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What circumstances must be met in order to destroy a planet with just a single laser beam hit?

If this is possible at all then, please, try to provide some possible details:

  • an approximate energy of such laser,
  • distance from which it should be fired,
  • what geological, physical chemical etc. steps or processes are involved in destruction act.

If this isn't possible at all then, please, provide some arguments to justify, that this isn't possible.

By destroying entire planet, I mean an example events that you can observe in some common SF literature (namely: the destruction of the Borg planet in the Scorpion episode of Star Trek: Voyager; and the destruction of Alderaan in Star Wars New Hope).

That is: ripping entire planet apart. Splitting it into small chunks or pieces of rock.

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    $\begingroup$ what-if.xkcd.com/13 $\endgroup$ – Aify Dec 18 '17 at 9:54
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    $\begingroup$ "its a friggen laser beam" the handwavium scifi weapon that magically achieves all effects, What are you even asking? $\endgroup$ – anon Dec 18 '17 at 11:10
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    $\begingroup$ Another alternative: The laser is modulated such that it creates massive vibrations with the resonance frequency of the planet. It shatters like a wine glass. $\endgroup$ – Muuski Dec 18 '17 at 18:18
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    $\begingroup$ @trejder Ah, that's clear now, thanks! A short-duration (seconds at much) laser beam that physically breaks apart the planet into much smaller fragments. That requires a ridiculous amount of energy, but there is a ridiculous amount of energy waiting to be tapped in nearby stars... $\endgroup$ – Eth Dec 19 '17 at 12:25
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    $\begingroup$ When people in sci-fi say laser, they basically mean a beam of energy. Actual lasers can't really work because they create plasma like answers below say. Also actual lasers are impossible to focus at such energy levels. In the case where you want to blow up the planet, you need to put the energy directly into the core. As such a beam of energy that only strongly interact with the planet's metal core can be the solution. $\endgroup$ – A. C. A. C. Dec 21 '17 at 18:03
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First, how much energy does blowing a planet up require?

What holds a planet together is gravity, and the gravitational binding energy of Earth is about 2.5 * 10^32 J. So an approximation, that's how much energy you need to turn the Earth into a bunch of debris. Of course, lighter planets like Mars require less energy, and a super-Earth requires more, at those scales, it won't make that much of a difference anyway.

There will also be losses, so let's round this up to 10^33 J. That's about the total energy output of the Sun in a month. Putting so much energy in a 1-second laser is going to be a problem, though not a physically impossible one.

When you want the biggest, baddest laser possible, the obvious solution is a Nicoll-Dyson beam - that is, a Dyson sphere acting as a giant laser emitter. Using short enough wavelengths, it can very well strike planetary-sized (or even smaller) targets. Problem is, it would need to store energy for a long time - for example, making a few trillion tons of antimatter - and then release it all in one second to generate the laser. Or they could a giant interstellar energy network in order to power a laser emitter. One option is to use the central galactic black hole as an power source (by throwing stuff into it) and interstellar gas clouds as maser transmitter.

Note that the emitter or transmitter will most likely be destroyed by the heat losses. Even if those are very small, a small percentage of apocalyptic energies is still a lot.

Anyway, we have our laser emitter, and it fires a 1-second long beam of 10^33 J at the planet. Then, weird things happen. The details would require simulations, but we can figure out the main effects.

When so much energy hit the atmosphere, it will instantly turn into plasma. If air is transparent to this wavelength (like, say, visible light), enough will hit the ground in the first millisecond to hit it with the force of countless nuclear weapons, but in any case the entire local atmosphere will turn into plasma and thus most of the second-long beam will be absorbed by what was formerly known as the upper atmosphere.

Air will also undergo nuclear fusion, which would add a bit more energy, which may or may not be negligible compared to the laser energy. There will even be some matter-antimatter pair production. This means that whatever the initial wavelength, some X- and gamma rays will be sprayed around, including back to space.

Plasma is opaque, and the laser would more or less be entirely absorbed by it before hitting the surface. That's similar to a nuclear fireball where most of the X-rays are more or less instantly stopped by the atmosphere, which then carry the energy as a giant fireball.

The plasma fireball will then do three things: it will reflect some of the laser back to space, it will expand as a shock-wave and it will radiate energy back all around. The planet is concerned by the last two.

The light will be strong enough to turn even more atmosphere into plasma, as well as a big chunk of the ground. Further away, it will be strong enough to instantly vaporise anything it lights. So expect the continent that is hit to instantly be caught into the plasma ball, and the hemisphere to be flash-vaporised by the fast-expanding plasma ball.

Due to the slight lens effect and the scattering of the atmosphere, the other hemisphere would also be exposed to the light. I would expect everything at the surface to also be instantly be burned, though the antipodean point my be slightly better off. Underground bunkers may also survive, if the detonation of the entire surface flash-vaporising doesn't compress them too much.

So yeah, a million voices suddenly cry out in terror and are suddenly silenced. And many more don't even have time to cry out.

