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In this case, I don't mean "make the Earth uninhabitable", I mean "make the Earth a debris field". This has been asked before, of course, but the answers always seem be be some form of "throw a really big rock at it". Although really big rocks are effective, they also get kind of boring after a while.

So, is there a way to destroy the Earth that doesn't throw rocks?

There are three rules:

  • This is meant to fit into an alien attack-style scenario (albeit a very loosely defined one). Any methods you come up with must be able to destroy the Earth fast enough that we would have no time to react - probably a few days, maybe a week at most.
  • Everything involved must be at least theoretically possible, without any blatant handwaving.
  • To keep things interesting, technology so advanced it would make this trivial is not allowed. Anything in Kardashev Tier 2 (or below) will most likely be fine, but Tier 3 and above is off-limits.
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    $\begingroup$ "technology so advanced it would make this trivial is not allowed" is totally incompatible with "Anything in Kardashev Tier 2 (or below) will most likely be fine". I'm not sure why people have this blindspot for the implications of a K2 technology base :-/ $\endgroup$ Aug 1 at 19:41
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    $\begingroup$ This is a very open ended question, especially since you're proposing an alien attack and don't tell us anything about the specific capabilities of the aliens in your world. We know that there exists in all of fiction ways to destroy planets without throwing rocks. Since you've left the technology unspecified each non-rock method would be equally valid. This results in a question that is far too broad to be a good fit for this site. $\endgroup$
    – sphennings
    Aug 1 at 19:42
  • $\begingroup$ this is a brainstorming question, which is off topic. $\endgroup$
    – John
    Aug 1 at 20:00
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    $\begingroup$ If nothing else, wanting to do the job in a week after having to wait decades, centuries or millenia to cross space between the source of the attack and Earth seems weirdly impatient. Possibly you assume the existence of FTL travel, and unless you clarify how that works then all bets are off. If there's no FTL, then the only realistic mechanism is the good old Nicoll-Dyson surprise, which is basically the Ultima Ratio Deum (Deorum?) here. $\endgroup$ Aug 1 at 20:08
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    $\begingroup$ Why do none of these alien civilisations just make a reasonable offer and buy the Earth. Are they a bit strapped for cash or something? $\endgroup$ Aug 2 at 2:13

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ZAP it with a lot of energy

The key to turning earth into a debris field is giving it enough energy to exceed its Gravitational Binding Energy, which is about $2 * 10^{32}$ Joules.

At the end of the day (or all days for earth) it really doesn't matter how you get all that energy into the earth. Big rocks are just good at carrying energy, which is why that's a common answer.

For a K2 civilization this amount of energy is significant, but not unreasonable. With a typical star (like the sun) yields about $4 * 10^{23}$ Joules per second. So it would take 500,000 seconds (5 days, 19 hours) for the star to output enough energy to make this possible. And since your K2 civilization has other energy expenditures, it could easily take twice this long or more.

After the energy is generated, it's just a matter of getting it to earth. Just exploding whatever energy storage device is used to store such a large amount of energy would do, but Lasers, antimatter, and just plain dumping superhot plasma are all on the table (since sending large rocks is off the table).

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  • $\begingroup$ Never underestimate the raw output of a star. They are truly astonishing! $\endgroup$
    – Cort Ammon
    Aug 1 at 22:14
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Help the sun grow

Once the Sun changes from burning hydrogen within its core to burning hydrogen in a shell around its core, the core will start to contract, and the outer envelope will expand.

How much will it expand? Enough to engulf the earth. How long do we have until that happens? about 7.59 billion years. But nothing stops the aliens from accelerating that process.

Increase the sun's gravitational force. Increasing its mass should do the trick (Honestly, I don't know how to calculate how much mass you would need)

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Although really big rocks are effective, they also get kind of boring after a while.

Throw a small rock at it. Really fast.

Aliens find an asteroid, paint it black, build a bunch of spaceship engines on it and start accelerating it. After a few years asteroid is going at 0.7c and when Earthling finally notice it it is an hour away.

There is no time to react. It is too close and too fast to divert or stop it.

As preparations can be started years in advance nothing stops aliens from throwing more than one asteroid so that they arrived simultaneously.

