How Did the Moon Form and Why Does the Earth Have Only One?

How Did the Moon Form and Why Does the Earth Have Only One?
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Our Moon shines so brightly and dominant at night that it gave birth to numerous myths, legends, and stories over the centuries. The nocturnal cosmic object has not only been the subject of fairytales, however, as astronomers have also done their job analyzing it in detail. Next year, we hope that NASA will once again send humans back to the Moon due to the forthcoming Artemis mission, but until that historical moment, we have to admit that the general public still doesn’t know a lot of stuff about our natural satellite.

The Earth is indeed a wonderful and unique planet in the Solar System, but the amazing thing is that other planets in our vicinity have many more moons than ours. For instance, Saturn is the champion in this category, as the gas giant and the Solar System’s second-largest planet has an incredible number of 146 confirmed natural satellites. Even Mars, on the other hand, a planet that is about two times smaller than Earth, has two moons revolving around it: Phobos and Deimos.

Until we try to understand why our planet has only one moon, we need to understand a bit about how our natural satellite formed in the first place.

How did the Moon form: one main theory

The main theory explaining how our Moon formed is the Giant Impact Hypothesis, which implies that our natural satellite formed after a Mars-sized cosmic object, often referred to as Theia, collided with the early Earth roughly 4.5 billion years ago.

Let’s have a look at a little breakdown of the process:

  • The initial conditions: in the early history of our Solar System, our planet was still in its formation process, surrounded by a disk of debris left behind from its accretion.
  • The collision: a protoplanet called Theia, which was about the same size as Mars, collided with Earth when our planet was very young. The collision caused large chunks of material to be blown away into orbit and eventually revolve around the Earth due to gravity.
  • Accretion disk: The disk of debris caused by the collision eventually coalesced because of gravity to form the Moon as we know it today.
  • Moon formation: Over time, the material from the debris disk began to clump together to form moonlets that merged to form our natural satellite.

Why is there only one Moon revolving around Earth?

There are a few factors that have likely played a role when it comes to answering the conundrum of why our planet has only one Moon.

Collision and rejection

The early Solar System was a very tumultuous place, as many collisions and interactions between cosmic objects occurred. There’s a possibility that other potential satellites were destroyed in collisions or ejected from the Earth-Moon system because of gravitational interactions.

Size and distance

Our planet has a relatively large size and distance from our star, the Sun. This situation might have made it less likely for extra large bodies to be captured into stable orbits around Earth. Let’s not forget that the Moon is already quite large compared to Earth. Our natural satellite is about a fourth the size of our planet in width.

Gravitational stability

The presence of a single large natural satellite may provide gravitational stability to the orbit of our planet, meaning that the likelihood of extra bodies being captured or forming stable orbits is reduced.

It’s obvious that our Moon, although it’s all alone in the orbit of the Earth, is an incredible cosmic object. Even alien life could be present on the Moon as well. It’s not only conspiracy theorists saying it, trying to justify why NASA hasn’t landed human crew there for so many decades since 1972; it’s NASA itself claiming it!


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Even since he was a child, Cristian was staring curiously at the stars, wondering about the Universe and our place in it. Today he's seeing his dream come true by writing about the latest news in astronomy. Cristian is also glad to be covering health and other science topics, having significant experience in writing about such fields.

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