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I'm thinking of a couple main ways, but they seem pretty unreliable for complete sterilization. I'm imaging a pretty reclusive hermit civilization around their solar system. They see intelligent life in a neighboring system, and they want to kill it.

Assuming realistic tech for a Kardashev 2ish civilization (i.e no tossing black holes), what would the most efficient and relatively quick (under a century) way of sterilizing the planet?

I initially thought bio-weapons, but they are unlikely to work on alien biology. Tossing a dwarf planet or asteroid might also be an option, but that requires a ridiculous amount of energy to accelerate it, and it would take too much time. I've also thought about lasers and nukes, and neither of them seem to work.

Tech level of the opposing system is sub-K1, about what humanity would be in 100 years.

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    $\begingroup$ Do you want to keep the planet for later use? $\endgroup$
    – vinzzz001
    Commented Jul 13, 2022 at 10:20
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    $\begingroup$ You need to think about what phrases like "ridiculous amount of energy" mean in the context of a K2 civilization which has ~4x10^26W of power to play with. The continuous output of a Sunlike star is a ridiculous amount of energy, and then some. $\endgroup$
    – Starfish Prime
    Commented Jul 13, 2022 at 12:14
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    $\begingroup$ Is it sterilising the planet or just killing the intelligent species? These are radically different goals, so it'll be good to know which you prefer. $\endgroup$
    – Trioxidane
    Commented Jul 13, 2022 at 12:18
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    $\begingroup$ Trioxidane makes a good point. On Earth bacteria exist living in rock a 3 miles below the surface. You would need something truly catastrophic to kill life that deep astronomy.com/news/2018/12/… $\endgroup$
    – Donald
    Commented Jul 13, 2022 at 14:10
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    $\begingroup$ "Nuke'em from orbit. Its the only way to be sure" is a line that comes to mind. $\endgroup$
    – Forward Ed
    Commented Jul 13, 2022 at 22:10

18 Answers 18

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Just accelerate a few 'ships' up to a significant portion of light speed and ram the planet with them.

The effect of just one moderately small vessel will be much the same as the meteor that wiped out the dinosaurs.

A dozen of them?

Each timed to impact just as all the 'dust' from the previous one causing a nuclear style winter is thinking of settling back out of the atmosphere?

That'll do the job.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch ♦
    Commented Jul 14, 2022 at 3:29
  • $\begingroup$ unless others live under clouds already or in other ways independent on sun light $\endgroup$
    – akostadinov
    Commented Jul 14, 2022 at 12:50
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    $\begingroup$ This would not sterilize the planet. It would be a mass extincition event, but much life like bacteria and insects would survive. $\endgroup$
    – Scorb
    Commented Jul 14, 2022 at 20:51
  • $\begingroup$ @Scorb Correct, but, given the context of the OPs reference to intelligent life and his statement that that is what they want to kill I thought it a much more than safe assumption his use of 'sterilised' is in the military sense rather than the literal, and have answered accordingly 🤗 $\endgroup$
    – Pelinore
    Commented Jul 15, 2022 at 0:09
  • $\begingroup$ I never found out what would happen, but in one Startrek book they discuss this to destroy a VERY dangerous planet at all cost, and figure out that this would leave an Enterprise-sized hole in the planet and that's it. Basically not enough exchange of energy. Maybe a question for physics.stackexchange. $\endgroup$
    – gnasher729
    Commented Jul 15, 2022 at 9:24
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Nicoll-Dyson beam

Use your Dyson swarm for a phased array laser. This is a light speed weapon, so it's the quickest way possible to hit a target.

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    $\begingroup$ This is the answer I'd have written (though I wouldn't have managed so few words), and is therefore clearly the best answer. If you can't make an ND-beam, you don't really deserve your Kardashev II badge, and if you can make them then you're a defacto galactic superpower... $\endgroup$
    – Starfish Prime
    Commented Jul 13, 2022 at 12:16
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    $\begingroup$ Again this assumes the target is defenseless. Because light + reflectors = 0. $\endgroup$
    – Vogon Poet
    Commented Jul 13, 2022 at 16:58
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    $\begingroup$ Even if the defender had mirrors, they'd need a reason to put them up. As soon as you see them laser beam it has already hit you, no time to put up they mirrors then. $\endgroup$
    – infinitezero
    Commented Jul 13, 2022 at 22:34
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    $\begingroup$ @Neinstein but to move “a significant amount of stuff” in the way, they have to have some warning. With a laser, the only warning is “oh crap our planet is being heavily irradiated from somewhere”, they wouldn’t (barring paranoia or foreknowledge) be able to prepare in advance $\endgroup$
    – fyrepenguin
    Commented Jul 14, 2022 at 17:09
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    $\begingroup$ i see. an edit. so that means they most probably cannot defend against this. they won't even know it's coming before they are toast. $\endgroup$
    – ths
    Commented Jul 14, 2022 at 23:06
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Use the planet's star

