-earth forming the moon, while the two cores sank to the bottom of what became Earth.
Since there are no -1: misinformation in the comment system, I instead reply to your post with wikipedia's article.
Antiope
Orbit of the Moon
Barycenter(Centre of mass)
They actually both revolve around a point that astronomers call the apotex, which lies about 187 million miles from Earth.
If I remember correctly, the center which they both orbit around is *inside* the Earth (but not at the center). Some have suggested that the difference between "moon" and "double planet" could be defined by whether the center of gravity is inside the larger body or on the outside (between them).
Table-ized A.I.
There's a book mostly intended as a resource for sf writers, What if the Moon didn't Exist? , which details many of the poor consequences for creatures like us given the absence of a large moon for earth. Its been years, but I remember two of the big ones.
First, without tidal interaction with a large moon, the earth would spin a great deal faster on its axis, resulting in much stronger and consistent winds. It would be hard for anything to be more than a few inches tall except in the windshadow of tall mountains, which themselves would erode much faster.
Second, without the tides, the transition from ocean life to land life would have been much more difficult, since there would be nothing like tidal pools to serve as temporary havens and opportunities for the creatures making that transition.
IMBO, the earth-moon system can still be called a binary planet as no other major body in the solar system except Pluto has a satellite with as large a mass fraction as the moon is to earth.
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