What if half a planet (not necessarily earth) was destroyed (not split)

Question

Basically I want to know what would happen to a planet if half of it was destroyed using explosives. Does the answer depend on the size/gravity of the planet and what would be the result on the environment and inhabitants?

Per the comments, this is an earth-like, earth-sized planet, and for the intents of the question the half of the planet just disappears.

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Ok so almost all of my question has been answered, but a small detail remains: when the hemisphere that remains collapses, must the resulting form be a regular sphere? This is only sci-fi but can it form, I don't know, a cube, a pentagon, or a weird irregular shape like that of a sea urchin? And must the hemisphere even stabilize, or can it breakdown to chunks or form an asteroid belt-like shape?

Updated: Thanks a lot. From what I collected from your answers is that it would be way more merciful if for example aliens destroyed the entire planet rather than the inhabitants live through that hell (I apologize in advance for the lame puns). I mean if you're gonna do a job, do it right - don't stop halfway, don't take it halfheartedly, and just because its all right doesn't mean its alright.

Answer 1

Setting

We find ourselves on a planet very much like earth. Suddenly out of nowhere disaster strikes and an annihilating explosion on planetary magnitude leaves the poor thing with half its mass practically evaporated away, leaving no debris and for some reason very little damage on the remaining part.

Storms, earthquakes and disappearing oceans

Even on the furtheres points away from the disaster, powerful earthquakes will shake the ground as the planet is adjusting to its new gravitational center. Storms will be building steadily as the atmosphere is rushing to fill the gap. People living by the ocean will soon begin to see the waterlevel drop slowly but surely. If they have the time to notice such things amidst the building storms and quakes.

All things bad

Depending on which part of the planet was exploded away the path it travels through space will shift. I'm not sure in which way, so let's leave that for now.

If the remaining half was left unhurt by the explosion itself it will not be so safe from the planetary scale event that is going out of control inside it. What once was the gravitational core of the earth is shifting towards what makes up the center of the remaning half.

This is where the remaining solid inner core wants to travel. (or you could say that the remaining planet is being pulled around the core, but the effect is the same) And in doing that traveling it will be displacing what is left of the liquid inner core and burrow into the mantle.

The displaced liquid core will now spill out over the edges of the mantle and crust in tandem with the edges of the crust and mantle beginning to collapse under the gravitational forces.

The lower and more liquid parts of the mantle will move inwards towards the new gravitational centre and as this happens the outer mantle will begin to break apart into fractions. When this happens there is no hope of saving the crust and what might live on it. Volcanic eruptions will fire in the faultlines following the fractures. While this is happening a lot of the atmosphere will burn and evaporate away.

Stabilization

When the planet stabilizes around its new gravitational center it has been reduced to an uneven ball of fire and smoke. Any life we would recognize from Earth would now be dead and most organic remains burned away. Who knows when life might thrive again, if ever. Depending on the new path around the Sun, maybe never.

Edit in response to edit of question

the initial collapse of the remaining planet doesn’t necessarily form a regular core (after all the earth itself is somewhat irregular) but whether it reshapes into an approximate sphere in time will to depend on how much was left of the planet after the explosion. The bigger it is the more likely it is that gravitational forces will work it into a sphere, but the smaller it gets there is a point where the gravitational forces of the core would no longer be sufficient to alter its outer layers. I’m not sure what the limit is here, but I would guess it would be rather small. Ex, look at the moons orbiting mars, or look at the dwarf planet Ceres.

As an irregular shape it would most likely keep more regular on the areas furtherest from the initial event and more broken and flat on the side of the event. It might look somewhat like a sea urchin at that.

As for whether it would break down into chunks I would assume that to be unlikely if the event initially left the remaining half of the planet relatively unharmed. If this was not the case and the initial explosion did impact the remaining half with considerable force then it is more likely that the remains would break into bits. In which case a lot of these bits would be liquid or molten rock bobbing and bubbling through space as they slowly cool down.

