When I was growing up, there were two or three theories in competition with each other. Proponents of the “Co-Formation Theory” suggested that both the Earth and the Moon formed, at the same time and near to each other but relatively independently, from the dense disk of gas, dust, and rocks that encircled the early Sun, out of which all the planets were born.
If that were true, though, we would expect the Moon and Earth to be composed of the same sorts of substances. But they differ in many ways. In particular, the Moon has much less iron, pound for pound, than does the Earth.
The “Fission Hypothesis” was popular for a long time. According to this idea, the Earth was molten at the beginning, and was spinning very fast. A glob of molten rock got flung out into space by centrifugal force; it cooled, and formed into the Moon.
We can do the calculations to see how fast the early Earth had to be spinning, in order to accomplish this. That original spin looks to be far faster than we think possible. Furthermore, how did the Earth get put together in the first place if its original spin was so fast? Or did something happen that spun the Earth up to that high speed? Nothing suggests itself.
Moreover, we would expect the resulting Moon to be revolving around the Earth in the plane of the Earth's equator, if it were ejected by the Earth's spin. But it's not. The Moon's orbit is almost in the same plane in which the Earth goes around the Sun, and those two planes are tilted with respect to each other by over 23 degrees. This implies the Moon's formation was related to something going on in the rest of the solar system.
More recently, the “Capture Theory” claimed that the Moon formed elsewhere in the solar system, as an independent small world. It then performed a close passage to the Earth, where it was captured by the Earth's gravity and became a satellite.
This is not easy to do. The proto-Moon would approach the Earth at some speed, but would then go by the Earth and back out into space at the same speed. In order to have the proto-Moon drop into orbit around the Earth, something would have to slow it down during its close passage.
One possible slowing agent would be the Earth's atmosphere. But the atmosphere would have to have been impossibly large and dense to affect the passing Moon.
The latest notion, the most intriguing one, is sometimes called “The Big Splash.” Soon after the formation of the solar system, according to this theory, an object about the size and weight of Mars collided with the Earth.
This is not an outlandish idea. The rocky worlds like the Earth were formed by the coming-together of dozens of such bodies. The Mars-sized object, sometimes called Theia (the mother of the Greek goddess of the Moon, Selene), might have just been the last large incoming body.
The energy released in the collision melted both Earth and Theia, or at least their crusts. Great masses of molten rock were splashed into space from the sheer force of the collision, and formed a ring of debris around the young Earth. Eventually, this ring cooled and collected itself into a young Moon.
If this theory is correct, the Big Splash occurred 30-50 million years after the formation of the Earth, about 4.5 billion years ago. Theia's metal core might have sunk rapidly down into the Earth, leaving the Moon with its observed iron deficiency.
The collision took place at relatively low speeds, no more than three miles per second-much slower than the Earth's orbital speed around the Sun, around 18 miles per second. It's not clear where the splashed material that formed the Moon came from. Was it mostly from the Earth, or mostly from Theia, or a mixture of the two?
There are other questions to be answered before the theory is firmly accepted. For instance, why does Earth have a moon, but Venus does not? The two planets are of similar sizes, and both were impacted by large rocky bodies during their formation phase.
This is a prime example of how science works. A scientist must have some imagination to come up with a theory about an observed object. Then she and her colleagues must calculate the implications of the theory, and see whether those match the observed features of the universe.
It's an inefficient process, especially for events that took place so long ago, but it's the most reliable way to uncover the truth, and usually it does the job. Who knows? We may see another theory emerge concerning the origin of this difficult celestial body!