As a type 2 civilization you may be able to affect solar flares and coronal mass ejections to reach the planet. In terms of efficiency, this method will probably be the easiest way, and over your 100 year period you could just put enough raw heat on the planet to liquify the rock on the surface which will likely do the job you want.

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    $\begingroup$ By far the most effective way to destroy something in a system is using power from their own star. Your K-II already knows a GREAT DEAL about star mechanics, they can trigger a coronal mass ejection or other solar phenomenon to cook the planet locally. No direct-kill mechanism from your star is really believable IMO. $\endgroup$
    – Vogon Poet
    Commented Jul 13, 2022 at 17:02
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Freeze it

Assuming propulsion allows physical access to the neighboring system, and the enemy is limited to a single planet.

Unfold a solar shade at the L1 point between the planet and Sun to put the planet in permanent shadow. Any burgeoning intelligent civilization (and most life) will be extinguished well within the century deadline.

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    $\begingroup$ Great idea +1 nasty trick :p especially if the victim civilization cannot organize a weapon in time. the solar shade will need to be BIG.. and defended against missile attacks, but it could work ! $\endgroup$
    – Goodies
    Commented Jul 13, 2022 at 22:24
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    $\begingroup$ A solar shade itself seems too much effort for this goal. However, if you just start making a Dyson sphere/swarm that during creation tries to keep the shade on the planet you kill 2 birds with one Dyson sphere. $\endgroup$
    – Trioxidane
    Commented Jul 14, 2022 at 11:06
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Give Them a Helping Hand
If they are pre-industrial, give them a leg up to industrial along the nastiest path possible with mutagens galore. Just don't tell them about medicine or even much along the lines of biology or environmental remediation.

If they are industrial or later, drop a few bombs that start the big war or split up their society into antagonistic factions. Give each side "help" in war technology, particularly nuclear, biological and chemical.

If there's no civilization, just dump mutagens targeting viruses in gigaton lots and let them do the job for you. Rinse and repeat every few hundred years.

Just never ever land there again once things get "warm".

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    $\begingroup$ AKA "When Monoliths Go Bad!" $\endgroup$
    – Tom
    Commented Jul 14, 2022 at 5:48
  • $\begingroup$ @Tom - I like that! You should write a parody series along those lines. The "Bad Monolith" series. Stay away from the ones with tatts and piercings! $\endgroup$
    – user53931
    Commented Jul 14, 2022 at 11:54
  • $\begingroup$ I like that! Instead of sending them Jesus and Leonardo, send them Stalin and Trump. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jul 15, 2022 at 17:19
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Pyrrhic Victory!

Assuming a Kardashev 2 civilization can harness the energy of their star, the fastest way to sterilize the nearby system is to trigger their own star to go supernova. For as much fun as blowing up a star is, it doesn't have to be a foolish move. If suicide isn't attractive, they could migrate out in ships shielded from the blast. For fun, you could even have riding the wave of a supernova be a very flashy way of them accelerating their ships up to a significant fraction of the speed of light. A civilization of this level need not feel tied down to any star or system.

This would have the added benefit that a sufficiently xenophobic species would wipe out all other potential life in the entire region. In fact, this may be integral to how they assure no other intelligent life is hanging around to bother them - travel, find life, blow it up, repeat. The neighborhood has gone to hell, so let's blow it all up and leave!

The blast wave and radiation would do a pretty thorough job of sterilizing any nearby system. Assuming the supernova can be triggered quickly, I can't think of any way faster to assure total destruction of another system.