Then, the fireball expands, and causes a giant shock-wave. At those scales, a planet acts like a big blob of liquid (hence its round shape), so it will look like a giant tsunami, but where the crust and upper mantle is making the wave instead of simply the ocean. The shock-wave is also travelling through the interior of the planet, so it will reach the other side before the surface wave. Doing so will cause earthquakes and volcanoes, in a sense that the Halifax explosion made a noise. That is, the crust will break apart and the mantle splash around. I would expect that to take around a few minutes, ten or twenty minutes at most.

This is also the point where the planet starts breaking apart, a bit like a big water drop being hit very hard. It fragments in gobs of varied sizes and shapes, from dust grains to, possibly, some as big as the Moon or Mars. Once settled, the big ones will settled back in round shapes.

Almost all of them are extremely hot, and will stay hot for a long time. Asteroid-sized ones will take years or centuries, the biggest ones may take millions of years to stop glowing - you can look at the history of our planets for details there.

In addition to the atmosphere and hydrosphere being blown to space, a chunk of the planet has been vaporised, and is also blown to space at great speed. In addition, the mantle and core were under tremendous pressure, which is suddenly released. This will make them splash back and/or vaporise with a vengeance. All this will cause the rest of the planet to be pushed in the other direction, as if the planet was a big laser-thermal rocket.

The fragments will, for the most part, still end up in a solar orbit. The push would have been enough to make the orbit vary a bit if they all stayed the same (the details of the new orbit depending on which side of the planet was hit). However, the fragments will not all stay in a roughly Earth-like orbit. Some will have been violently ejected by the energy of the laser, or by how the shock-waves interacted. Other will be destabilised by gravitational interaction with the other fragments.

Starting a few centuries or millennia down the line, and possibly lasting millions of years, the other planets of the system can expect nasty meteoric impacts, some very fast and some very big. Not enough to break another planet apart, but enough to definitely ruin most planetary surfaces at some point.

But even before that, those planets have other problems.

Anything not in the shadow of the planet will have been exposed to the light of the laser being scattered at impact. This scatter, for one second, is emitting as much energy as about a million times the Sun.

If there is something like the Moon and it was in the shadow of the planet, it will take a fair amount of debris in the face, probably breaking bits of it and sending them flying as well. The entire surface is, of course, turned upside-down into a lava ocean.

If this Moon is exposed to the flash, the exposed hemisphere literally explodes into plasma. The vaporised plasma pushes it in the other direction and send it in a slightly more different orbit than the debris. The shock-waves will turn the other hemisphere upside-down as well, and debris will bombard it for a long time, probably starting the following years if not sooner. Again, bits may be sent flying.

The exposed hemispheres of more distant planets will be burned to a crisp, as if by a close nuclear explosion. Atmospheres will be partially blown away. Ice surfaces will explode into steam, some of which may keep around as atmosphere. Surfaces may be vitrified. On the non-exposed hemispheres, all this may also cause earthquakes. The details depend heavily on the planet type and its distance from impact.

There won't be much effect on the local star(s), though weather patterns may be affected in weird ways. Maybe there will be more solar flares in the future, though at that point, few would probably care.

Exposed neighbour star systems will see the flash easily. At a million times the intensity of the Sun, assume a visible wavelength for the laser, anyone looking at the sky will have a hard time missing it, even for a one-second flash. It will be as bright as a thin Moon crescent, but concentrated in a single point, and with a distinct colour.

With access to modern astronomical instruments, probably half the galaxy can see it (by the time the flash arrives, obviously), and possibly neighbouring galaxies as well if they're lucky. Between a very distinct signature and the probably hard-to-miss stellar engineering next to it, it won't take long for them to guess what it was.

Special mention for the Death Star:

Star Wars is apparently taking place in an universe with different physical laws than ours. There is pressure in the vacuum (hence walking on an asteroid with only oxygen masks, flying around as if in atmosphere, space fireballs when things explode...) Flying requires very little energy (no visible radiators or incinerating exhausts). Planets may be smaller, and stars are. Asteroid fields are much, much denser. Star systems are closer together.

In this case, planets are probably much easier to blow up, and their debris cool down near-instantly afterwards.

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    $\begingroup$ Someone has a very vivid imagination. $\endgroup$ – Justin Thyme Dec 23 '17 at 7:56
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No this is impossible
Such a weapon is preposterous and would not work. Any laser beam would need to be unrealistically powerful and the mode of destruction would not work as you envisage.

The immense energy input at the surface would create massive amounts of gas and plasma which would absorb the beam and radiate it away in all directions. Ultimately the planet would be melted from the outside in not the inside out, but as I said that’s just not going to happen.

If you had a laser powerful enough it would be so massive that it would be easier to destroy the earth by a colliding with it than by firing at it.

