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I like superhero movies and Superman is one of my absolute favorites.

And one stable thing in that universe is always that Krypton goes kaboom, the reasons for this are multiple: from the original one where Krypton's core was made out of uranium, to the planet's shifting its orbit (from what I can assume that would mean it was ripped apart by the sun's gravity) to some evil scientist deploying a doomsday weapon that slowly changed to composition of Krypton's core so it went radioactive and then it eventually exploded.

We have also seen various depictions of how it would look like for the kryptonians as their planet exploded around them.

But if we where to apply real life science to all the various scenarios (and just assume that somehow they where plausible and the planet was blowing up), what would it look like for anyone standing on the surface of Krypton in the final hours leading up to the explosion of Krypton?

Would it start with slow tremors, that steadily got worse and worse until the planet ripped itself apart or would it be more like the cartoon show and Man of steel movie, where we see various geysers of Lava spewing up coupled with some earthquakes before the planet exploded.

Also how long into the destruction of krypton would kryptonians survive?

Lets assume that they did whatever they could to secure themselves, e.g. if the planet got closer to the sun, they would seek to go underground or something like that.

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  • $\begingroup$ I think you need to be a bit more specific concerning what is happening and how quickly it is happening. If the core was somehow made of fissionable Uranium then the whole planet is gone in a a few seconds, if it gets too close to the sun then the question is how quickly and how close? If its a doomsday weapon how quickly does it explode? $\endgroup$
    – Slarty
    Dec 3 '20 at 17:51
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    $\begingroup$ Relevant quote from XKCD: "You wouldn't really die of anything, in the traditional sense. You would just stop being biology and start being physics." $\endgroup$
    – Mark
    Dec 3 '20 at 22:04
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In order to explode an earth-like planet so that it is destroyed, i.e. reduced to rubble so that it would not re-coalesce, it would be necessary to have most of its parts leaving its centre of gravity at a speed equal or greater than its escape velocity.

From what I remember of Krypton, it was a roughly earth-like planet orbiting a red star, and its sentient population were human-appearing beings without super-powers, unless they spent a lot of time upon a planet orbiting a sun with a considerably higher optical temperature than Krypton's - such as Kal-El spending fifteen years on Earth.

So, we can conclude that since Kal-el/Clark Kent grew up without being either hyper-muscular or gangly and weak in appearance after a childhood spent largely without super powers, Earth and Krypton must have surface gravity within around 10% of each other's. Since Earth has an escape velocity of 11,190 m/s, Krypton's is likely in the range of 10,071 to 13,309 m/s.

This means that whatever event led to the destruction of Krypton, it would of necessity have been extremely violent in order to propel the pieces of Krypton away from one-another at a minimum velocity of over 10 km/s.

Any event capable of propelling the pieces of an entire earthlike planet at over 10 km/s would most likely have been extremely abrupt. If the planet's core somehow exploded, there may possibly have been readily observable warning signs, but in all likelihood, to the inhabitants of Krypton, it would have been 'here one second, gone the next'. The acceleration necessary to propel the planetary crust fast enough to destroy the planet would have likely killed them instantly.

Alternatively, a supernova that produced enough energy to destroy the planet would have killed anyone on the daylight side of the world instantly, and likely anyone within hundreds of kilometres of the twilight area, and anyone on the night side would likely die shortly thereafter as the atmospheric temperature rose to the point where it would incinerate every living thing. Those who hid deep underground might survive a bit longer, until the planetary crust melted around them, or the pressure of energetic photons and expanding gases might simply blast away the ground above and around them.

If Krypton was knocked out of its orbit by some event - perhaps its orbital energy was stolen by some rogue planet or star - then that would offer the kryptonians the most drawn-out demise of all the options. Even if all of Krypton's orbital energy was removed, Krypton would still take time to fall into its sun, though the likelihood is much greater that krypton would spiral into its primary rather than making a right-angle turn and dropping in on a straight-line course.

The effect would be similar to that of a star changing from burning hydrogen to burning helium, but over a shorter time scale. Temperatures would rise, the seas would boil away, the atmosphere would be driven off, the ground would begin to melt, and then the molten rock would begin to boil away, until the planet had become part of the star.

People in such a situation who remained on the surface would have time to see their sun grow larger until the heat killed them, which could take some time, depending upon the speed at which Krypton was headed toward its star. Those who hid below ground could survive until the ground became hot enough to kill them by transmitted heat, perhaps many times longer than someone on the surface.

