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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.

JYelton
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Aravind Karthigeyan
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    Do you want to keep the planet for later use? – vinzzz001 Jul 13 '22 at 10:20
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    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. – Starfish Prime Jul 13 '22 at 12:14
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    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. – Trioxidane Jul 13 '22 at 12:18
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    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 https://astronomy.com/news/2018/12/scientists-discover-staggering-amount-of-life-deep-within-earth – Donald Jul 13 '22 at 14:10
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    Quickly, "a few light years away" won't be reached by anything at all in under hundreds of millennia if you are limited to "realistic tech" even if you have the ridiculous K-II energy levels. A K-II can move planets but the power comes from the star. Sure, U can throw something big at them, your stated time scale just can't work. – Vogon Poet Jul 13 '22 at 16:49
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    What kind of transport to the other system is available? And does the enemy have access to space travel? – Michael Richardson Jul 13 '22 at 19:41
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    @MichaelRichardson The enemy is about sub-K1. What Humanity would be in about 100 years. The fastest way would be light sails, which can reach 25% of c. But they are mainly used for intel and relaying information – Aravind Karthigeyan Jul 13 '22 at 20:45
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    @vinzzz001 No, the planet is of no use. – Aravind Karthigeyan Jul 13 '22 at 20:47
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    @Trioxidane after thinking about this, I would be fine with sterilizing multicellular life. But I would like to see both solutions – Aravind Karthigeyan Jul 13 '22 at 20:55
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    "Nuke'em from orbit. Its the only way to be sure" is a line that comes to mind. – Forward Ed Jul 13 '22 at 22:10
  • Possibly relevant novel: "The Killing Star" by Pellegrino and Zebrowski. – The_Sympathizer Jul 15 '22 at 02:38
  • "I initially thought bio-weapons, but they are unlikely to work on alien biology." There are stories of this precise thing happening. The protomolecule in The Expanse is one such case. It effectively figured out how to harness the present biological organisms and repurpose it. The protomolecule had other plans, but one could create a similar nanotech whose sole purpose is to learn about and then wipe out the organisms. – Flater Jul 15 '22 at 12:29
  • Can you clarify what you mean by most efficient? Using the least amount of energy? Killing the most things? Please define how you would measure efficiency in this use case. – Mathaddict Jul 15 '22 at 13:48
  • Where is that coming from, please? Is it reasonable to assume the only way to sterilize a planet from light-years away is with a beam weapon? – Robbie Goodwin Jul 16 '22 at 19:57
  • When you're "… thinking of a couple main ways…" can you Post them here? If not, would you rather withdraw that whole idea, or stand wholly unsupported? – Robbie Goodwin Jul 16 '22 at 20:03
  • How might a hermit civilization matter?

    Why might being a hermit civilization mean seeing intelligent life in a neighbouring system, and wanting to kill it?

    Could you drop all the preconceptions, and work with what's left?

     – Robbie Goodwin Jul 16 '22 at 20:08

18 Answers18

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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.

Pelinore
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  • Comments are not for extended discussion; this conversation has been moved to chat. – L.Dutch Jul 14 '22 at 03:29
  • unless others live under clouds already or in other ways independent on sun light – akostadinov Jul 14 '22 at 12:50
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    This would not sterilize the planet. It would be a mass extincition event, but much life like bacteria and insects would survive. – Scorb Jul 14 '22 at 20:51
  • @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 – Pelinore Jul 15 '22 at 00:09
  • 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. – gnasher729 Jul 15 '22 at 09:24
  • @gnasher729: Assuming "commonplace" space travel in their own solar system and the kind of ship that this would require (i.e. non-Hohmann transfer "I go and hover where I want" style spaceships), which is pretty much a given at the point of being a K2 civ; the lack of mass can be offset by the century you can spend accelerating towards your target. It's about the energy transferred, so F=m*a very much applies. Smaller m times bigger a = same F. I suspect they did not have the luxury of a century's worth of acceleration in the book you're referencing. – Flater Jul 15 '22 at 12:33
  • @gnasher729: Just for clarity: I'm also working on the supposition of the size of ship you'd need to construct your solar-harnessing equipment needed to become a K2 civ. That's not going to be a small dinghy, and its mass will significantly approximate a planetkiller asteroid (within a handful of orders of magnitude), enough so that the century's worth of acceleration will rapidly outpace it. I'm not assuming that they're going throw the equivalent of a scooter, but rather that they throw a cargo container ship at the problem. – Flater Jul 15 '22 at 12:48
42

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.

