Science Questions

How do we know that another planet collided with the Earth?

Sun, 22nd Feb 2009

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Peter Robinson asked:

How do we know that another planet collided with the Earth?


I saw a TV programme that said during the Earth’s evolution another planet called Thea smashed into it. How do we know this when it doesn’t exist anymore?



Chris - Well, we have to go by what the models are telling us and what samples we’ve got.  We’ve got a very large moon around the earth.  In fact, it’s unfeasibly large for a planet of our size.  Why have we got such a big moon where it is?  What the prediction is, and based on what we know about the composition of the moon from samples that the Apollo astronauts have brought back is that the moon is made of exactly the same stuff, give or take, as the surface of the Earth – the Earth’s crust.

The big question is how did something made pretty much of the same material as the Earth end up orbiting the Earth unless something bashed it an put it up there?  The best suggestion that scientists can come up with, based  on all the evidence we have, is that during the early phases of the formation of  the solar system (something like 4.5 billion years ago) there were two planets.  One a future Earth, one another planet which they’ve notionally called Thea.  These were very similar in terms of their orbital pattern.  One ran into another – it was like cosmic billiards that went on.  As a consequence of their massive great collision the cores of both planets effectively fused.   In the course of this collision a lot of the surface material from the  Earth got ejected into space and it formed a sort of shroud around the Earth which slowly coalesced in the same way that rings around Saturn have coalesced to what would have originally been an envelope.  They then coalesced and aggregated to form the moon.

It’s on the basis of there’s no other better explanation than that one to explain why we have this phenomenon of this big moon and what the moon’s made of.


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peter Robinson asked the Naked Scientists: A TV programme stated that during Earth's evolution another planet, Theia?, smashed into it. How do they know this, how do they know when and how do you name a (now) non-existent thing? What do you think? Pe-Pe, Fri, 19th Dec 2008

We don't know it because there was no one there to witness and record it.  However, the consequences of it are apparent today and these form a record of evidence for which the planetoid collision scenario is by far the best fit solution. LeeE, Fri, 19th Dec 2008

LeeE: Is correct;too see this evidence you simply have to look up.The answer to this question;began with another question.Where did the moon come from? Not only do we have our own moon but we only see one side of it the reason for this is in its early stages of formation the moon was molten like lava in fact it was lava. The liquid moon began to cool at the same time as it orbited our planet the gravitational pull of the moon on the earth so close to our planet in its original orbit slowed the rotation of the earth. The way this works is through the tidal forces that still pull at the earth today;Kind of slowing the planet like dragging A boat paddle throw the water to slow A canoe. At the same time the earth is doing the same thing to the moon;as the moon cooled it became more solid and eventually locked its rotation to its orbit around our planet. Soil samples from the moon have been studied and are known to be made up of mostly the same material as found on our planet.So Either some thing come along and sliced A moon size chunk off A much bigger earth;(man that's A big knife!)or A smaller earth collided with A smaller planet that was moving in just the right trajectory to hit the planet not dead on;that would have resulted in A situation where the material would have globbed together into A single planet;but at an angle that could leave enough molten material in orbit after gravity and friction had it out;too form our moon... Hope this helps TECHFACTOR:OUT TECHFACTOR, Fri, 19th Dec 2008

You can find all the evidence of Thea in a book called; the cataclysmic impact/planet earth's most violent intersect. It clearly provides an abundance of evidence, concentrating on Earth's biggest and most profound impact crater. Vredefort, Barringer, and Chicxulub craters combined, would not equal a quarter of this impact's enormity. DR, Wed, 23rd Dec 2009

OK, seems logical- but then where did the rest of Thea go to?? Wells, Tue, 12th Jan 2010

The Earth is believed to have absorbed the other planet which had been in the same orbit as Earth. It slowly caught up to the Earth. It was a Mars sized planet. They collided at small angle instead of a direct collision. This allowed the Earth to absorb the other planet. Most of it is believed to have gone into the core of the Earth making it just right for us today. The entire planet was molten for quite awhile. The collision is also believed to have blew off excess water into space so we ended up with the perfect amount for us today. The collision also threw out a lot of rocky type debris which eventually setteled into the large moon we now have that is so essential to keep Earth on a perfect axis as well as regulate tides and other things. Personally, I give The Creator credit for such a fine tuning event that made it possible for us to have intelligent life on Earth today. There is an abundance of other fine tuning facts that make the chances of this all accidentally happening numerically so small that it equals zero. Carl, Wed, 20th Jan 2010

We do not know... The idea that the Moon was made by a glancing strike on this planet makes no sense. The reason Moon Rocks are like Earth rocks is because they are Earth Rocks. Man has not set foot on the moon. Probes, Yes.. Buzz and the boys did not pick up rocks and bring them back to Earth. Remember back in the 90's when they claimed the found a Mars rock on Earth? Well would it not be more likely be a Rock from Theia? or Thea... the "Mars size" planet that struck this world? Nope.. they said that rock came all they way from Mars... Such Junk Science. Our Moon is more likely to be a planet that was formed between Earth and Mars and drifted toward the bigger planet and had its orbit around the orrery captured the bigger Earth... Any theory that requires you to believe man walked on the moon 11 times is junk. Bones, Tue, 25th May 2010

Bones, you're an imbecile! CB, Mon, 14th Feb 2011

Bones, what an idiot! Does he live in a cave? Muppet lol, Sat, 22nd Oct 2011

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