We know it happens occasionally, or there would be no Superman and Khan might have ended his days happily, but how? I mean, exploding is pretty much the opposite of what an Earthlike planet does, which is persist as a huge mass pulled into near-spherical shape by its own weight in its own gravity field. What does it take to overcome that?* Is nuclear fission at the core on sufficient scale conceivable – say, from fissionable metals in the crust or mantle somehow and for some reason coming together in critical masses as they never have before in planetary history? If not that, then what?
Even that beam from the Death Star should only have overcome it momentarily. I mean, you would expect Alderaan to collapse back into a (lifeless) sphere of (nearly) its original mass almost immediately, wouldn’t you?
It would come back together because of a little force called gravity. Voyager long ago reached escape velocity and gravity is not strong enough to attract it back to Earth. It would take a titanically enormous force to fling the fragments of a planet far enough away so they would not be re-attracted to one another (much greater than the force required just to blow the planet into fragments).
Superman comics actually discussed this in a “return to Krypton” story. The planet is described as suffering massive disruption and upheaval but from a distance, looked whole. Only a deeper scan showed that the mass was still roughly planet-shaped but consisted mostly of mountain-sized asteroids bumping against each other, tightly orbiting a molten core. As best I can recall, the author (John Byrne) had Superman observing:
“It’s been fifty years since Krypton exploded. It might take fifty million years for a solid planet to condense out of that mess. Another billion for the first chance at life…”
Planets are held together by surface tension, much like a droplet of water. That whole gravity thing is just a theory. Once the surface tension is broken everything goes ‘splat’ or '‘blewie’ and comes apart.
Hey, I checked, and this is not in General Questions.
Here’s my totally unsupported fan theory for mysterious explosions like what happened with Ceti Alpha VI in Trek. Recall the mirror universe? They are very different than the “regular” universe, but despite that there are many similarities like the mirror-characters that shouldn’t be possible thanks to the butterfly effect. The universes are connected somehow; some force keeps certain aspects of each universe identical or close to it no matter how others diverge. Well; if you blow up a planet in such a mirror universe, what happens to the mirrored planet? If a “regular” planet explodes, perhaps it’s because in a linked mirror universe, some space fleet made their version of that planet explode; and whatever force it is that keeps the universes mirrored tore apart the “regular” planet.
If you want to fanwank it, you could say Krypton was very rich in radioactive isotopes, resulting in an abundance of natural nuclear reactors, such as we had at Oklo 2 billion years ago. Some sort of geological event could have injected lots of water, which would have acted as a neutron moderator. This would increase the reaction rate, leading to an explosion.
I don’t think a natural nuclear explosion is completely impossible, just very unlikely. Early in the history of the solar system, there was a much greater abundance of radioactive isotopes, which have since decayed.
I did the calculation a while back on how much energy it would take to give escape velocity to every part of a planet (so it wouldn’t reassemble after the explosion). Here’s what I wrote:
For objects large enough to be rounded by gravity (into which category objects considerably smaller than the Moon fit), the dominant force holding them together is gravity. It’s surprisingly easy to calculate how much energy is required to overcome that force: start at the outside of the object and calculate how much energy it takes to accelerate a infinitesimal shell of material to escape velocity, and iterate for shells closer and closer to the center of the object until there’s nothing left (the onion-peeling approach to planetary-level destruction). If you assume that the object has uniform density, the answer is 3/5GM*M/R where M is the mass of the object, R is its radius and G is the universal gravitational constant (G=6.67300 ? 10^-11 m^3 kg^-1 s^-2). This is a lower bound for three reasons - first, it ignores other forces holding the object together, second, big objects will tend to be denser in the middle, which increases the energy required to pull them apart, and third, inefficiencies (like parts of the planet colliding with each other) will naturally exist - but a lower bound can be useful…