Are there other fragments aside from the Moon that resulted from the collision of Earth and "Theia" (as it's called in recent main-stream press stories)? And if not, why not? And what happened to "Theia" (like where did it go)?

Has there been any consideration or hypothesizing that Earth may have had a larger orbit (say between Mars/Jupiter) until a large body (Theia) bashed into it, sending Earth closer to the sun along with its new satellite, while also creating the asteroid-belt? Could something like Ceres be the missing "Theia"?

I know it sounds a bit far-fetched, but then so does the whole concept of the Moon being born from a planet collision with Earth? (With very little evidence to back it up, at least till recently).


2 Answers 2


Let's start answering your question in reverse. Ceres could not be "the missing Theia", because of its shape. If an object hit the Earth at an angle (as is currently thought), it would be pretty deformed, if it managed to stay together. If it hit the Earth head on . . . Well, it would almost certainly not survive. So if it did survive the impact, it would be in pretty bad shape. Where would such an object go?

Let's investigate the idea that it went into the asteroid belt. That begs a question: Where is it today? Ceres is the largest object in the belt (and we've already ruled it out). Theia had a mass about that of Mars (see Canup (2012)). It's safe to say that if Theia survived with much of its mass intact and then disappeared into the asteroid belt, we should have observed it by now.

Now for your belt-formation theory. It is, admittedly a very cool idea. The mass of the asteroid belt is about 4% that of the Moon - so it could be the result of the planetary crash. And some theories say that the belt is, in fact, the remains of a planet. There is, of course, one problem - the different composition of the asteroids. There are three types, differentiated by their composition - C, S, and M. So it is very unlikely that they came from the same planet. Meier et al. (2014) show evidence that indicates Theia's composition would have been more like Earth, containing enstatite chondrites instead of carbonaceous chondrites.

Finally, let's look at your idea about the Earth having an orbit farther out. This one seems to have a good chance of being right. The early protoplanets in the Solar System were very - very - unstable in their orbits because of the frequency of collisions. So the Earth could have been in a different orbit early on, and then migrated inwards or outwards, though planet-planet scattering or perhaps gas disk migration.

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    $\begingroup$ I don't think the shape of Ceres is an issue. It's big enough for its own gravity to pull it into a sphere. $\endgroup$ Aug 11, 2014 at 20:51
  • $\begingroup$ So, @KeithThompson, you're saying it could have re-formed? $\endgroup$
    – HDE 226868
    Aug 11, 2014 at 20:52
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    $\begingroup$ I'm saying that if a body the size of Ceres were deformed into some non-spherical shape, it would re-form itself into a sphere over time. Any significant irregularities would not hold up against gravity. We have at least one Hubble photo showing it to be close to spherical; this is not a coincidence. I don't suggest that it's plausible that Ceres is a remnant of the Theia/Earth collision, but its shape doesn't argue against it. $\endgroup$ Aug 11, 2014 at 20:54
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    $\begingroup$ At the moment, it appears Ceres survived the early solar system: en.wikipedia.org/wiki/Ceres_(dwarf_planet)#Origin_and_evolution. Edit: Posted this before I saw your last comment. $\endgroup$
    – HDE 226868
    Aug 11, 2014 at 20:55

I hope it's OK to answer an old question.

First, let me say that your proposal is creative. You've explained the possible creation of the asteroid belt (by collision - not impossible), and you've combined it with the formation of the moon by impact. That 2nd part is a little more problematic.

There's a principal in scientific theory and it applies to astronomy that as you add requirements to a scenario, the probability decreases significantly. (I don't know if there's a name for that principal but I've read it, I'm not making it up).

Your hypothesis:

Earth was once outside of Mars. Earth was struck by a large object creating the Moon That impact blew off matter which became the asteroid belt The impacting object may have become Ceres The impact pushed the Earth into a lower orbit.

That's a LOT of events strung together, which raises the improbability of your scenario. That's not to say it's impossible, and improbable things can happen, but you need a LOT to fall into place for the theory to work.