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    $\begingroup$ You assume the star can be made to supernova without an enormous, external source of energy. And if if you have the right kind of magical source of energy, there are probably more useful things you can do with it that suicide... $\endgroup$
    – Starfish Prime
    Commented Jul 13, 2022 at 12:08
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    $\begingroup$ you could supernova their star. $\endgroup$
    – Tom
    Commented Jul 13, 2022 at 14:58
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    $\begingroup$ @Tom I considered that. It depends on their level of technology and capabilities. I assumed they could easily blow up their own sun, but it might be harder to blow up another's sun. Besides, if you blow up theirs, the blast will still be coming at you from the other system. $\endgroup$
    – DWKraus
    Commented Jul 13, 2022 at 16:01
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    $\begingroup$ @Tom I think my way is funnerer. Most funnest? Cooler, which is to say insanely hot. Explosions are fun. $\endgroup$
    – DWKraus
    Commented Jul 13, 2022 at 18:36
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    $\begingroup$ @DWKraus Not only is all of that entirely theoretical handwavium, it's also not K2-level. You want to apply kinetic energy to every particle in the star, each in a unique direction, of a sufficient magnitude to artificially cause a supernova? How do you propose to do that? What could you do with K2-level technology and energy consumption that could possibly reach hundreds of thousands of miles into the star's core? Not to mention that unless you're within ~50 ly of the target, you'll have to do it to their star, without them noticing. Even if that were possible, its way above K2. $\endgroup$
    – Ethan Maness
    Commented Jul 14, 2022 at 20:36
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Grey Goo

Just spread some samples around their world and watch it work. Since the speed at which the goo multiplies rises exponential with the amount of Goo available, under a century seems very plausible. Bonus points for them seeing their inevitable doom coming while becoming increasingly unable to stop it.

Kind of dangerous for your civilization too, but that isn't the point. It is very energy efficient and nearly undetectable in the early stages. Just use some form of particle dispersal device (read dirty bomb) to spread it around their atmosphere, and watch them go.

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    $\begingroup$ Ehh. Since this is tagged with hard science, I'm going to have to point out that grey goo is no better than any other machine at thermodynamics. Where does it get it's power from to replicate and then eat life-forms? One of the major problems with nanotech is the furious handwaving around power requirements. They don't magically change just because the machines are tiny. $\endgroup$
    – user53931
    Commented Jul 13, 2022 at 15:42
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    $\begingroup$ The problem with grey goo is that it's got some pretty stiff competition from the green goo that's already infesting the place. $\endgroup$
    – Mark
    Commented Jul 14, 2022 at 2:53
  • $\begingroup$ @CTeegarden It doesn't need to eat any life-forms, it just needs to make the planet uninhabitable. The planet in question is going to be in its star's goldilocks zone, meaning the nanomachines can use solar power. It might be slow, but that doesn't matter. Once it's propagated into the oceans there is nothing that can be done; it's only a matter of time. $\endgroup$
    – Ethan Maness
    Commented Jul 14, 2022 at 20:06
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Kill it with fire from the Heavens.

A K2 civilization can harness the power of a star. Beam energy into the planet until the atmosphere reaches the Nitrogen-Oxygen ignition temperature, then keep the beam on to keep the chain reaction going.

It is not self-sustaining (ref) so you need to keep beaming thermal energy (IR light) into the planet.

Basically microwave them into extinction from afar.