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    $\begingroup$ So in other words the way Krypton is shown blowing up in various media, such as Superman from 1978, Superman TAS (The last son of krypton) or Man of Steel is completely fictional, any "real life" Kryptonians wouldn´t have that much warning or such a "quick" or drawn out death before the planet blew up? Though i don´t know if you could say Krypton in man of steel blew up as it seemed that only the middle half blew up. $\endgroup$
    – Gridlock
    Dec 3 '20 at 15:12
  • $\begingroup$ NOTE: A central part of the Krypton Mythos is that it had a gravity much greater than the Earth’s. $\endgroup$ Dec 3 '20 at 21:53
  • $\begingroup$ This make sense to me. AFAIK there's no known mechanism for a solid planet to "explode" apart without some sort of external impact or large body approaching to tear it apart with tidal forces. $\endgroup$ Dec 3 '20 at 21:56
  • $\begingroup$ @RBarryYoung Perhaps... but comic book fiat doesn't fit the observed traits of the young Clark Kent/Kal-El. He supposedly developed super powers in his late teens, and prior to that, he didn't show any superhuman strength or develop into a skinny, gangly, super-tall person either, both of which would be expected had Krypton had high gravity. The skinny gangly thing might not happen, as Kryptonians were genetically engineered and vat-grown for the most part IIRC - Kal-El was supposedly the first naturally born Kryptonian for quite some time. $\endgroup$
    – Monty Wild
    Dec 3 '20 at 23:09
  • $\begingroup$ @jeffronicus I agree that there is no known mechanism for a planet to simply explode naturally, but my answer was made on the assumption that Kryptonian super-tech might have accidentally or deliberately made it do so. $\endgroup$
    – Monty Wild
    Dec 3 '20 at 23:16
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There is a theory that planets which don't have plate tectonics undergo periodical resurfacing - the heat from the plant's interior is slowly gathering under planet's mantle "lid" till it boils over and the mantle is replaced completely. Hard to say how exactly it would look to an observer. If it involves global explosion of superheated water (if there are oceans, there's likely plenty of water in the mantle), then it's like thousand Krakatoa explosions going at once, and the observer is going to be incinerated or torn apart in seconds.

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  • $\begingroup$ The entire planet would be vaporised. No surface no mantle and no core, just an expanding ball of plasma. Worse than a thousand Krakatoa explosions by more than 15 orders of magnitude. It will be a bigga badda boom. $\endgroup$
    – Slarty
    Dec 3 '20 at 13:41
  • $\begingroup$ No, in this scenario only the surface boils over and no nuclear explosion is involved. It supposedly did happen on Venus several times and the planet is still there. If you need the planet to explode completely, more exotic explanation is necessary. $\endgroup$
    – Juraj
    Dec 3 '20 at 14:18
  • $\begingroup$ It's happening on Earth as I write, and has been for billions of years. $\endgroup$
    – Monty Wild
    Dec 3 '20 at 14:21
  • $\begingroup$ If the surface just "boils over" we need more details of what makes it boil over and how quickly, otherwise how can any one answer the question? $\endgroup$
    – Slarty
    Dec 3 '20 at 17:41
  • $\begingroup$ What kind of details? $\endgroup$
    – Juraj
    Dec 4 '20 at 12:29
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I don't think that making the core of a planet out of fissionable material such as certain isotopes of Uranium is physically possible. But assuming by some hand waved method such a large quantity were to be assembled the energy released would destroy everything within millions of miles of the planet utterly within seconds. The inhabitants would just be ionized along with the rest of the planet and possibly the star and rest of the star system as well.

The Earth's core makes up around one third of the mass of the Earth and in a nuclear explosion around 0.1% of the fissionable mass is converted to energy. As the Earth's mass is around 6 x 10^24 Kg the loss of matter directly to energy would be around 0.001 x 0.333 x 6 x 10^24 or roughly 2 x 10^20Kg

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Okay, something happened to Krypton that destabilized its core. It is known that Krypton orbited a red star, meaning that its sun was smaller and dimmer, but longer-lived than Earth's, and was the home planet of the Kryptonians. Kryptonians are stronger than humans, due to Krypton having more gravity, and Krypton was denser than Earth, so let's put its radius at 1.25 Earth radii, and its mass at 2 Earth masses. That means that the amount of energy needed to destroy the planet would be 7.17*10^32 J. It's possible that to heat up the planet's core, in a misguided effort to gather more geothermal energy, a large amount of uranium as deposited there.

The energy is equivalent to converting 7.96*10^15 kg of matter. That would require literal mountains of uranium deposited into the core, and if that were the case, the first sign of the apocalypse would be an increase in the movement of tectonic plates, which means that earthquakes and eruptions happen more often and with increased intensity. Tsunamis would flood coastal cities, earthquakes would occur in locations where there's little to no historical seismic activity, new volcanoes would appear, and ash clouds would block off the Sun. Krypton would act like a giant, unstable Demon Core (which is not an anime). Eventually, the activity will increase until the planet rips itself apart, and if the reaction is slow enough, surface radiation levels will rise.

