How the Moon was Formed

To understand how the moon was formed we have to take a step back, actually a really, really big step back. Let’s go back nearly 5 billion years ago.

The Birth of a Solar System:

It was nearly 5,000,000,000 (billion) years ago that the Solar System we live in was formed. This occurrence took place through the collapse of a smaller section of a giant molecular cloud.

The majority of this matter collected into a mega cluster which would eventually form to create our Solar System’s star, the Sun.

However, all the remaining matter spread out and even flattened out into what is called a protoplanetary disk, this is rotating and dense gas. Protoplanetary disks form; planets, moons, asteroids and other smaller objects which make up a Solar System.



“Protoplanetary-disk” by NASA – NASA; Licensed under Public Domain via Wikimedia Commons –


Where do Babies Moons Come From?

So now that we have a basic understanding of how Solar Systems and more importantly, planets and smaller solar objects are created; we can start to understand how the Moon was formed.

Since no one was around to actually record this event, Scientists and Astronomers have to use the information and data they have to create hypotheses around possible explanations for how the Moon was formed.



Photo by H. Raab


The most recent and most popular theory around how the Moon was formed comes from what is known as a Giant Impact. Before we can understand what this impact of giant proportions consists of, we must introduce a one of our Solar System’s long lost relatives; Theia.

Allow me to Introduce Theia:

Theia, the planet, is thought to be one of the early Planets that formed in our Solar System, shortly after the formation and birth of our Solar System took place nearly 5 billion years ago.

Theia is thought to have been around the same size as Mars (today), but that’s still up for debate. It’s pretty clear that Theia’s size wouldn’t have been larger than the Earth, so something around the size of Mars is the most commonly agreed upon size, at the moment.

Theia gets her name from Greek mythology. Theia was the mother of Selene, who was the goddess of the moon. It’s fitting that Theia’s presence in our Solar System is thought to have given birth to our Moon through what we refer to as the Giant Impact Hypothesis.

The Giant Impact Hypothesis:

The Giant Impact Hypothesis is the current and most agreed upon theory of how the Moon was formed. While the specifics haven’t been worked out entirely, the general idea and principals are pretty clear.



“Artist’s concept of collision at HD 172555” by NASA/JPL-Caltech – Licensed under Public Domain via Wikimedia Commons –


What’s thought to have occurred is during the Hadean eon, which took place a long time ago, roughly 20,000,000 to 100,000,000 years after the creation of the Solar System took place, a massive collision occured.

This collision happened between two planets; one of the planets was the Earth and the other was Theia.



At this time, the Earth would have looked much different. In fact, you wouldn’t even recognize it. Because of how young the Earth was, it would have been extremely hot and constant volcano eruptions, miserable for life. These frequent volcano eruptions would have created a molten surface layer, which would have endured collisions from other objects from our Solar System, pretty regularly and often.

The most beneficial of these collisions was that with Theia, the collision which started the chain reaction for how the moon was formed.

How the Moon was Formed:

This precious chain reaction is nothing short of a miracle, which is fairly simple to understand. Without it, our Moon likely wouldn’t have formed and life most likely wouldn’t exist.

To start, Theia’s orbit around our Sun would have been disrupted, likely by an external gravitational force. This alteration in Theia orbit put her on track to collide with Earth.

Even though the collision didn’t occur at high speeds (by astronomical measurements), it’s thought to have taken place at roughly 8,928 miles per hour (2.48 mps). Now, this sounds pretty fast, but in astronomical terms, it really isn’t. To put it into perspective, the International Space Station orbits around the Earth at 17,136 miles per hour. For two planetary bodies to collide with one another at roughly half that speed, seems like a snail colliding with a rock.



Photo by Rufus Gefangenen


Post Collision Solar Momma Drama:

After these two planetary bodies collided, huge fragments of matter would have been ejected out from each object’s orbits. It’s believed that between 50% to 70% of the Moon is made up of Theia. There’s a lot of debate as to what the materials and the types of matter found on the Moon suggest; but that’s part of what makes Science and Astronomy so exciting.

Over time, the Earth and Theia’s two iron cores would have started to merge and fuse together. It’s likely that the Earth was much smaller during this period of time and the collision of Theia helped increase its mass.

As the Earth and Theia did their thing, the fragments from both Earth’s and Theia’s mantle which was spewed out into space would have done one of two things.

The first, all material which had the ejection speed exceeding escape velocity would have been flung out into the Solar System and likely pulled towards the Sun through the force of its gravity. The second, material which would have only had the speed of orbital velocity, would have stuck around and started to orbit around the newly fused planet.





All the material remaining close to Planet Earth would have begun orbiting around Earth rather quickly, so quickly that it began to condense and create a circular object. Thus, the moment and event of how the moon was formed.

Scientists believe the moon would have formed between a very short window of time. In fact, on an astronomical scale, it’s a blink of an eye; the moon likely formed between 1 month and 100 years after the collision between Earth and Theia. Can you believe that? If a human was alive on Earth, they could have literally witness the creation and entire formation of the Moon in their lifetime.

The Blessing and the Curse:

Well, the Great Impact would have been a sight for sore eyes. With a collision of unearthly proportions, destruction and devastation would have only been the beginning, but after the drama would have settled down, things would have started looking up… pun intended.

After the Earth and Theia collided, both the Earth and the Moon would have started rotating faster. The collision would have been the cause of a massive momentum and mass transfer for the Earth and the Moon would have been rotating from the condensing and creation of itself in orbit around the Earth.

Scientists believe that the Earth would have rotated so quickly after the Great Impact that an Earth day would have been roughly 5 hours long.

Over the course of time, the Earth would have slowed down the Moon’s rotation – the Moon would have also slowed down the Earth’s rotation and begun the next miraculous chain of reactions, life.



Photo by Bluedharma


The next time you look up in the sky, think about Theia and what the part of our Solar System would look like if she didn’t give birth to the creation of the Moon.


Featured image by Rufus Gefangenen,

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