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  • $\begingroup$ Microwaving a planet from many light years away is going to require a very precise beam. I doubt that a beam would be able to make it that far without diffusing too much to be effective. $\endgroup$
    – Ethan Maness
    Commented Jul 14, 2022 at 20:07
  • $\begingroup$ @Ethan a K2 civilization could surely manage the details of focusing light sufficiently over a couple of light years. They can surely build lenses that can encapsulate their own star, by definition. Our star's diameter is about 1 million miles, but some get up to half a billion. Hand wave the lens diameter to about 1 billion miles. 1 light year is about 6 trillion miles. That means you need a focal accuracy of about 1/6000 per light year. Seems pretty doable. $\endgroup$
    – BlackThorn
    Commented Jul 14, 2022 at 22:18
  • $\begingroup$ @BlackThorn It's not about if you can make the beam, its about whats between you and the target. The beam must pass through our solar system's interplanetary dust, the Oort cloud, lightyears of interstellar space, another Oort cloud, and more interplanetary dust, before finally reaching the target. For example, if you are firing the beam at an Earth-sized planet from 100 ly away, even an average diffusion angle of 0.000000000001 degrees will reduce the beam's intensity by 99%. Every zero you remove from that number is another 99% loss, because inverse square law. Good luck frying anything. $\endgroup$
    – Ethan Maness
    Commented Jul 15, 2022 at 14:31
  • $\begingroup$ @Ethan is your concern obstruction by interstellar matter or is it the diffusion? If the former, it is literally not a problem. Any matter hit by a beam of this power will turn into a rocket until it exits the beam's path. If the latter, I think your concern is overblown. Modern astronomy mirrors are precise down to the 10s of nanometers. Add to that the ability to put a lens/mirror several light minutes or hours away from the source in order to refocus or lase the light and that will refine the beam's precision by orders of magnitude. $\endgroup$
    – BlackThorn
    Commented Jul 15, 2022 at 17:01
  • $\begingroup$ @BlackThorn My concern is the diffusion of the beam as a result of matter, like a laser pointer through fog. The fog is thin, but the distance is very far. You're right, the beam will carry any particles it hits with it, but matter necessarily travels slower than light and thus will constantly be blocking photons for the entire journey, deflecting them out of the beam. Making the beam thinner won't help as the reduction in surface area is counteracted by the increase in energy blocked per particle. The only affecting variable is the interplanetary dust density, which can't really be changed. $\endgroup$
    – Ethan Maness
    Commented Jul 15, 2022 at 18:03
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KISS: Hydrogen bombs

Let's rule a few things out. If you have a K2, then you have the full power of the local star at your disposal, but not the power of multiple stars. Even if you focused the entire output of your star at the neighboring system, it would diffuse too much over the light years to do more than provide a beacon.

This means you have to send a ship to their system. Just sending a guided missile would work to deliver a payload, but you'd probably want to send someone to do a little analysis and figure out where the population was concentrated.

From there you have many options. Fifty or so hydrogen bombs should do the trick pretty readily, and it's a fraction of the power required to get you to the other system "quickly." You could also drop rocks on them from the nearest asteroid belt, or small moons from a local gas giant.

Realistically, the energy to change their orbits to something that intersected the planet would be greater than the energy released by the hydrogen bombs in most cases.

Higher tech aliens could engineer viruses that would target specific lifeforms, like all primates, but it wouldn't be nearly as quick.

You could have them use their K2 tech to build a giant mirror and raise the global temperature, wiping out their ecology, but you could drop the hydrogen bombs and wait for the radiation to decay in the time it would take climate change to kill them.

If your race has nanotech like gray goo, then you're beyond the horizon of speculation.

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    $\begingroup$ Fifty hydrogen bombs will absolutely not sterilize a planet. That wouldn't even be enough to drive humanity extinct. Best it could do is set us back 5,000 years. $\endgroup$
    – Ethan Maness
    Commented Jul 13, 2022 at 20:24
  • $\begingroup$ @EthanManess Agreed. Robert should say "5 billion hydrogen bombs, each terratonne". After all, that isn't much harder. I think that would even boil the oceans. Earth's oceans take 10^27 J to boil (roughly). A KT is 4*10^12 J; need 10^15 KT of nukes to boil a planet's oceans. (TT is 10^9 KT, a billion of them is 10^18 KT, so a 100x safety factor). We could already build TT nukes; making a TT nuke factory in their system seems plausible. $\endgroup$
    – Yakk
    Commented Jul 13, 2022 at 20:55
  • $\begingroup$ I was thinking of destroying mankind. You couldn't kill the microbes found five miles underground, even with billions of hydrogen bombs, but I doubt that the aliens would find them a threat. My solution would wipe out the entire chain that starts with photosynthetic plants, but mushrooms would probably survive. $\endgroup$
    – Robert Rapplean
    Commented Jul 13, 2022 at 22:26
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Ultra-long gamma-ray burst

When a stellar object falls into a black hole, it propels relativistic jets from its poles, in the form of a lot of ionized matter and gamma-rays. Like, a lot. Just pick any active black hole within 10,000 ly, rotate it so one of its poles points towards the target, and push a star into it.

