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First, is it possible for a planet hit be a meteor to shatter into two pieces?. Then, can the two pieces remain in orbit? Especially, can they develop a sort of co-orbit around each other?

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  • $\begingroup$ Sure why not you might even get 2 ring planets. $\endgroup$ – user6760 Apr 18 '15 at 9:21
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    $\begingroup$ Sorry a meteor you say that's impossible as it is smaller than typical asteroid but superweapon maybe. $\endgroup$ – user6760 Apr 18 '15 at 9:25
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I like TimB's answer discussing the Giant Impact Hypothesis. I see that in a comment on his answer, you wrote

Thank you! I actually had this Moon origin theory in mind. But I'm actually wondering about an inhabited planet, in the immediate run. Is there any possibility of survivors, say, after the shattering? For a while, at least? Any idea?

The answer is most certainly no. I'd like to go in-depth as to why that won't be happening.

In an answer to one of my questions, Serban Tanasa discussed the conditions of impacts. He cited Stewart et al. (2015) (which cites an earlier work by the three, Stewart et al. (2014), giving the formula for the energy released in an impact, $Q_S$, as $$Q_S=Q_R(1+M_p/M_t)(1-b) \tag{1}$$ $Q_R$ is calculated as $$Q_R=\frac{0.5 \mu V^2}{M_p+M_t} \tag{2}$$ In this latter paper, they mention that for grazing impacts, $b>\frac{R_t}{R_t+R_p}$, so we'll assume here that $b<\frac{R_t}{R_t+R_p}$.

You mentioned a meteor. A meteor will do next to no damage to a planet. In this case, $R_p \ll R_t$, so we're left with $b<1$. To do some real damage - enough to do what you want - we need $R_t \approx R_p$. So $b<\frac{1}{2}$. We can, though, say that the projectile isn't too massive. We'll toe the line and have $b=\frac{1}{2}$.

Assuming the densities are the same, $R_p \approx R_t \to M_p \approx M_t$. We now have $$Q_S=Q_R(1+1)\left(1-\frac{1}{2}\right) \to Q_S=Q_R$$ This means that $$Q_S=\frac{1}{4} \mu V^2 \tag{3}$$ $\mu$, the reduced mass, is $$\mu=\frac{M_pM_t}{M_p+M_t}$$ Assuming both bodies are Earth-like, this means that $$\mu=2.985 \times 10^{24}$$ It doesn't matter what $V$ is. Looking at one of the graphs, I see that whatever $V$ is, $Q_S$ is going to be off the charts:

We're all going to die.

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Yes, in fact that is one theory of how the Earth and our Moon came into being. A large body hit the earth, ripped off a section of it, the smaller section ended up in orbit around the larger.

Note that gravity of the chunks will always try and pull them back into a spherical shape though, so you will end up with two smaller spherical bodies orbiting each other. You won't end up with "shattered chunks".

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  • $\begingroup$ Thank you! I actually had this Moon origin theory in mind. But I'm actually wondering about an inhabited planet, in the immediate run. Is there any possibility of survivors, say, after the shattering? For a while, at least? Any idea? $\endgroup$ – Sykik Apr 18 '15 at 8:31
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    $\begingroup$ Actually, Tim B is wrong if your question refers to splitting the planet into two pieces (immediately after the collision). The collision produced a massive debris field in orbit around the earth, and the debris fairly quickly coalesced to form the moon. The short explanation is that the collision would act like an enormous bomb, and the strength of the planet is not high enough to produce two pieces which go their separate ways. Instead, you get shrapnel on a planetary scale. As with the Moon, stuff can solidify into a body, but it doesn't start out as two pieces. $\endgroup$ – WhatRoughBeast Apr 18 '15 at 15:47
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    $\begingroup$ @Sykik No survivors. The Theia event would turn both bodies to magma. $\endgroup$ – Loren Pechtel Apr 18 '15 at 19:44
  • $\begingroup$ Agreed, the Theia event liquified both bodies which then took a long time to both coalesce and cool. I'm not disagreeing with @WhatRoughBeast said - he meant quickly on a geologic or astronomical time scale. $\endgroup$ – Jim2B Apr 19 '15 at 4:14
  • $\begingroup$ See this presentation for current models. It is gas, not (just) magma. An extended body that behaved as an atmosphere. $\endgroup$ – JDługosz Apr 19 '15 at 6:02
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Given the amount of energy needed to destroy a planet, even sending another planet on an impact orbit will not do what you want (as a BTW, the impactor that smashed into Earth and created the Moon is estimated to have been the size of Mars).

For the amount of energy you will need, a futuristic super weapon is the tool required. Cribbed from another website we get the amount of energy needed:

The gravitational binding energy of a planet of mass M and radius R is given by the formula $$E=\frac{3}{5} \frac{GM^2}{R}$$. For Earth, that works out to roughly 224,000,000,000,000,000,000,000,000,000,000 Joules. The Sun takes nearly a WEEK to output that much energy.

So a relativistic hyperweapon approaching the Earth at a very high fraction of the speed of light is probably the best way of going about planet "cracking", but you need to calibrate the impact rather carefully. Too "slow" and you get a ball of molten magma that gradually collapses back into a planet sized ball (or maybe a planet and one or two moons). Too "fast" and you end up with a cloud of gravel forming a new asteroid belt around the Sun. Even more energy and the pieces reach Solar escape velocity and exit the Solar System for a lengthy tour of the Galaxy.

