Ahh, the moon.

Earth’s steadfast companion, bringing light to the night and  inspiring oodles of art and music. It’s always been a central  character in our cultural stories, whether as a spiritual figure or as a symbol for the race  toward scientific progress.

For as long as we can remember, the  moon has also inspired questions. Early on we wondered, “where does the moon’s light come from?” And as we learned more, we asked more complex questions, like, “how does the moon affect the tides?” But there’s one big, unanswered question  that’s still bugging us to this day: Where did the moon come from? [intro] The first thing you have to understand  about our moon is that it’s weird. Our moon is about 81 times  less massive than Earth.

And that’s really strange because  most moons in our solar system are truly tiny compared to their planet. Like most of these satellites orbit planets  more than one thousand times their mass. The only other oddball in our Solar  System sits at the other extreme: the dwarf planet Pluto and  its so-called moon, Charon, which is only about eight  times smaller than Pluto.

They’re not really like a planet and a  moon, where the moon orbits a planet. Instead, they act as a binary system, where  they both orbit a point in between them. But besides them earth and our moon really stand out We can’t glean any clues from the rest of  our solar system regarding lunar origins, because there are no other satellite-planet  relationships quite like ours.

We need to get creative to come up with  theories about how the Moon formed, where it came from, and why its  relationship with the Earth is so peculiar. Now, if you’ve ever learned  anything about the moon’s formation, it was probably something about  the Giant Impact Hypothesis. That’s the most commonly accepted  theory on the moon’s origin.

We've made several videos about the hypothesis, including one that had a pin to go with it As the story goes, some four  and a half billion years ago, ancient Earth was minding its business, doing its gooey-lava-ancient Earth thing when a huge chunk of space  rock very rudely interrupted. We call the intruder Theia, and she is assumed to have  been about the size of Mars. When Theia collided with Earth, the debris from this spectacular  collision eventually coalesced to form what we now call the Moon.

Sounds pretty reasonable, right? The solar system certainly  had a chaotic, youthful phase before it mellowed out into the relatively  quiet neighborhood we know today. We’ve seen the aftermath of big  crashes in other young star systems, even in systems relatively close  by, like in the Aries constellation, just 300 light years away from us.

So it’s not too hard to imagine  that an enormous interloper may have impacted our own  planet in its early days. Besides the fact that it  makes some intuitive sense, the Giant Impact Hypothesis also  successfully explains some weird Moon quirks that other theories struggle with. For example, recent lunar exploration has shown that in its earliest days, the moon was covered in magma.

Samples recovered from the  near and far sides of the moon have shown similar composition, meaning it’s likely those samples formed in one big, Moon-wide molten sea. That’s puzzling because we typically  think about planets and satellites forming from the slow accumulation of  tiny space dust into a big hunk of rock, like rolling a massive snowball! But that snowballing process can’t  account for the amount of heat necessary to create a layer of molten rock  all over the Moon’s surface.

However, the Giant Impact could  have created all that heat, like striking flint against  steel multiplied by a gazillion. Plenty of energy to create a  magma ocean on the moon’s surface. There are also striking similarities  between Earth rocks and Moon rocks.

Scientists have analyzed lunar samples  brought back by Apollo missions, and found they’re almost identical  in composition to Earth-side samples. But if the Giant Impact  caused pieces of ancient Earth to get mixed up in the newly-formed moon, then it’s certainly plausible  that their rocks would be similar, even billions of years later. At this point you might be thinking the Giant Impact Hypothesis is a home run.

We know where the Moon came  from, score one for science! But hold your horses, I’m gonna poke some holes. Because while the Giant Impact model has been the prevailing theory  for over forty years now, it has some major weak spots.

And I’ll tell you all about those weak  spots right after this short break. Thanks to JMP for supporting this SciShow video! JMP was designed with scientists and engineers in mind, but it’s great for anyone solving problems with data.

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JMP works with the systems you already have in place. To reap the benefits of visual statistics for yourself and get a 30-day free trial, visit jmp.com/scishow. The truth is, we know surprisingly little about the reality of the Giant Impact  and its supposed instigator, Theia.

The Giant Impact theory assumes that Theia would have been about the size of Mars, or about one-tenth the mass of the Earth. That’s the most popular version of this theory, but it’s not the only possibility. Researchers have run computer simulations  over a range of masses for Theia, from Mars’ size to over double that And they’ve found that despite  the range of mass inputs, many of these scenarios could have  created similar impact outputs.

We also don’t know where Theia actually came from. Recent research suggests that Theia  formed in the inner solar system, based on the chemical composition of samples taken from the Earth, the moon, and meteorites. Theia may have even formed  closer to the sun than the Earth.