However, unless fractured and broken by the explosion itself I would assume the gravitational forces of the remaining body would be too great for it to simply crumble into pieces that drifted away from each other.

Answer 2

Any celestial body over a radius of a few hundred kilometers will collapse into a sphere. Even solid stone cannot hold against gravity and it will break apart or "flow" as the atoms themselves are forced apart.

It is safe to assume that your planet (having an atmosphere and all the usual goodness that makes it suitable for life) has a radius at least 10 times that.

If you suddenly take away half the mass of the planet, it will still be way over the limit, so it is going to shift until it is once again a sphere. This is likely to be more violent and spectacular than even Roland Emmerich can imagine. Not only will the molten core be exposed and mixing with the outer crust, the friction from all of that shifting mass is likely to heat the crust itself to high enough to set things on fire everywhere or even to start melting itself.

It's unlikely there will be any inhabitants alive after the planet settles into its smaller self, and if there are any that survived the cataclysm, they can then gasp for air (in a much thinner atmosphere) while they enjoy a good view of the shrinking sun as the planet zooms off into a new orbit much farther out, or even escapes into deep space.

Answer 3

Well, LOOOONG-term the remaining half of the planet will form back into a spherical shape again, if there's enough mass left... but that's probably not in a timescale that'd interest the environment and inhabitants ;)

If you remove half the planet with a method that magically disappears it, you'd still expose its core - which, depending on its age, may well still be liquid. The entire mass of the planet will try to get back into a spherical shape - a liquid core will make that happen faster than if the planet had cooled and solidified completely, in which case I'm not sure but it might be able to hold its hemisphere for a while, possibly even a geological-scale while.

If there's a liquid core and mantle, I'd imagine that flowing away from the outer parts of the remaining shell to try and get spherical (but smaller) again in the middle - leaving the crust of the planet without the warm bed of magma it was floating on. So the continents that people, animals and plants live on would collapse into caverns that are still half-filled with magma. Though depending on how fast the magma flows and how long it takes the crust to collapse, the fall might be enough to kill them before they actually hit the hot stuff ;)

Either way, any oceans that are at all connected to the new free path "down" would become beautiful (to an outsider) waterfalls, and drain over that edge. Even worse for any inhabitants, ditto for the atmosphere - the air on that planet will try to form an atmosphere as close to the center of gravity as it can, meaning the hot swirly ball of magma forming in the middle with the steam formerly known as oceans will have breathable air around it. The maybe-still-holding crust? not so much.

Oh, a small fun fact: lower gravity. Less planetary mass will do that.

So, the inhabitants will find themselves weighing less, but this will likely not help them get over the insane storms produced by the entire atmosphere getting closer to the new, smaller planet forming inside the hollow shell of their former home. The oceans draining down there won't really matter anymore to all the suffocated people, but the waterfalls might be nice to look at for anyone who survives long enough by hiding from the storms somewhere sturdy and getting into something space-suit like... at least until the now-hollow crust collapses into the new, smaller and likely liquid form of that planet and produces nice splashes on a titanic scale.

(if the planet didn't have a liquid core, the atmosphere and ocean bit would still happen)

Oh, but as long as there's still a piece of core left that forms a valley or bowl, there might be atmosphere and water caught in there. Depending on how long the collapse takes, it might allow survivors to live there for... I'm not sure. Months? Years? Decades or centuries even, if it was a planet with a cooled, solid core? Though with no connection to planetary weather patterns, problems might arise from THAT.

Answer 4

I see that this has been well answered; however, I did want to add one note. If this were to happen, the first thing that life forms on the surviving half would experience would be a cataclysmic windstorm. The atmosphere over the surviving half as well as the oceans would spill over the edge toward the now exposed core and the center of the earth's gravity. I don't believe that much would survive the event and anything that did which needed to breath gaseous air would likely suffocate as the air became too thin (I am assuming that when you remove half the earth that you also remove half the atmosphere with it as that is a part of the earth...),