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    $\begingroup$ The laws of physics being what they are, one does not simply accelerate a projectile to relativistic velocities without it being very, very visible from a very long way away for many years before impact. The very kinetic energy that makes the projectile so deadly also makes it extremely vulnerable to countermeasures, and the long observation time gives lots of opportunity to launch some. Relativistic bombardment is not necessarily the unstoppable superweapon everyone seems to think it is. $\endgroup$ Aug 2 at 8:30
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    $\begingroup$ Hmm, such a rock might just get destroyed long before by colliding with neutral gas atoms, heliosphere, solar winds and other similar objects. At this speed the amount of collisions could grow fast enough to turn the accelerated rock into plasma, which will disperse in space long before delivering energy to its target. $\endgroup$
    – Vesper
    Aug 2 at 8:51
  • $\begingroup$ @Starfish Prime - we are talking about aliens who arrived to Earth, I would expect them to have some expertise in building space ships. "for many years before impact" - it is a relatively small object with tiny cross section, painted and disguised to be unnoticeable. Which laws of physics would make it "very, very visible from a very long way away"? "very kinetic energy ... makes it extremely vulnerable" - if Earth had lots of has time to react. Make it 0.99 c ant there will be barely any time. $\endgroup$
    – D'Monlord
    Aug 2 at 9:13
  • $\begingroup$ @Vesper - we are talking about space traveling aliens here. Ok, they didn't just add spaceship engines, they also added some spaceship shields. They got to Earth somehow, right? $\endgroup$
    – D'Monlord
    Aug 2 at 9:16
  • $\begingroup$ Well, they could travel by some conventional means like a generation spaceship, that is, slowly but "surely", so no relativistic shielding was involved. Also why then accelerating a rock for several years, they could just build a mass driver with the length of Jupiter, and accelerate things from LEO downwards, at the same relativistic speeds. $\endgroup$
    – Vesper
    Aug 2 at 9:22
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A Nicoll-Dyson Beam could work. After a Dyson Swarm/sphere is created (A Dyson Swarm is a large number of solar-collecting satellites around a sun). The star's energy could be stored into a beam and sent to the target of your choosing. Of course, lasers only travel at the speed of light. The aliens would have to build the Dyson Sphere around The Sun first or otherwise it would take a minimum of four years for the beam to hit Earth.

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ANTIMATTER!

Produce a tight beam of a LOT of antimatter, and aim it at the center of the planet. This will (ignoring the strength/slump resistance of the mantle, which is left as an exercise for the reader) bore a hole toward the center. At the approprite time, send down a bigger bolus of antimatter which will impact the center and cause a massive explosion.

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Kugelblitz!

Going off of @Mathaddict's answer, if you want flashy (and you're a K2 civilization with access to 10³² Joules of energy), make a Kugelblitz (a black hole made from energy instead of mass).

10³² Joules of say, photons (though anything will work), concentrated into a point less than 2 picometers across will create a black hole with a mass of roughly 10¹⁵ kg. You don't need to worry about Hawking Radiation—a black hole of this size lasts at least a couple billion trillion years.

So, you just make a black hole and fling it at the Earth. Humanity won't see it coming, and they couldn't stop it if they did. Unless your black hole is traveling at relativistic speeds, the rapid mass gain upon entering the Earth will slow it down enough to destroy the planet.

If you want there to be some rocks left for dramatic effect, shoot it fast enough that it eats a cone out of the middle of the planet but leaves some crust and mantle behind. That will collapse into a debris field.

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    $\begingroup$ Amazingly, this doesn't work. There's another question on here about it that does some of the calculations--such black holes eat very slowly. $\endgroup$ Aug 2 at 16:48
  • $\begingroup$ Yeah, you need a surprisingly big black hole to eat a planet promptly, as the OP would like... a 10^15kg black hole has a Schwarzschild radius of only 15 picometers, which is smaller than the radius of a hydrogen atom, and it'll be radiating at about 10 million kelvin, which will discourage stuff from falling in it. A series of smaller black holes that evaporate more promptly might be a way to deliver enough boom to dust the planet, though. $\endgroup$ Aug 2 at 17:40
  • $\begingroup$ @StarfishPrime The temperature of its radiation is irrelevant because it is not radiating many particles (luminosity is 0.015 W). Additionally, it starts at 15 pm, but it will grow linearly while inside the Earth (both Schwartzchild radius and rate of consumption scale with area). In the case where it doesn't consume the entire planet, it leaves a cone-shaped hole. The angle of this cone depends on the velocity, so like I said as long as the black hole isn't going too fast it will destroy the planet. $\endgroup$ Aug 2 at 18:14
  • $\begingroup$ @EthanManess I have a somewhat higher luminosity than you, but no matter: the volume of matter around the hole is very small, and the hole is very hot, and this will help drive matter away. Sure, the hole will eventually eat the Earth, but the OP specifically requested timescales no longer than a week. Having a race with the Sun leaving the main sequence doesn't quite fit that criterion. $\endgroup$ Aug 2 at 18:18
  • $\begingroup$ Yeah, I'm getting a Bekenstein-Hawking luminosity of ~356W for a 1e15 kg hole. And a temperature of 118 million klvin (as I'd accidentally had 10e15 kg as the mass the first time round). I've used the wikipedia formula and not actually checked its correctness, so I'd be interested to know where your 0.015W came from. $\endgroup$ Aug 2 at 18:23

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