But if they formed so close to each other, why didn’t they collide sooner, before they were both so big? Some scientists have proposed  that Theia got caught between a space rock and a hard place. Theia could have been pinned between the gravitational  forces of the Earth and the sun, at a point of gravitational  equilibrium called a Lagrange point.

It’s possible that it hung out there, gradually accumulating dust and space junk, until it got massive enough to escape and set a collision course with Earth. This theory is possible, but other studies suggest perhaps Theia formed in the outer solar system, after all. The jury’s still out.

Partly because the biggest remaining question is just a major problem with the  Giant Impact theory in general: What was Theia made of? This problem is called “the isotope crisis” because, if you’ll recall, we still don’t know why Earth and Moon rocks share so much material with each other. That’s not so much of a crisis if the Moon was made mostly of Earth stuff  that got blasted apart by Theia.

But the most widely accepted computer  model of the Giant Impact Hypothesis, which was published in 2001, predicts that the proto-Moon debris was actually made mostly of Theia, not Earth. So if the Moon is made mostly of Theia, it looks pretty suspicious and  crisis-y that Theia and Earth would have had such similar compositions. For a lot of researchers, that level of coincidence is just too outrageous.

So experts have scrambled to propose solutions to the “isotope crisis”. One potential scenario is  that the Earth-Theia collision was actually a “hit-and-run”. The canonical Giant Impact model  depicts the collision as a “slow graze”.

But “hit-and-run” proponents say it’s possible that Theia hit Earth fast and at a steep angle. That could have churned up more Earth debris, leading to a Moon comprised  more of Earth than of Theia. Another option is the “merger” model, which assumes proto-Earth and Theia were actually two bodies of similar size, instead of Theia being much lighter.

This would result in a more powerful collision that thoroughly intermixed the  material between those two bodies. Then both the Earth and the Moon would have formed from that  mixture of the same basic “stuff.” The final option is a “bigger bang”. The Canonical Giant Impact Hypothesis is based on a relatively tame collision.

But if Earth and Theia crashed into each other with enough speed and angular momentum, there would be enough energy in that  collision to basically vaporize them both. This cloud of Earth-and-Theia vapor would then have formed a new kind of  planetary structure called a “synestia.” That name was coined  specifically by the study authors to describe this new shape they noticed when running their computer  models of the collision. It’s a combination of the Greek prefix “syn”, meaning together, like in “synthesis”, and the name of the Greek goddess Hestia, who represents home, hearth, and architecture.

So “synestia” means the assembly of our home! That’s so sweet! Anyway, this energetic vapor  cloud would be spinning so fast that the synestia would become a donut-y shape, where the material is denser around  the rim and emptier in the middle.

As the spinning donut cooled down, it would have eventually  settled into Earth and the Moon. And, much like the merger model, they would both be made from the same stuff, explaining their similar compositions. These are all essentially variations  of the Giant Impact model, tweaked to patch the original model’s leaks.

But some researchers are thinking outside the box. In a paper published in 2025,  one team is boldly asking: What if the moon wasn’t formed  by a single big collision at all? Perhaps the moon formed over  a longer stretch of time due to multiple small  intruders colliding with Earth.

That team’s model shows how each small collision could have chiseled off a bit  of proto-Earth’s material, producing relatively small chunks  that they’re calling “moonlets.” another cute name. We have some cute names in this episode! These smaller impacts wouldn’t  have generated enough momentum for the moonlets to escape, so a moonlet would have hung around  Earth’s orbit for a couple million years until the next impact generated a new one.

Eventually, gravity would pull all these  chips off the old block into one big

Moon: the one we know and love today. And because the moonlets are just  chunks chipped off proto-Earth, it would explain the “isotope crisis” pretty well. Now this team isn’t the first to  propose a “multiple impacts” scenario, but it’s the most realistic one yet. This model demonstrated that  three moonlet-generating impacts could have led to our Moon’s formation.

Whereas other simulations required twenty or more impacts to make this work. And that’s a lot of gravitational chaos for all those poor untethered  moonlets to withstand. Basically, fewer impacts implies larger moonlets that are less likely to  accidentally escape Earth’s orbit and be lost forever in the solar system.

After all, we would need them to  have stayed close enough for gravity to do its moon-making job. Three impacts might just be the sweet spot theory that accounts for the strange  quirks in our moon data. Without needing anything too outlandish to happen.

But of course, this still isn’t a  definitive answer to the question “Where’d the moon come from?” As computer models get more advanced, we’ll be able to run whatever  new scenarios we concoct, or reanalyze the chemical  composition of old samples. And it’s possible that bringing back  more physical samples from the moon might confirm or disprove our existing theories. But it’s exciting to have  a promising new explanation after decades of the Giant Impact Hypothesis.

The three impacts hypothesis is  a compelling alternate theory for scientists to consider. The moon continues to inspire us, even if it remains kind of a mystery. [ OUTRO ]