Jupiter has 66 known moons, Saturn has at least 62, Uranus is known to have 27, and Neptune has at least 13. For smaller objects, Mars has 2 moons, and going out into the Kuiper Belt, Pluto has at least 4, and two moons are known to orbit Haumea. Yet Mercury and Venus have no moons, and Earth has only one, albeit a pretty large moon in comparison to the planet. Sometimes pseudoscientific stories will pop up about Earth’s “dark moon” (which I’ve seen called “Lilith” by some astrologers, though I suspect that’s a little kooky even by astrology standards) and other such garbage, it’s certain that nothing of substantial size orbits Earth other than the obvious Moon.
Well, let’s amend that slightly: Earth has only one permanent moon. On the other hand, there are a lot of little asteroids known as near-Earth objects (NEOs), and in 2006, an asteroid euphoniously known as 2006 RH120 was found to actually be in orbit. After 11 months as Earth’s second moon, it moved away again, but it will be back in 2028 for another sojourn. Asteroid 2006 RH120 is pretty dinky: it’s only about 5 meters across, and nobody has taken a clear photo of it (or else I’d include that picture in this post). However, that’s typical of Earth’s temporary second moons: according to a recent paper in Icarus, at any given time a small chunk of rock one meter may be in orbit around us. These irregular satellites may be no larger than 1 meter across
The general idea: we know there are a lot of NEOs, whose orbits are shaped by gravity both from the Sun and Earth. Sometimes Earth may snag one of them, causing it to fall into orbit around us instead of a direct orbit around the Sun. Of course, if you trace its entire trajectory, it’s still orbiting the Sun: it’s merely a detour to loop around Earth one or more times! While we are conditioned to think of orbits as being stable and permanent (or at least as permanent as possible in our evolving Solar System), most types of orbits aren’t. The best way to guarantee a stable orbit is for the satellite to form together with the object it orbits, or at least to be torn off from it, as our Moon probably was. Some satellites—especially in the outer Solar System—may have been captured objects, and some models argue that Mars snagged a passing asteroid to explain Phobos’ odd orbit.
However, in Earth’s case, the Moon makes things more difficult. After all, any object in orbit around Earth will also feel the pull of the Moon, so unless the orbit is either very small (as with most artificial satellites) or much larger than the Moon’s orbit, it’s going to be yanked around substantially. A very large orbit would make it very much like a circumbinary exoplanet, but in that case the tug of the Sun is also going to influence it. In other words, there isn’t much chance for a long-term, stable orbit for an asteroid grabbed as it passes by.
This discussion makes me think I need to write a general “three-body problem” post. Generally speaking, it’s easy to solve the problem of two objects interacting under their mutual gravitation—as long as gravity isn’t too strong, in which case you need general relativity, and things become messy. However, if you throw in a third object, there isn’t a complete solution: the equations are too complicated to solve exactly, so we have to resort to approximate solutions or using computers to simulate the system. If two of the objects are a lot more massive than the third (as in circumbinary planets), approximate solutions may be good enough. in any case, there’s a lot of fun to be had in looking at the three-body problem in general. If you want a preview, check out my earlier post “Centrifugal Forces and Trojan Horses“; the famous Lagrange points are part of the three-body problem. And expect more soon!