The mystery of how the moon formed is still puzzling scientists. The secret could lie deep within our planet.

• Our moon has been our constant companion for billions of years. 
• But we're still not sure how it came to be, though scientists have a few theories. 
• The most outlandish theory, in which an alien planet crashed into Earth, is also the most likely.

With the likes of NASA and Elon Musk vying to land on the moon within the next five years, you would think we would know everything about our satellite.

But there remain a vast number of unanswered questions, including one of the most fundamental: how did the moon first come to be?

That has been surprisingly difficult to answer, and scientists still aren't certain how it happened.

But they have theories, the most prominent of which is arguably the most bizarre as it involves a theoretical alien planet crashing into the Earth — then disappearing —  more than 4 billion years ago.

Here are four leading theories of how the moon was formed, and why the secret to uncovering the truth could lie deep within our planet.

The moon wandered by the Earth and was captured into its orbit

According to the capture theory, the moon was wandering through the universe like a giant asteroid. One day, it passed within the vicinity of Earth and was captured into its orbit, staying with it ever since.

This isn't altogether impossible. After all, this is how Mars is thought to have captured its moons Phobos and Deimos.

In the case of the Earth, however, the capture theory is very unlikely. One reason is that our moon is very big — almost the size of Mars itself.

An object of that size "would be more likely to impact into the planet rather than just to be captured in orbit," Sara Russell, a professor of planetary sciences at the Natural History Museum, told Business Insider.

Lunar samples brought back during the Apollo missions put a final nail in that hypothesis.

Scientists analyzed the isotope makeup of these samples. Isotopes are forms of atoms that have the same chemical function, but carry a different number of neutrons.

"We use isotopes as a fingerprint, which are very distinctive," Russell said.

When space rocks form independently, it's easy to tell them apart with isotopes. A Mars rock looks different from an Earth rock for instance.

But the moon it turns out, is made up of the same isotopes as the Earth.

"That means they must be genetically linked somehow because every other body in the solar system has got a different isotope signature," she said.

The moon formed alongside the Earth

The accretion hypothesis ties the moon to the birth of the Earth. This theory suggests both objects emerged from the same cosmic cloud of dust around 4.6 billion years ago.

The Earth, per that theory, would have been the greedier twin, pulling more material to its side during its early accretion, which led to its bigger size, molten core, and stronger gravity. The moon, then, would have been pulled into Earth's orbit.

The problem is that while the moon and the Earth share isotopes, the way they put them together is very different. The moon is made up of much less sodium and iron than the Earth, for instance.

"You would expect that if it formed from the same cloud, the same mixture of material, it would end up with roughly the same composition, it might just be smaller," said Russel.

"But actually, it's completely different," she said.

The moon was a glob of material shed by the Earth as it started spinning

Another theory for the formation of our constant companion is not very flattering. This one argues that the moon is just a bit of excess baby fat our Earth shed early on in its life.

Per the fission theory, as the Earth started spinning faster, part of its material spun off, and then miraculously stopped exactly where it needed to be to create a body in Earth's orbit.

In this case, physics is not on the side of this theory.

It's very difficult to shed such a big mass of rock, no matter how quickly the planet was spinning at the time, said Russell.

"It might happen for an asteroid, but not something as big as the planet," she said.

Another issue with that theory is that the moon would have been much more likely to fly off toward the sun or fall back to Earth than to stay in orbit.

A Mars-sized planet crashed into the early Earth

That leaves us with one last hypothesis, the giant impact theory. While this hypothesis has the most scientific support, it is arguably the one that requires the biggest leap of the imagination.

The theory suggests that around 4.5 billion years ago, a Mars-sized planet called Theia crossed paths with a new Earth that had just started accreting.

The collision between the two planets created a humongous explosion, 100 million times bigger than the impact that wiped out the dinosaurs.

Per this theory, the blow was so brutal that Theia kicked some of the Earth's material into space, which eventually turned into the moon. Theia disappeared by melting into the mess of molten rock that would eventually become the Earth and the moon as we know them today.

The subtle difference between the Earth and the moon, then, would be explained by Theia grazing the Earth's mantle but avoiding its iron core, which could explain why the moon is so iron-poor, per Russell.

The whole process may have taken only a few hours, according to NASA.

While this sounds far-fetched, thanks to the Apollo samples, it has quickly become the leading theory for how the moon formed.

"That is one of the absolute triumphs of the Apollo missions," said Russell. "Before Apollo, people honestly had no clue which of these models was the cause of the formation of the moon," she said.

"Apollo showed clearly that the moon has such great similarities to each other, that they must be somehow linked in some way," she said.

Still, the giant-impact theory is not without its issues. We have no idea where Theia came from and the planet left no evidence behind.

Another issue is that it's not quite clear why the moon and the Earth would be so similar if they picked up bits of Theia along the way. Models suggest more than 60% of the moon should be made up of Theia, per Space.com.

One possibility is that at the point of impact, Theia mixed in so thoroughly with both the Earth and the moon that these are virtually indistinguishable.

Another possibility is that Theia was almost a twin of the Earth, created in the same corner of the solar system.

Scientists are working to reconcile these finer points of the giant-impact theory. Modeling can explore the angle at which the planet hit the Earth and how its material mixed in with the Earth's mantle.

A recent study suggested that continent-sized chunks of an alien world may be hiding deep within our planet — and it could explain the weird structures that appear to be surrounding Earth's core.

Hongping Deng, a professor of computational physics from the Chinese Academy of Sciences, ran computer simulations tracking 100 million virtual particles from the moment Theia may have crashed into the Earth through to millions of years later.

Using these simulations, the scientists were able to show that some fragments of Theia — roughly 2% of the whole mass of the Earth —could have survived inside our planet, slowly sinking through the mantle.

As humans return to the moon, they can bring back precious samples

An upcoming slew of planned missions to the moon, including NASA's Artemis mission to bring astronauts to the lunar surface for the first time in more than 50 years, could also offer opportunities to return more samples from the moon.

"Everyone's really excited about Artemis at the moment and having NASA and ESA, returning to the moon," said Russell.

"There's loads of stuff that we're gonna find out in the next decade or so," she said.