In the decades and even centuries before the Apollo missions, scientists weren’t really sure where the Moon came from. It’s weird; for just one example we have the largest moon-to-planet ratio in the solar system (they didn’t know about Charon back then, and most people don’t count Pluto among the major planets anyway), implying something odd happened to create it. But what?

One thought was that it was captured by Earth somehow. It formed elsewhere in the solar system, passed too close to Earth, and we snagged it. A major problem with this idea is that just falling into orbit around a planet is essentially impossible; the conditions have to be fairly exacting and objects getting close to us tend to be moving too quickly to be captured.

Then Apollo happened. Mineralogy of the Moon became a thing, and scientists found a lot of similarities between the Earth and its satellite. Lots of differences, too, but the similarities were weird. So to explain that a new idea was proposed: the Giant-Impact Hypothesis.

Nowadays, if you ask essentially any planetary scientist on our particular planet how the Moon formed, they’ll tell you tales of a Mars-sized planet, an immense collision, debris splashed into low-Earth orbit, material coalescing into the Moon, and then the interaction of gravity pulling the Moon away from Earth to its current position, with the Moon’s current makeup a mix of ancient Earth and the planet that hit us, posthumously named Theia. The captured-from-elsewhere hypothesis was tossed into the cosmic dustbin.

Having said all that by means of an intro, a new paper maybe — maaaaaayyyyyybe — breathes some new life into it [link to journal paper].

The idea is that there are lots of binary objects out there — the aforementioned Pluto and Charon, asteroids with moons, Kuiper Belt Objects with big moons, and so on. The authors posit that Back In The Day (a few billion years ago), it’s possible there were binary objects where one of the two objects was relatively big. Not planet-sized, but getting there. What they show is that it’s possible, during a very close pass of a binary with a larger planet, the latter could capture one of the two objects gravitationally, while ejecting the other. This has been known for a long time, especially in the outer solar system where there are giant planets and conditions for such an event make it a lot easier. What’s new in the paper is that they show a satellite as large as 0.1 times the Earth’s mass could be captured by an Earth-ish planet. 

The Moon is only 1/80^th Earth’s mass, so this result is interesting. Mind you, the idea that Earth captured the Moon in this way is not the main point of the article at all; they are showing that capturing biggish moons is possible this way for a terrestrial (rocky, roughly Earth-sized) planet in the inner solar system. They do speculate about our own Moon, but don’t draw any conclusions.

This idea would solve one thorny question: Why does the Moon orbit Earth closer to the plane of Earth’s orbit around the Sun, and not over our equator? An impact would blast material into space, which would accrete and form the Moon, but even with a tilted impact, tides would tend to force the Moon to orbit over the equator. It doesn’t, which is harder for the impact idea to explain.

The binary capture idea does this somewhat naturally, since most encounters will happen in the plane of Earth’s orbit; most planets and objects orbit the Sun in very roughly the same plane.

So is this a viable hypothesis? I’ll let planetary scientists argue the details, but my impression is that it’s possible, but not likely.

For one thing, the impact hypothesis already explains a lot of stuff pretty well. The orbital plane issue is a problem, sure, but it’s not a deal breaker. You also have to start with a binary object in the first place — the protoMoon orbiting another big object — and that had to have formed somehow. So in that sense it’s removing the problem one step (similar to the “who created God” answer you can give to creationists who question how the Universe was formed*). Mind you, we know such binary objects exist, so it’s not a big problem, but it adds a layer of complexity.

Also, the authors mention some problems with the impact hypothesis that could be answered by capture, but to be honest they’re not big problems, and are already possible under an impact scenario (for example, the spread of isotope abundances on the Moon which differ from Earth’s, implying it came from elsewhere in the solar system, but remember Theia came from elsewhere, so those isotopes could be from Theia).

I suspect the impact hypothesis will still be standing strong for a while yet. This new idea is possible, but did it actually happen? Those are two different things, and it may be a while yet before more evidence comes in to support it. So, in the meantime, in my opinion the answer is, “probably not”. But, like any good scientist, I’m willing to be swayed by evidence.

* To be clear I’m not comparing this capture idea to creationism! The capture hypothesis is scientific, based on evidence and physics and honest observations; creationism is not that.

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