abestrange
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ths
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    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... – Starfish Prime Jul 13 '22 at 12:16
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    Again this assumes the target is defenseless. Because light + reflectors = 0. – Vogon Poet Jul 13 '22 at 16:58
  • "Starfish Prime" ND-beam assumes that it can be precisely targeted. This is plausible, however not granted for Kardashev II civilization. – Alexander Jul 13 '22 at 17:00
  • @VogonPoet The defender doesn't have perfect mirrors. At atmosphere destroying levels, you need to put something on the order of an entire planetary atmosphere in the way. – Yakk Jul 13 '22 at 20:52
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    I was also going for this, and it does seem the best option. Most energy for the least momentum. But, I was still wondering if the energy density would be enough after the light "diverged" from its original beam path because the beams won't exactly be parallel. Guess more brute force – Aravind Karthigeyan Jul 13 '22 at 20:53
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    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. – infinitezero Jul 13 '22 at 22:34
  • @Yakk all the other planet has to do is move a significant amount of stuff there. Beam will plasma it, and plasma absorbs light pretty nicely. – Neinstein Jul 14 '22 at 02:00
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    @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 – fyrepenguin Jul 14 '22 at 17:09
  • unfortunately. OP didn't specify the targets level of technology. if it's K2 as well, there's probably no way to eradicate them at all as they will be spread out. if it is at our level, it certainly won't be able to block the beam. – ths Jul 14 '22 at 17:15
  • @ths OP did specify the target's level of technology, last line of the question: "Tech level of the opposing system is sub-K1, about what humanity would be in 100 years." – Esther Jul 14 '22 at 18:56
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    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. – ths Jul 14 '22 at 23:06
  • Would this work though with only passive optics? Because there is diffraction limiting, no? You need a lot of focus to get things toasty at your destination if it's many light years away, even with a star's worth of power. Hence short wavelengths (XUV and/or X-ray) are ideal. – The_Sympathizer Jul 15 '22 at 02:34
  • Direct Nicoll-Dyson Beam fire is wasteful. Use the phased array to speed up relativistic kill missile swarms. Has the added benefit that Those gigatons of matter bearing down on the opposition can be steered and you can take out space infrastructure as well. – TheDyingOfLight Jul 15 '22 at 16:08
21

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.

Mathaddict
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    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. – Vogon Poet Jul 13 '22 at 17:02
14

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.

Michael Richardson
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    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 ! – Goodies Jul 13 '22 at 22:24
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    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. – Trioxidane Jul 14 '22 at 11:06
11

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.

DWKraus
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    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... – Starfish Prime Jul 13 '22 at 12:08
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    you could supernova their star. – Tom Jul 13 '22 at 14:58
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    @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. – DWKraus Jul 13 '22 at 16:01
  • @DWKraus I'm thinking if you can make a distant star go supernova, you might know how to shield yourself from the radiation. – Tom Jul 13 '22 at 16:57
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    @Tom I think my way is funnerer. Most funnest? Cooler, which is to say insanely hot. Explosions are fun. – DWKraus Jul 13 '22 at 18:36
  • There's not really a way to make a star go supernova. Any energy you pour into it just makes it bigger. Stars go supernova when they stop making energy and collapse on themselves. – Ethan Maness Jul 14 '22 at 20:01
  • @EthanManess Simple enough, you just (insert K2-level effect here) and the star collapses in on itself, exploding all at once. See? Sure, WE can't, but we're talking borderline magic. Alter atomic valence shells, or change the vector of every atom in the star inward. Lense gravity so the star behaves like it doubled its mass. Quantum-teleport a fine misting of strange matter into the star so it decomposes rapidly. I was listening to a debate today about if black holes even actually exist as we understand them. These people know all that. – DWKraus Jul 14 '22 at 20:11
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    @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. – Ethan Maness Jul 14 '22 at 20:36
  • @EthanManess I would have considered that in the category of star lifting https://en.wikipedia.org/wiki/Star_lifting but I'm glad someone around here knows what a K2 civilization can and can't do. https://en.wikipedia.org/wiki/Kardashev_scale – DWKraus Jul 14 '22 at 21:08
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    @DWKraus There is a very, very large difference between slowly skimming small amounts of matter at a time from the surface of a star, and instantaneously triggering a supernova collapse. – Ethan Maness Jul 14 '22 at 21:34
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    @EthanManess It may not be that hard, and we just haven't though of how to do it. If you were a xenophobic race and had been planning for this for a while, you could probably do it with about a moon-sized ball of antimatter. Or perhaps a giant dump of positrons to dampen the nuclear reactions long enough for the star to collapse due to it's own gravity. Or maybe it takes a lot less in explosions in a star to make it implode in on itself (raise internal temp until you have 511,000 electron-Volts per photon, they break into electron-positron pairs and the photon pressure collapses.) – DWKraus Jul 14 '22 at 22:31
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    @EthanManess Is it wrong that it's so much fun to casually be trying to come up with ways that an advanced civilization could blow up a star? I suspect my wife wouldn't approve. – DWKraus Jul 14 '22 at 22:54
  • @DWKraus Oh, not at all! That's the whole point of this website. We come up with a bunch of answers and figure out which one are viable. I'm just helping with the second part. Which, by the way, a moon-sized ball of antimatter is not. Even at 100% efficiency that would require nearly 10⁴⁰ joules of energy to create, which beyond the scope of even a Kardashev-III civilization. Dumping positrons also wouldn't do much--again, antimatter is expensive, plus the Sun already produces boatloads of positrons, and they would harmlessly annihilate on the surface anyways. – Ethan Maness Jul 15 '22 at 14:33
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    One of the alien groups that came to the solar system in Stephen Baxter's novel Manifold: Space did travel between the stars by triggering stars to supernova to propel their light sail craft. – Dan Is Fiddling By Firelight Jul 15 '22 at 14:33
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    @DWKraus As for raising internal temp, the required energy is actually 511 million eV, which would require an average temp of 1.2 billion K. The Sun by itself is only ~5800 K on the surface and 15 million K at the core. So, you'd need thousands or millions of stars' worth of energy to superheat the whole thing enough to get 511 MeV photons. Which, again, necessitates K3. – Ethan Maness Jul 15 '22 at 14:47
11