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The most plausible way to explode a planet is by a collision with another large astronomical object.

I don't know how large an impacting object would have to be to totally shatter an Earth-like planet.

The origin of the Moon is a scientific mystery since every theory of its origin has problems. the theory with the fewest problems which is most accpeted at the present is the giant impact theory.

The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, suggests that the Moon formed from the ejecta of a collision between the proto-Earth and a Mars-sized planetesimal, approximately 4.5 billion years ago, in the Hadean eon (about 20 to 100 million years after the Solar System coalesced).1 The colliding body is sometimes called Theia, from the name of the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.2 Analysis of lunar rocks, published in a 2016 report, suggests that the impact may have been a direct hit, causing a thorough mixing of both parent bodies.[3]

So if the impact theory is correct an impact between an Earth sized world and a Mars sized world not always be enough to totally destroy the Earth sized world. Depending of variables such as the angle and the speed of the impact, the Earth sized would would sometimes or always survive more or less intact, though of course its surface would become an ocean of molten lava.

It is possible that a Mars size impactor would sometimes suffice to shatter an Earth size planet, depending on the angle or velocity of the impact. Or maybe an impactor would always have to be more massive than Mars to totally shatter an Earth sized planet.

Of course many present theories of solar system formation involve planets migrating inwards or aoutwards from the orbits where they formed. Thus it is possible for an Earth sized planet to collide with another Earth sized planet or with a super Earth, or a giant planet.

Thus it is possible to imagine as violent a collision as is necessary for an Earth sized planet to be completely shattered or to be swallowd up by a larger planet.

However, such collisions would have been far more likely to happen early in the history of the solar system, when gravitational interactions between forming planets caused some to be ejected from the solar system, others to fall into the Sun, and others to collide with other planets, until the only planets left were ones in orbits which would be stable for billions of years.

And of course a planet with human like inhabitants would have to be bilions of years old, since it should take bilions of years for intelligent life to evolve on a planet. And by then the probability of a collison between planets happening during the short period of maybe a few million years when there is intelligent life on one of those planets would be very small.

Of course there are countless billions and trillions of stars and planets in the universe, and in DC Comics like Superman a rather high percentage of them have planets with intelligent life, so statistically a planet with intelligent life would sometimes be destroyed by a planetary collision in the fictional universe of DC Comics. Whether such a collision would be likely to happen so close to Earth that people escaping from it would land on Earth is another question.

And there are rogue comets, asteroids, planets, and brown dwarfs which have been ejected from solar systems as mentioned above or which formed in interstellar space.

Just as it is possible, though statistically unlikely, for another star to collide with the Sun and destroy the solar system, it is possible for a rogue astronomical object large enough to destroy an Earth size planet if it hit it to pass thorugh the solar system and possibly collide with an object in the solar system.

In 2017 an interstellar rogue comet or asteroid called Oumuamua passed through the solar system, and in 2019 an interstellar comet called C/2019 Q4 (Borisov) was discovered passing thou rough the solar system.

https://www.syfy.com/syfywire/wait-another-interstellar-object-is-passing-through-the-solar-system[1]

https://en.wikipedia.org/wiki/Interstellar_object#Confirmed_objects[2]

In the novel Fifth Planet, 1963 by Fred and Geoffry Hoyle, a bright blue star is detected approaching the solar system. It passes so close to the Sun that several planets are switched between stars as they pass. I found it very hard to believe that any bright star which was going to come that close to the solar system within the next few millennia would not already be discovered and already be known to me.

But an interstellar comet or asteroid could enter the solar system and be detected only a short time before it makes its closest pass by the Earth - or collides with Earth. And since interstellar objejects would often have several times the relative speed of solar system objects a collision with an interstellar asteroid or comet, an impact from one of them might be several times as devastating as an impact with one that comes from our solar system.

And since rogue planets as large as Earth or bigger have been detected, it is certainly possible that one of them could enter the solar system unexpectedly and collide with Earth with comparatively little warning.

So it certainly would be theoretically possible for Kyrptonians to detect a rogue planet entering their solar system from interstellar space, and calculate that it would collide with Krypton, and perhaps some would try to escape from Krypton to other worlds in spaceships.

And as far as I can tell, that is just about the only scientifically possible method for a habitable palnet with an intelligent species to be shattered into pieces that would never rejoin.

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    $\begingroup$ This doesn't really address OP's question though. $\endgroup$ Dec 3 '20 at 22:25
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    $\begingroup$ yeah, its a great answer, just not this question $\endgroup$
    – Topcode
    Dec 4 '20 at 16:33

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