How Was the Moon Formed?

The Moon has been our celestial neighbor in space for as long as history is able to indicate. The Moon has had an effect on the formation of life on Earth and still has an impact on our planet today. Our world would be different without the Moon, but how was the Moon formed in the first place?

How Was The Moon Formed

No One Knows for Sure

Not every question has a concrete answer, especially when it comes to the mysteries of our Solar System. In fact, it’s safe to say there are many more questions than answers.

With its active volcanic interior, the Earth has seen a lot of reshaping over the course of the last 5 billion years. As a result, there’s almost no evidence of our ancient history on our relatively young surface.

The Moon, on the other hand, has floated rather dormant in space for an incredibly long period of time. With no volcanic activity to speak of, a look at the Moon is like looking potentially billions of years into the past.

While the Apollo missions were a huge breakthrough for space travel, it was also a huge breakthrough to understanding more about our satellite.

A Few Theories on the Formation of the Moon

Moons of the Solar System To Scale
A size comparison of the moons of the solar system. (Image credit: “Moons of the Solar System To Scale” by Kevin Gill on Flickr CC BY 2.0)

Our Moon is rather unique because it is, by comparison, much larger than the moons of other planets in the Solar System. This has led to the understanding that it must have become our satellite in an unusual way.

Over the years, scientists have developed a few theories that could explain how we got our Moon.

Giant Impact Theory

Giant Impact Theory
Giant Impact Hypothesis . (Image credit: Citronade on Wikimedia Commons CC BY-SA 4.0)

The early Solar System was essentially a giant ring of dust, rock, and gas spinning around a newly formed Sun. This debris coalesced into the planets we see today, including our Earth. However, our eight planets and various moons weren’t the only objects that formed.

The giant impact theory follows the belief that an object the size of Mars, called Theia, found its way to Earth.

Theia, wandering without a stable orbit through the newly formed Solar System, smashed into our Earth long before any life existed on the planet. This impact caused massive amounts of the Earth’s crust and mantle to be thrown out into space.

The Earth and Theia would have melted during the impact and reformed as one planet. While the Earth healed, the material that broke free from the Earth would have been caught in orbit around it. This material would have then orbited our planet like a ring. It would then have slowly drawn together by gravity to form the Moon we know today.

For this theory to hold true, the Moon would need to be made up of the lighter elements of both Theia and the Earth that would have been present in their crusts. The dense cores of these worlds would have been compact enough to mostly survive the impact and combine into one core.

No one knew what the Moon was composed of until the Apollo astronauts reached the lunar surface just over 50 years ago. Not only did astronauts bring back lunar samples, but they also set up seismometers on the Moon’s surface to detect information about each layer.

Astronaut Edwin Aldrin walks on lunar surface near leg of Lunar Module
Astronaut Edwin E. Aldrin Jr., lunar module pilot, walks on the surface of the moon near the leg of the Lunar Module (LM) “Eagle” during the Apollo 11 extravehicular activity (EVA). (Image credit: NASA)

Scientists were able to analyze these samples to determine that the Moon’s composition is very nearly identical to our own. The Moon is made up almost entirely of the lighter materials that would have been a part of Earth’s crust and mantle. This theory also backs up why the Moon has such a small core of denser metal.

It would also make sense that the Moon would settle in orbit around the Earth’s equator. It also matches the belief that the Moon was able to pull away from Earth as it formed, causing the Earth’s rotation to slow. The Moon is still drifting away from Earth by a few centimeters per year.

The big question still lingering about the giant impact theory is what happened to Theia. If this planet came from somewhere else in the Solar System, it would make sense for it to have a different chemical composition than what is shared between the Earth and the Moon today.

It’s possible that Theia was made up of the same elements as our Earth when the collision happened and was lost entirely to the Earth and the Moon after the impact.

One variation of this theory suggests that rains of cosmic debris crashed down on the Earth instead of one large object. Such an event could still potentially send pieces of the Earth’s newly formed crust into space to create the Moon.

Co-Formation Theory

Earth and the Moon
The co-formation theory explains that the moon was formed out of the primitive solar nebula at the same time as the Earth.

Another interesting take on the Moon’s formation centers around the thought that the Moon and Earth simply formed from the same gas and dust, but as two separate entities. Since the Earth was much larger in size, it grabbed onto the Moon and held it into an orbit as the two objects cooled.

This lines up with the fact that the Earth and the Moon are made up of the same materials, but it doesn’t explain why the Moon would form with such a small core.

It’s also been suggested that two planetary objects larger than Earth collided, and the two objects fused together to form the Earth. The resulting space debris coalesced to make up the Moon. Since both objects came from the same source material, their chemical compositions would match like the Earth and the Moon do.

Capture Theory

As is the case with many other moons in our Solar System, it is possible that the Earth was able to snag the Moon as it passed by our planet. This capture theory makes the most sense when objects are quite small, like the two Moons of Mars.

This theory doesn’t do much to explain the similarities of the two objects, though. The Moon would also have to have been traveling at a pretty slow speed to get caught up in Earth’s gravity.

Which Theory Makes the Most Sense?

Each of these theories has some merits, but the co-formation and capture theories still leave many questions about the Moon’s origins. Although it has not been proven, the giant impact theory is the most widely believed today.

Simplistic representation of how the Moon formed as per the Giant Impact Hypothesis
Simplistic representation of how the Moon formed as per the Giant Impact Hypothesis. (Image credit: “GIH-simple-model” by Jatan Mehta on Flickr CC BY-SA 2.0)

Could the Earth Survive Without the Moon?

The Earth would be a much different place without the Moon.

Our neighbor’s gravity helps hold our planet at a 23.5-degree tilt, allowing us to have the seasons and weather we’re used to. Without the Moon, the Earth’s tilt could rapidly change, leading to no seasons or extreme ones.

The Moon also slows our rotation. We currently experience a 24 hour day, but scientists predict that a day on Earth would go twice as fast if the Moon disappeared.

Tides would drastically drop to about a third of what they are now. This would have huge repercussions on aquatic environments and life.

We would also find nights to be much darker than they are now. While more difficult for humans to find our way, nocturnal predators may find it that much more challenging to locate their prey.

See how different Earth would be if there were no moon.


Although we don’t entirely understand how the Moon formed, it is clear that the Moon and the Earth have a special relationship. The good news is that the Moon shows no signs of changing at any time in the near future. However it got there, we can gaze up at our heavenly neighbor and continue to find comfort in the support it provides.

About Noah Zelvis

Noah is a content writer who has had a love of all things astronomy for as long as he can remember.
When not reaching for the stars, you’ll likely find Noah traveling or running.