This isn't necessarily the fastest method, but it will definitely sterilize a planet. When the beam hits the planet, the number gamma-rays that reach the surface depends on the atmosphere but would probably be minimal. Instead, the surface will be bathed in extreme ultraviolet radiation for hours or days, killing anything that can see the sky. Meanwhile, the gamma-rays will cause all kind of bad reactions in the atmosphere. The ozone layer will be annihilated, the clouds will turn to acid rain, and the entire planet will be plunged into a very long cosmic winter by nitrogen oxide smog. So, anything underground will freeze to death, and the ocean will become uninhabitable from the drop in pH. Without ozone, the planet will be permanently barren.

EDIT: Based on the discussion in the comments, this probably isn't possible for your civilization's current scale.

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    $\begingroup$ "Fast" and "10,000 ly away" don't really mesh. Let alone manipulating a black hole and using stars as ammunition after traveling 10,000 ly. This is a K2 civilization, not a K3 one. $\endgroup$
    – Yakk
    Commented Jul 13, 2022 at 20:53
  • $\begingroup$ @Yakk Yes, I acknowledged that this wasn't going to be fast. The reason I posted this answer is because the other answers either would not actually sterilize a planet, or would require the attackers to have a prolonged presence in the victim's star system without being intercepted, which is not feasible. A K2 civilization has enough energy to nudge celestial bodies, it will just be a tedious process. This is the only way to achieve what OP requested from a safe distance. $\endgroup$
    – Ethan Maness
    Commented Jul 14, 2022 at 15:01
  • $\begingroup$ The weapon involves converting the mass-energy of a star into a beam and controlling said beam. That is 2*10^30 kg, which is 10^47 J. A K2 civilization, by definition, is limited to 4*10^26 W; the weapon you are using is 10^20 times the capabilities of the civilization in question, or (by definition) requires 10^20 seconds of effort to pull off. This is 10^12 years, or many times the lifespan of the universe. In short, your answer reduces to "become a K3 or K4 civilization and swat them". $\endgroup$
    – Yakk
    Commented Jul 15, 2022 at 1:07
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    $\begingroup$ The difficulty here is hand waved in the "control the black hole", "redirect a star". And you could argue that "having a weapon capable of producing X energy" isn't the same as controlling it. But my basic issue is that this looks like a K3 civilization (or above) weapon, moreso than a K2 weapon. $\endgroup$
    – Yakk
    Commented Jul 15, 2022 at 1:08
  • $\begingroup$ @Yakk You are not pulling an E=mc² on a star. You are only using enough energy to push it into a black hole at the right angle to spin the black hole towards the target planet. However, doing the math on it, I'll admit that you are still correct in that a K2 civilization cannot move the star on a timescale as short as a few millennia. $\endgroup$
    – Ethan Maness
    Commented Jul 15, 2022 at 14:59
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Ammonifying bacteria

True sterilization under a hard-science tag will probably require some high energy solution, there are many answers.. but when the goal is to kill off the enemy population and any other higher lifeforms in a few years, there's ways you could derange the biosphere of a planet like e.g. Earth. Destroy plant life and the entire bottom of the food chain:

Ammonification is the conversion of organic nitrogen to ammonia and ammonium ions.

A K2-civilization can genetically engineer an ammonifying bacteria or fungus that will spread very fast and in huge quantities, consuming all plant life it encounters, converting it to harmful ammonia gas. The world would be deprived of oxigen and warm up quickly, as a result of CO2 not being absorbed anymore.

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    $\begingroup$ There are some nitrifying bacteria that go against this process. Of course, you would still screw up the biosphere, even if it is just by destroying a very large part of the biodiversity. If you are genetically modifying a bacteria anyway, you might want to try to make it produce HFC's as they would REALLY screw up the atmosphere, being about >10.000x as effective as CO2 for global warming. $\endgroup$
    – vinzzz001
    Commented Jul 14, 2022 at 8:59
  • $\begingroup$ @vinzzz001 nice addition, thx for the comment. $\endgroup$
    – Goodies
    Commented Jul 14, 2022 at 10:57
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It would probably not be the quickest way, but a very fun one: Find a rogue planet or use a planet of the solar system of the planet you want to sterilize.

Change its orbit so it slingshots the target planet out of its solar system.

Without a sun all life on the planet is doomed. An intelligent species could utilize the powersources it has left and probably survive longer than 100 years, but it wouldn't really get them anywhere.

No sun, no other planets and no asteroid belts means they are restricted to the resource they have on their freezing planet. So even with advanced tech, I doubt that they would find a way to travel to any nearby star.

