In order to estimate the scarcity of "double planets," we need to understand how the Moon formed. For many years, the formation of the Moon was one of the longstanding problems of planetary science. Three mechanisms were proposed: co-accretion, in which Earth and Moon formed at the same time from the gas and dust in the solar nebula; fission, in which Earth formed first and was spinning so quickly a large piece of material tore away and formed the Moon; and capture, in which the two objects formed at different places in the solar nebula, and then the Moon became trapped in orbit after straying too close to Earth. All three mechanisms had difficulties in explaining several important features of the Earth-Moon system, but it was hoped that analysis of lunar rocks brought back from the Apollo missions would vindicate one of them. Instead, it became clear that none of these ideas worked. A new theory of lunar formation was needed.
In 1975, two groups of American scientists independently proposed the impact hypothesis for the Moon's origin.211 They postulated that an object with the mass of Mars struck the infant Earth in an off-center impact. The unimaginably violent collision ejected a mixture of terrestrial and impactor material into orbit around Earth, and this material quickly coalesced to form the Moon. Now, scientists generally dislike having to resort to cataclysmic or unique events to explain their observations, but at least in this case computer models can simulate possible Moon-forming collisions. Although the details of the impact are still in dispute — for example, recent work suggests that the impactor may have been more massive than previously thought — the hypothesis
figure 57 Pluto and Charon combined have less than 5% of the mass of Mercury, the next-smallest planet.
explains many of the observed facts about the Earth-Moon system. Furthermore, there is other evidence (from the tilts of the planets, for example) that violent collisions were not uncommon in the early Solar System. The impact hypothesis has gained a large measure of consensus among planetary scientists.
If our Moon was indeed the consequence of a giant impact, then the uniqueness of the Earth-Moon double planet within our Solar System need not surprise us. Although collisions Solar System objects were common, such cataclysmic Moon-forming collisions may have been scarce; perhaps the infant Mercury, Venus and Mars were simply fortunate enough to dodge the larger missiles. (It has been suggested that Venus once had a large satellite, which was formed in the same way as the Moon, but which followed a retrograde orbit: in other words, it orbited Venus in the "wrong" direction. Such an orbit could certainly occur if the satellite was created through an impact event. However, whereas tidal forces are causing our Moon to move away from Earth, in the case of a retrograde orbit those forces would act in the opposite direction. A satellite in a retrograde orbit moves toward the planet and is eventually destroyed. This is the fate of Triton, the largest of Neptune's satellites.) Furthermore, the Moon-forming collision
occurred at a critical time. Had it happened much earlier, when Earth was less massive, then most of the debris from the collision would have ended up in space, and the Moon would have been much smaller than it is. Had it occurred much later, then Earth would have been more massive, and its greater surface gravity would have prevented the ejection of enough mass to form a large Moon.
Whereas the original scenarios for lunar formation implied that our Moon was almost a natural by-product of planetary formation, the impact hypothesis suggests that the Earth-Moon system may be exceptional. Imagine a collection of primordial stellar nebulae, each identical to the nebula from which our Solar System formed. Perhaps only 1 in 10, or 1 in 100, or 1 in 1000, would generate an Earth-like planet with a Moon as large as ours. Perhaps the figure is 1 in 1,000,000. We have no idea — and it will take huge advances in observational astronomy before we discover whether extrasolar terrestrial planets possess satellites as large as our Moon. With our present knowledge, however, it is entirely possible that Earth is unusual in possessing such a large satellite.
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