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".

10

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.

vinzzz001
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    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. –  Jul 13 '22 at 15:42
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    The problem with grey goo is that it's got some pretty stiff competition from the green goo that's already infesting the place. – Mark Jul 14 '22 at 02:53
  • @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. – Ethan Maness Jul 14 '22 at 20:06
10

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.

Mindwin Remember Monica
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  • 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. – Ethan Maness Jul 14 '22 at 20:07
  • @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. – BlackThorn Jul 14 '22 at 22:18
  • @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. – Ethan Maness Jul 15 '22 at 14:31
  • @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. – BlackThorn Jul 15 '22 at 17:01
  • @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. – Ethan Maness Jul 15 '22 at 18:03
  • Just a FYI, I didn't state a distance nor the Q demands one. You are reading too much into it, @ethanmaness. They could very well send a swarm to enemy's stars and then beam stuff inside the system. – Mindwin Remember Monica Jul 15 '22 at 20:37
6

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.

Robert Rapplean
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    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. – Ethan Maness Jul 13 '22 at 20:24
  • @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. – Yakk Jul 13 '22 at 20:55
  • 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. – Robert Rapplean Jul 13 '22 at 22:26
5

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.

Ethan Maness
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    "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. – Yakk Jul 13 '22 at 20:53
  • @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. – Ethan Maness Jul 14 '22 at 15:01
  • The weapon involves converting the mass-energy of a star into a beam and controlling said beam. That is 210^30 kg, which is 10^47 J. A K2 civilization, by definition, is limited to 410^26 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". – Yakk Jul 15 '22 at 01:07
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    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. – Yakk Jul 15 '22 at 01:08
  • @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. – Ethan Maness Jul 15 '22 at 14:59
5

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.

Goodies
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    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.

     – vinzzz001 Jul 14 '22 at 08:59
  • @vinzzz001 nice addition, thx for the comment. – Goodies Jul 14 '22 at 10:57
4

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.

Mathaddict
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Trioxidane
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    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. – Cris Luengo Jul 13 '22 at 16:08
  • @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. – Trioxidane Jul 13 '22 at 17:51
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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.

elPolloLoco
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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.

Haven
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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.

KaffeeByte
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  • This may not work with the pacifist people of the Kumbaya system. – DWKraus Jul 13 '22 at 18:44
  • 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. – KaffeeByte Jul 13 '22 at 19:07
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    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. – Ruther Rendommeleigh Jul 13 '22 at 21:36
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    " 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.

     – Daniel B Jul 14 '22 at 03:15
  • Hell, I spent four hours yesterday trying, unsuccessfully, to get quicktime to open a file I knew was a valid mpeg4. – Daniel B Jul 14 '22 at 03:17
  • Related: https://worldbuilding.stackexchange.com/q/221067/42571 – Ruther Rendommeleigh Jul 14 '22 at 19:34
  • yeah, I changed the answer, the idea of a virus would still work, but it would need prior knowledge of their computers, and the virus would be coded on some system that is compatible with their computers. – KaffeeByte Jul 15 '22 at 18:13
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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.

CaffeineAddiction
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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.

JohnHunt
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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.

Zags
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