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Let their star do the job

When the world started to use CFC's in spray cans we quickly found out a bad effect. The gas would come in contact with our ozone and change it into other gasses. This is bad, as the ozone layer protects us against harmful UV light. If we would've continued we would kill our ecosystems and drastically reduce our longevity even if we had the food. Taken to the extreme the UV can sterilise anything the star can shine upon.

There is an incredibly large chance that the intelligent species will have some protection against the dangerous radiation of the sun. From the satellites we currently have in orbit (like James Webb) you can gather information about the composition of the other planet. A Kardashev 2 civilization should definitely be able to analyse a weak point. When this is found they can make chemicals (or biology!) that will neutralise the radiation defences of the target planet. Make the weed killer(s) and send them on their way.

When it'll arrive it is spread the stuff around the planet and the star will do the rest. Harmful rays will beat down on the planet, scouring it clean. The radiation can also damage many other things. It isn't impossible to survive this if you live on the planet. If the technology is advanced enough they might survive longer, but even then there are many reasons for the survivors to perish.

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    $\begingroup$ UV light only kills land dwellers, not much will reach the bottom of a puddle, and none of it will reach into deeper waters. Note that the Earth had no oxygen (and hence no ozone layer) when life first appeared. It was life that created the oxygen atmosphere and the ozone layer. $\endgroup$
    – Cris Luengo
    Commented Jul 13, 2022 at 16:08
  • $\begingroup$ @CrisLuengo yes it does. That is why it us important to look at the whole spectrum and not just the example. Even so it has a good chance to destroy the targeted sentient life. Especially with what we encounter in literature and other media. $\endgroup$
    – Trioxidane
    Commented Jul 13, 2022 at 17:51
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Could very well use a combination of destroying the Ozone layer and intense heat, perhaps from focused lasers over time. If the Ozone layer is destroyed, the job will be done on it's own but perhaps you'd want something quicker than that, like using some sort of highly powerful laser that can be parked in orbit and sweep over the planet several times. You want to keep the planet itself intact.

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Give Them A Computer Virus

Send them an advanced computer Virus, and since your level 2ish civilization, why not slap on a self-perfecting AI? (note: the virus needs to be compatible with the civilizations computers, so prior knowledge is needed)Then send it over to them and let it cause mayhem.

There are many possibilities, why not make it get the nuclear launch codes and kill the planet with nukes? or shut down power across the planet, maybe cause nuclear reactors around the planet to fail and cause hundreds, if not thousands, or Chernobyl's all at once. Have it screw with the GPS, cripple global, national, and maybe even local communications, screw up the data sent by artificial satellites, have it leak government secrets or turn off the internet. Send satellites careening into the atmosphere and burning up. Have it release dangerous viruses from laboratories, or make dams fail, Like I said, endless possibilities for destruction.

Or what if they were a multiplanetary civilization? the virus can kill all the inhabitants on bases located on other planets by shutting off vital systems, like shutting off the oxygen supply, or water filtering. It can cut off communications to the main planet, it could strand colonists on uninhabitable planets, or blow up moon bases, so many options.

Or why not think bigger? If they were Dyson swarm civilization, you could cripple the entire civilization in one fell swoop by sending the orbitals in all directions, dismantling the entire swarm. Or what if this civilization consists of uploaded consciousnesses living in a virtual reality world in a Matrioshka brain, giving the virus to them could mean instant game over just by corrupting the entire thing or even shutting it off entirely.

The are endless possibilities with a Virus, And as an added bonus, the virus is more destructive the more advance the targeted civilization is. and compared to the other answers, this is relatively very simple, No Lasers or giant structures or impacts needed.

edit: as it was pointed out, it would be near-impossible to deploy this virus without prior knowledge of the other civilizations computers, so that part was removed. The virus would still work, but needs to be coded to already be compatible with their computers.