An entertaining synopsis can also be found here: https://what-if.xkcd.com/20/

You will see according to his calculations, you need to impact the Earth at 0.9999999*c* to get to the effects that you are looking for, and far beyond any sort of engineering that a Type I civilization would be able to unleash on its unwary interstellar adversaries.

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No

You could not split a planet in the manner you described as several answers here pointed out.

Maybe
However, there is a special case which might result in a split body.

Consider all bodies in the Solar System are split into two groups: 1) the roughly spherical ones and 2) the non-spherical ones.

  1. Spherical

    Spherical bodies are in a state called isostatic equilibrium, which is a fancy way of stating that their shape is governed by their own self-gravitation. These bodies could NOT be split by a meteor / asteroid / weapon impact. If you supply enough energy to split them, you will instead disrupt them.

    In the case of the Theia event, the Earth and Moon formed out of the debris produced by the disruption.

  2. Non-spherical

    Non-spherical bodies are shaped by things in addition to their own gravitation or at least gravity isn't the biggest factor in their shape. Instead the strength of the material is sufficiently great to resist the gravity of the asteroid and maintain its non-spherical shape.

    If you struck a rock asteroid with enough force to cleave it and impart enough momentum, you might get the two halves of the body rotating around each other.

    Given some of the odd, peanut, shaped asteroids out there; it's possible this has already happened many times and we're just now learning about it.

    Asteroid Itokawa may have started as one of these. Asteroid Itokawa

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    $\begingroup$ The bar or peanut shaped asteroids are contact binaries. I suppose if you hit one lobe you could make it come apart again. $\endgroup$ – JDługosz Apr 19 '15 at 5:56
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    $\begingroup$ I didn't make it clear... I meant some contact binaries could have started as a cleaved asteroid. Others (like the one depicted), the different masses indicate different origins. $\endgroup$ – Jim2B Apr 19 '15 at 15:15
  • $\begingroup$ What would make it cleve and then come back together, and how could each piece be round if it was produced by chopping a round object? $\endgroup$ – JDługosz Apr 19 '15 at 19:19
  • $\begingroup$ Anytime you get two objects orbiting each other (other than an impact), it takes 3 bodies and a transfer of momentum. For objects in the inner solar system, one of those bodies can be the sun. To cleave an asteroid and get them orbiting would require time and a chance interaction with the gravity of some other object. Objects become round through abrasion (microscopic meteor impacts) over geologically long periods of time. $\endgroup$ – Jim2B Apr 20 '15 at 4:10
  • $\begingroup$ So the contact binary was an object that was cleanly cleaved while leaving them in mutual orbit, then the pieces became rounded, then came back together carefully? What's wrong with just having two unrelated asteroids end up in similar orbits and then come together? $\endgroup$ – JDługosz Apr 20 '15 at 4:26
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No, it's not possible. You can have something like Tim B's answer regarding the Theia impact but that is not a case of a planet shattering into two pieces, but rather a grazing impact where the impactor broke up but went into orbit rather than merging with the planet. (Yes, there was substantial mass exchange, but the moon is mostly impactor, the Earth is mostly Earth.)

To actually split a planet is another matter--the basic problem here is that the heat of vaporization of the planet is less than the gravitational binding energy of the planet. A simple application of brute force can't break it without destroying it.

Even if you have some sort of gravity generator to pull it apart without smashing it in the process you'll have the problem that the pulled-apart pieces are massive enough that they're going to turn into spheres. That will be cataclysmic for anyone on them. Not to mention that the atmosphere is more mobile than the rock, it's going to fall off the edge of the world. Everyone suffocates.

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    $\begingroup$ Got a reference on "but the moon is mostly impactor, the Earth is mostly Earth"? I thought the lithosphere was pretty well mixed and the cores coalesced. Here's mine current as I write this. $\endgroup$ – JDługosz Apr 19 '15 at 5:59
  • $\begingroup$ I just gotta say, I loved your reference @JDlugosz! It takes an hour, but I thought it was well worth the time. $\endgroup$ – Jim2B Apr 20 '15 at 4:07
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An "advanced interstellar weapon" would need to carefully cut the globe in half without heating past the edge, while simultaneously shoring up the exposed inside half so it doesn't pull itself back into a ball.

Digging a trench allmthe way through, in and of itself, is just an engineering problem. Self-reproducing machines could dignand build more with the removed material. One could calculate the energy needed to move a half-mile wide curf out of Earth's gravity.

Sealing the cut end, and building a wall to hold the air, probably involves materials stronger than what is physically possible. However, it does not have to be a ultra-thin covering. The half of the world does the job now, passively. Make an extended slab that is dome shaped but much smaller than the original half it's replacing, so it has bulk (hundreds of miles) especially towards the center. It allows for active cooling too, so it doesn't need to withstand the temperature like a passive material.

That would cause the half-world to cool much more rapidly, and the gravity woukd be all messed up so even with the cap (with a lip) the oceans and air would fall towards the middle of the hemisphere, as the thin lens edges don't have enough attraction. The rocks would do likewise: even with a strong cap over the cut, it would collapse in the other direction. The cap needa to be made with ultradense material to provide gravity, matching the original missing half. It would be less massive than the original because it is smaller, so not 4000 miles away in places.

I think you should try it with an airless world first. If you slice the moon, you could get away with hollowing it out too.

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