How Earth destroyed the ninth planet

When NASA scientists began to analyze samples from the moon landing in 1960, they discovered something fascinating. The moon rocks the team had brought back resembled the rocks and dust on Earth. In fact, aside from lower water and iron content, the moon’s chemical composition was almost identical to the Earth’s. This was huge.

In case this does not seem significant, let’s get some perspective on the matter. The moon is without a doubt the most studied astronomical body in history, but by the end of the nineteenth century, scientists still had no idea how it got there. Theories had ranged from the plausible (the Earth and the moon formed together) to the farfetched (the Earth used to have a really big atmosphere and was able to capture a rogue, pre-formed moon), but further scrutiny uncovered holes in them all. By 1900, the predominant though mathematically questionable explanation involved centrifugal force from a fast-spinning Earth throwing off material that coalesced into the satellite.

In 1947, geologist Reginald Aldworth Daly offered a different solution. The moon, he proposed, was formed by the impact of the Earth with another body. The collision had destroyed the foreign object and sent dust and debris into space, where it eventually fell into orbit and coalesced into the moon. Furthermore, this was no comet or asteroid. According to his calculations, the foreign body had to be much bigger: planet sized.

Fast-forward to our NASA researchers, and we can see why the moon rocks’ composition was so extraordinary. The chemical similarity implied the Earth and moon shared a common origin, supporting Daly’s impact hypothesis. At the same time, new theories of star formation suggested these types of collisions were common in the early solar system when Mars-sized protoplanets occupied overlapping orbits.

Theia Moon Giant Impact Hypothesis Crash Collision Picture Artist's Impression Weekly Show

An artist’s impression of Theia crashing into the Earth. The collision vaporized the ninth planet, blasting debris into space and leaving much of Earth in shambles.

Though Daly’s theory did not gain widespread recognition until the 1970s, the giant impact hypothesis has become the standard model of the moon’s formation. Known as Theia, the Mars-sized body slammed into the Earth some 50 million years after the birth of the solar system, flinging debris into space and disintegrating much of both planets in the process. Models suggest the debris coalesced within a month into two moons, which eventually came together as one. The collision greatly increased Earth’s mass and angular momentum. In fact, after the impact our planet experienced a day approximately five hours long.

The hypothesis explains most but not all of the moon’s features. As mentioned earlier, it accounts the Earth and moon’s common origin, and it stands consistent with samples showing the moon was at one point molten.  It also shows why the Earth’s spin and the moon’s orbit are so similar and why the moon is less dense and iron deficient compared to the Earth. Two main problems with the hypothesis are that it suggests the Earth would have been covered in an ocean of magma after impact, which doesn’t show up in geological records, and that Venus experienced a massive collision and does not have a similar moon (fun fact: Venus spins backwards compared to everything else in the solar system, suggesting it was hit by something so big that it reversed its spin).

So that’s how Earth destroyed the ninth planet and the origin of the moon. This is one of my favorite models in astronomy. It’s crazy to think imagine the Earth colliding with another planet. Is this as cool as I think it is? Leave a comment, yo. As always, please like, share, or reblog this post if you enjoy it. That small click really helps me out! Be sure to check me out on Twitter and Facebook as well. Thanks for reading! Don’t forget to subscribe for new content every Wednesday! IT’S FREE!

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Comment question of the week

Is this giant impact hypothesis cool or what? Can you imagine Earth crashing into another planet?


2 thoughts on “How Earth destroyed the ninth planet

  1. Interesting theory. Seems very chance-like that the moon just so happened to be far away enough to not collide again and close enough to stay in orbit. But good nonetheless! I wrote a recent article on the anthropic principle, check it out!

    • Thanks for the comment, szea426. Sorry for the delayed response–I’ve been swamped with midterms the last few days. You make a good observation. I’ve read a few articles that discuss it, but I haven’t been able to find anything in depth. From what I’ve read it seems most of the debris would have either fallen back to Earth or shot out into space, but what eventually became the moon coalesced far out enough to keep a stable orbit. It’s worth pointing out that the material fell into orbit before forming the moon, so it shouldn’t be too surprising that it remained there once coalescing.

      Another big question about the theory is whether it can explain why the Earth and moon have such similar compositions. Calculations suggest the moon would have formed out of debris from both Earth and Theia (about 40% from the latter), but samples from the moon are incredibly similar to the Earth. One study I came upon suggested that the debris and remains of Earth formed a sort of channel between them and homogenized the system before the moon coalesced, but more work needs to be done to see if this is possible. All in all, it’s a pretty exciting theory!

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