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  • $\begingroup$ This may not work with the pacifist people of the Kumbaya system. $\endgroup$
    – DWKraus
    Commented Jul 13, 2022 at 18:44
  • $\begingroup$ If their pacifist, I am still certain that a super-virus could still do the job, how about it causes nuclear reactors to fail around the world, causing hundreds of Chernobyl's. The radiation from Chernobyl alone had reached throughout the northern hemisphere, imagine hundreds. Or what if they were a multiplanetary civilization? the virus can shut down bases on other planets to kill their inhabitants. Or why not think bigger? If they were Dyson swarm civilization, you could cripple the entire civilization in one fell swoop by sending the orbitals in all directions, dismantling the entire swarm. $\endgroup$
    – KaffeeByte
    Commented Jul 13, 2022 at 19:07
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    $\begingroup$ I suspect that "millions of versions" wouldn't cut it for brute-forcing a virus - by so many orders of magnitude that not only the targeted systems' processing power was insufficient, but also the bandwidth afforded by inter-system radio transmission. You can push these limits with clever engineering, but the more sophisticated the algorithm, the less margin for error you have (i.e. more iterations needed) - and you won't get any timely response telling you when/if you get close. $\endgroup$
    – Ruther Rendommeleigh
    Commented Jul 13, 2022 at 21:36
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    $\begingroup$ " eventually one version will be compatible with the civilizations computers" this is, and I apologize for the strong language, poppycock. For a sense of scale of how unlikely this is to work, imagine never having heard of the PDF file format, and then trying to create a binary file that, when opened in Adobe Reader, would successfully render a well-formatted manuscript. "Millions" of attempts would fall woefully short, and what you're proposing is orders of magnitude more complex than that. $\endgroup$
    – Daniel B
    Commented Jul 14, 2022 at 3:15
  • $\begingroup$ Hell, I spent four hours yesterday trying, unsuccessfully, to get quicktime to open a file I knew was a valid mpeg4. $\endgroup$
    – Daniel B
    Commented Jul 14, 2022 at 3:17
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Dennis E. Taylor did this in style in book 3 of his Bobiverse Series

-------- SPOILER WARNING --------

accelerate 2 stellar bodies into an orbit that will collide w/ the target systems star at stellar north/south at the same time. Then kick back and watch the fireworks.

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  • $\begingroup$ I think that's the second best solution after nudging them to self destruction. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jul 15, 2022 at 17:21
  • $\begingroup$ any books where one world nudges another into destruction intentionally? $\endgroup$
    – CaffeineAddiction
    Commented Jul 15, 2022 at 18:44
  • $\begingroup$ I remember a short story where secret agents of a foreign race made people build prominent houses on hills all wrong, violating Feng shui principles and thus bringing misfortune and discord over an Asian city. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jul 15, 2022 at 19:28
  • $\begingroup$ In one of the later Lensman stories (EE Doc Smith), they destroy the enemy planets (a full galaxy of them) by teleporting two planets and crushing the enemy planet like a nutcracker. $\endgroup$
    – NomadMaker
    Commented Jul 20, 2022 at 12:50
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Strangelets would be the quickest way to destroy the planet. From Wikipedia:

"If the strange matter hypothesis is correct, and if a stable negatively-charged strangelet with a surface tension larger than the aforementioned critical value exists, then a larger strangelet would be more stable than a smaller one. One speculation that has resulted from the idea is that a strangelet coming into contact with a lump of ordinary matter could convert the ordinary matter to strange matter.[16][17]"

https://en.wikipedia.org/wiki/Strangelet

Stranglets are real science, though they haven't been produced or even detected yet. The effect of stranglets interacting with normal matter would be akin to an anti-matter chain reaction; as though anti-matter would produce more anti-matter as it annihilated. The conversion of normal matter to strange matter would take place at close enough to the speed of light, but, since stranglets are unstable, the reaction ends when the matter becomes too dispersed; i.e. you run out of planet. The planet is gone, but the universe remains, as it were.

A tiny amount of strange matter, a quick flash of light and your problem is solved.

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You don't need to throw one of your asteroids at them; use one of theirs. Most solar systems will have asteroids, so I'm assuming the target system does.

If you have a Dyson swarm capable of delivering targeted interstellar bursts, you should use it to ablate their local asteroids or comets, altering their orbits such that they will intersect the planet. You then get a dinosaur-killing extinction event. You probably want to go for some of the asteroids/comets with eccentric orbits that pass near the planet already rather than one of the ones in a belt.

This method required advanced computational ability, but I assume that's trivial for a K2 civilization. It's much more power efficient than trying to boil their oceans/atmosphere with your Dyson laser, which you presumably need for other things like accelerating spacecraft and doing astro-engineering.

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