Could Earth have once been outside Mars? Sure, theoretically. Planets have been known to migrate, though how common this is isn't well understood and if Earth and Mars passed close to each other, Earth could have given Mars a kick outwards. It's possible that the inner 4 planets may have swapped orbital positions in the past.

The giant impact that created the Moon is an accepted theory, though there's some variation on the precise mechanism, it's generally accepted and it explains a few things like the similarity of isotopes and nearly identical age of the Earth and Moon and the high angular momentum of the system and the Moon's very large relative size. This part of your model is strong.

The impact blew off debris that became asteroids - I've not seen any mathematical models for how much debris could have been blown outside the gravity well when the giant impact happened, but if I was to guess, I think it's very reasonable that the giant impact left a debris field of asteroids as a result of the impact.

Theia became Ceres. - This is a problem, because the material that makes up planets behaves like a liquid when they smash together. If it had been a glancing blow, then maybe, but there's the problem that the moon is much more massive than Ceres. A glancing blow wouldn't create a larger collection of debris and leave a comparatively tiny impactor. The Moon is like 70 times the mass of Ceres, so that just doesn't work.

Now if you'd said Mercury may have been the impactor and Mercury had a glancing blow, in the process, Mercury losing much of it's outer shell as it nicked the Earth - creating the Moon in the process, it would still be unlikely but at least the masses would work. Mercury could in theory, set the Earth spinning and in the collision lose a lot of it's outer material. I'm not saying that happened, only that the mass of mercury is close to what you'd need to set the Earth/moon system spinning so fast.

It's generally assumed that Theia was absorbed and completely destroyed in the impact, but a glancing blow by Theia becoming Mercury .... just maybe.

The Earth's orbit changed - this is also a problem. With any giant impact, some of the energy is lost in debris flying away and some is converted to heat and angular momentum, but generally speaking, momentum is still concerved. In other words, to slow the Eareth down and drop it into a lower orbit, the two objects would need to be moving in different orbits, that is, the smaller object (Theia) would need to be orbiting the wrong way around the sun. While that is possible (perhaps from a gravity assist around Jupiter), it's unlikely. What's more, opposite orbits greatly increases the speed of impact, so for the Earth to survive, you need to decrease the Mass of Theia if you increase the velocity of impact.

An impact changing Earth's orbit would send Earth into a more eccentric orbit, which then could theoretically interact with Mars and circularize over time, but the idea of a planetoid with a wrong way orbit hitting Earth increases the improbability.

Don't get me wrong. I like where your theory is coming from. It's just when the details are examined, it appears improbable.

That said, as asteroids in the belt are examined, if a number of them are shown to have earth and moon like isotope ratios, that might lend support to aspects of your proposal, though Ceres having once been Theia is highly unlikely, both because it's too small and Theia is believed to have not survived and because Ceres has an enormous amount of water. Ceres is more like an outer solar-system object, like a formation moon of one of the outer planets or a dwarf planet in the Kuiper belt, that by some mechanism, probably a gravity assist, may have migrated inwards.

Keep coming up with theories. I like your approach.

  • $\begingroup$ "to slow the Earth down and drop it into a lower orbit" What do you mean? If by "lower orbit" you mean one with a smaller mean radius, then the Earth would need to speed up, not slow down. $\endgroup$
    – PM 2Ring
    Sep 27, 2019 at 8:20
  • $\begingroup$ @PM2Ring smaller mean radius, yes. More specifically smaller semi major axis or shorter orbital period. (The semi major axis may be the mean radius, I'm not 100% sure). But on slowing down/speeding up - it does both. Earth orbits the sun at an average velocity of 30 km/s. If you speed that velocity up, Earth will move away from the sun, but in moving away it will slow down. If you slow down the 30 km/s, Earth will fall closer to the sun and in the process, speed up. $\endgroup$
    – userLTK
    Sep 27, 2019 at 13:24

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