Or has it been gravitationally steered that way because of how it was formed?
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4 Answers
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No, this is not entirely a coincidence, though perhaps not in the way you expect (and other answers have indicated): it may have been coincidal at formation, but is required for long-term stability of the Moon-Earth system itself and, possibly, Earth climate.
The formation mechanism of the Moon (or the Moon-Earth system) is still debated. The favoured model involves a collision (between a pre-Earth and a Mars-sized object), when the resulting orientation of the Moon-Earth orbit depends critically on the impact parameters, which are essentially random, i.e. coincidal. Conversely, if the Moon-Earth system formed by some form of capture (possibly involving some surrounding gas to absorb the excess energy), a orientation in the eccliptic is more likely.
Howerver, the formation mechanism is not all there is to this. It is also required that the Moon-Earth system remains stable over 4.5 Gyr. This excludes any orbit initially circular and inclined by more than $i=\arccos\sqrt{3/5}\approx39.2^\circ$, when the Kozai mechanism would result in a collision of the Moon with Earth within a few years. The critical incliniation for a non-circular orbit is even smaller.
Even an inclination $i\approx30^\circ$ may not be viable, since it may imply that the Earth's spin axis (which is after all tidally coupled to the Moon's orbit) would be/have been tilted considerably stronger than by merely $\approx23^\circ$. A strong tilt of the Earth spin axis would have significant consequences for the Earth's climate and may well have prevented the formation of higher live forms. This is essentially a weak form of the antropic principle.
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$\begingroup$ Shouldn't the inclination of the Moon at formation be roughly the inclination of Theia? The highest inclination planets today is Mercury's 7 degrees, even Pluto i only 17 degrees. The rest are at less than 2.5 degrees, half that of the Moon. Isn't it unlikely that Theia had higher inclination than that? $\endgroup$ Sep 14, 2015 at 16:24
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1$\begingroup$ @LocalFluff No, only the total angular momentum is conserved, i.e. the inclination of the combined Moon-Earth system is about the same as that of the (mass-weighted) mean inclination of the pre-Earth and Theia. But the orientation of the resulting Moon-Earth orbit is not contrained by angular momentum conservation (the angular momentum in the relative orbit is tiny compared to the orbit of the Earth-Moon system around the Sun). $\endgroup$– WalterSep 14, 2015 at 19:53
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From what I can find out on-line (Wikipedia, etc.) most natural moons tend to be in the equatorial plane of the planet (formed in a circumplanetary disc). The Earth's moon is commonly thought to have formed in a collision with a Mars size object. It probably settled initially into an orbit inclined about 10 degrees to the equatorial plane of the Earth due to accretion disc processes. Since then, tidal forces from the sun (due to Earth's proximity) have brought the Moon more inline with the ecliptic.
answered Sep 13, 2015 at 16:25
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$\begingroup$ I don't think that tides with the Sun will necessarily bring an inclined moon into the ecliptic. Do you have any reference/evidence for this? $\endgroup$– WalterSep 14, 2015 at 19:57
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$\begingroup$ @Walter I may have been mistaken here. I read that "tidal evolution calculations" show that the moon's inclination changed from 10 deg. from the equatorial plane to it's current 5 deg. from the ecliptic. I couldn't find my original source, but hou.usra.edu (3041.pdf) "The Lunar Inclination as a Monitor of Late Stage Terrestrial Accretion" by K. Pahlevan et al. (2015) says the same thing. I think this may be referring to planetary tidal forces as the moon changed from 10 R(e) to 60 R(e). Coincidence that it lined up with the ecliptic?? $\endgroup$ Sep 16, 2015 at 1:11
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The Earth and Moon are rather better thought of as a two planetoid system both orbiting the Sun, than a planet and a satellite (the Moon does indeed orbit the Sun as well as the Earth). The Moon orbits Earth at a point where Earth's gravitational pull is of the same order as the Sun's.
We also believe that the Moon was formed as a result of a catastrophic event involving the proto-Earth - ie one planet split into two.
As such it is not a surprise that it has an orbit close to the plane of the ecliptic.
Other planets' satellites are not the same - the gas giants dominate their satellites and in Mars's case it seems likely that Phobos and Demos are captured planetoids from the asteroid belt rather than anything much to do with Mars as such.
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It is not a coincidence at all! (There was, however, some statistical chances involved.) A key question to answer before answering your question, though, is Why is there an ecliptic at all? It is fair to see all the planets and a majority of the smaller bodies existing in a nice flat plane (the ecliptic) and think that it didn't have to be that way. Why not have different planets orbiting in different planes, or even backwards? To get the answer, we have to go back to the origins of the Solar System, when it was still a collapsing cloud of interstellar gas and dust. As gravity collapsed the cloud, the small amount of angular momentum it had became more significant, much as an ice skater pulling her arms in while spinning will make her spin faster. So, with the help of friction, the early Solar System flattened and had a 'spin' in a preferential direction.
As the Solar System evolved, planets formed, and all of them had ingrained in them this spin. Today, we see that in not only the existence of the ecliptic and the fact that planets orbit in the same direction, but in the fact that most planets rotate in the same direction as the sun does (Uranus and Venus being notable exceptions) and most large moons orbit their planets in the same direction (Triton around Neptune being a notable exception).
What about the moon? A leading theory for its formation is that it formed from the debris that resulted in a collision between a still-forming Earth and another planet-sized object. Statistically, both these objects would be most likely to have similar angular momenta to that of the overall Solar System (though it was by no means guaranteed), so their resulting debris would have it as well. When that debris coalesced into our moon, it too would also keep the original angular momentum 'signature,' and its orbit would thus be comparable to that of the ecliptic.
Of course, this is all a very chaotic and statistical process. While angular momentum is always conserved, there are ways of exchanging it between bodies (via gravitational interactions & collisions). Consequently, there are variations away from the ecliptic: the moon is inclined 5 degrees from the ecliptic; Earth's rotation is inclined 23 degrees from it; smaller bodies like comets will sometimes orbit well outside the ecliptic; etc. But the general trend holds, and is a marker you can see present throughout the solar system.
As an interesting aside, while all major planets in our Solar System lie roughly along the ecliptic, that is not always the case for other planets in other star systems. Wasp 17-b, for example, is observed to have a retrograde orbit around its host star, while others have been observed to orbit outside of their own system's ecliptics entirely.