(This joke is a variant of one originally used during the development of Ronald Regan’s star wars program. It was said that the size of a laser powerful enough to shoot down incoming missiles would be so great that it would not need to work as it could simply be dropped on the enemy as a weapon in its own right)

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  • $\begingroup$ Unfortunately, your answer misses formatting and some breakpoints or my language is not enough good to understand it. How should I understand your last sentence? "If you had a laser powerful enough (...) it would be easier to destroy the earth by a colliding with it (...)". Colliding Earth with what? With that laser? $\endgroup$ – trejder Dec 19 '17 at 12:06
  • $\begingroup$ @trejder Sorry let me reword it to make it clear. $\endgroup$ – Slarty Dec 19 '17 at 12:49
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    $\begingroup$ Please don't answer questions that you VTC. $\endgroup$ – sphennings Dec 19 '17 at 13:01
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    $\begingroup$ @PieterB Vote To Close. It's part of the community moderation process where users with over 3k reputation can participate in the closure of questions. It takes 5 votes to close a question and 5 votes to reopen. It's sends mixed signals to new users when someone simultaneously thinks a question is too broad to answer but still answers it. $\endgroup$ – sphennings Dec 19 '17 at 13:13
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    $\begingroup$ @Slarty no, it does not depend. If you are making answering impossible for others you also shouldn't answer yourself. $\endgroup$ – Mołot Dec 21 '17 at 15:08
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There are three crucial criteria in this question, and one that is not specified.

One is that the destruction has to be initiated by a laser beam. That means a light beam, not a directed energy beam. This means the answer is limited by the properties of light, not other forms of directed energy, such as the wavelength and the 'penetrating power'. Light penetration is limited by opacity, and energy transfer is limited by how photons react to the material that is being struck.

The criteria NOT specified, is whether or not this explosion is generic to ANY planet, or is it applicable to only one specific planet with unique characteristics, composition, or development that makes it susceptible? For example, a planet filled with propane?

Second, the process has to occur in a second or so. Prolonged heating effects, and progressive destruction, are not allowed.

Third, the planet has to explode. That is, there has to be some huge force generated from within the planet, sufficient that it instantaneously creates enough expanding gas or other pressure to catastrophically propel the planet's substance into space.

Lasers produce local heating. The only explosive gasses that are created are from the expanding air due to heat and of the material melting and creating gases itself. In application, lasers are used for making precise cuts, because the heating effects are localized. They do not heat up the material slowly, they apply heat quickly enough and intensely enough in a very limited area that it does not spread through the material before the cut is made. They create very intense local melting. The damage is localized, limited to the width of the beam.

The beam width would have to be humongous in order to melt enough material to create sufficient explosive gases to blow a planet apart, perhaps even the size of the planet itself. Like other posters have said, a platform capable of supporting a device capable of creating a beam this wide would itself be a kinetic weapon. And even then, the destruction would not be explosive. It would be from some form of atomic or sub-atomic decomposition.

Unless the laser is directed at something that itself is explosive, like a propane filled balloon, there is no explosion. That is why lasers are so inefficient at missile defense - they punch a single hole, or perhaps a cut line, in the missile, but unless that hole is in a critical area, or ignites the fuel, it does no good. There is nothing in the laser beam that is inherently explosive itself. The missile is not explosively destroyed, it is simply disabled.

I would like to remind the reader that, even when laser beams are used to initiate nuclear fusion, they are directed at a specific target which itself has been processed and refined to produce the required reaction. It is the target that is crucial, not the laser.

In the case of the planet, it would be the material of the planet itself that is ignited and caused to explode. Not a likely scenario. Very few solid planets are made of explosive substances. Perhaps you might end up with a very deep hole in the planet, but no explosion. And if there WERE a specific target on the planet - say a particular deposit of enriched nuclear fuel - the laser beam would first have to GET to the target, if it were buried. A one second burn would not be a dependable weapon, except in a very specific circumstance on a particularly vulnerable planet.

So it would seem that either the planet destruction would not be explosive in nature, it would not occur from a one second application of a laser beam, or the beam itself would not be a laser beam, but of some other directed energy beam, capable of creating sub-atomic interactions other than that from photons.

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To sum up my answer, a Death Star like laser would need to produce and transfer enough energy into a planet to coax the planet's core into producing fusion reactions. The outwards pressure generated by the fusion would need to counteract the gravitational pull of the planet to cause it to expend its outer layers in an explosive-like manner.
You'd basically be momentarily turning the inside of a planet into a star.

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I don't believe it's possible to do an Alderaan. Eth gives a good start on it but there's a big problem with his analysis: As he says, the plasma is opaque. Virtually 100% of the incoming energy is going to be absorbed before it reaches the planet--you're going to have to destroy the planet with the energy re-radiated from the plasma. However, given the short time interval involved everything is an insulator. I seriously doubt you can dump those 2.5E32 J into the mass of the planet. You'll get a very big boom on the surface but the energy from it will depart into space--the blast wave won't go around the planet, but straight out. Even in the realm of the largest nuclear weapons we see this happening--making the warhead more powerful doesn't increase the destruction.

To actually destroy the planet will take a much more sustained application of energy.

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