Space faring advanced species needs to 'disinfect' planets in habitual zones around stars. That's all life. They can wait a few thousand years to come back and seed the planet with a new ecosystem. My 'Gama ray' bombs wouldn't irradiate far enough underground. Any good ideas how my aliens could fully clean off a planet and maybe still have water, hydrogen, carbon, and other useful elements for an alternative carbon based form of

I am looking for a semi-hard sf solution. Ie no life-killer-beam-weapons. Needs to sound sciencish.


  • 1
    $\begingroup$ Oxygen should do it, awful stuff, starts fires you know. Hehe, that wasn’t very helpful. Sorry, I’m trying to think of extreme fast acting solutions that a planet could physically recover from. I’m a bit stumped, sensible non-threatening solutions are either desperately slow or just permanent. I’d still be looking for an answer in biology though. $\endgroup$ Commented Dec 11, 2019 at 23:13
  • $\begingroup$ What can you tell us about the existing ecology? Perhaps we can poke the weak spots? I’m thinking you should start with the oceans if they exist. $\endgroup$ Commented Dec 11, 2019 at 23:16
  • $\begingroup$ Control, Alt, Delete? $\endgroup$
    – user57780
    Commented Dec 11, 2019 at 23:43
  • $\begingroup$ Does it have to work for planets like Earth, with deep oceans and active plate tectonics? $\endgroup$ Commented Dec 12, 2019 at 0:09
  • 1
    $\begingroup$ Believe it or not, possible duplicate of several questions $\endgroup$
    – Cort Ammon
    Commented Dec 12, 2019 at 5:02

5 Answers 5


To get underground enough on your average planet, so as to exterminate all forms of life, you need:

  • excavators. Very powerful machines; they've found Archaea three kilometers underground (a very long time ago, those areas were probably near the surface).
  • sifting and hunting robots to examine the soil.
  • some way of insulating the already disinfected area from airborne fungi, molds, sporae, insects, flying critters, seeds and so on.
  • disinfecting agents (probably just concentrated UV and microwaves will do)

How to proceed: a beachhead will be established somewhere where suitable construction materials are available. Excavators and sifters will start working, processing the planetary crust up to a suitable depth. Meanwhile, atmospheric processors will start extracting hydrogen and oxygen, mixing it up with carbon. Silicium and aluminum will be harvested from the soil. Nano-etched silicon will be used to build solar collectors, while ordinary glass and aluminum will be organized in ceramics and used to build tetrahedral and octahedral air-tight boxes (one of the sturdiest geometric shapes) where the sterilized waste materials will be stored.

Gradually, the whole central pit well into to the bedrock will fill with armored sealed boxes full of compressed waste, sealed and sterile. All around, behind a series of plastic barriers, excavators will attack the intact crust and replicate themselves.

After many thousands of years, the whole planet will be an almost airless smooth ball of rock covered in a 3 to 5 kilometers thick layer of inventoried tetrahedrons and octahedrons containing everything necessary to rebuild the biosphere.

At this point, the whole operation can be run in reverse, incubating the new biota from chosen areas and spreading outwards.


A quicker way is using neutrino projectors. A sufficient neutrino density can sterilize a planet, and a neutrino beam will blithely go through the whole planet without missing a beat.

The amount of energy required is staggering - to do so from some 400 million kilometers would take a supernova according to Randall Munroe - and to do it from orbiting satellites you would need 36 millionths of that same energy, which is still staggering (it would literally require a Kardashev level II civilization, or even more, since we need a very special level of stellar energy).

Nonetheless, it stands to reason that a sufficiently advanced civilization might be able to do so using "neutrino bombs" - devices designed to maximise the output neutrino flux while yielding negligible unwanted radiation.

Current nuclear reactors produce 185 MW of neutrinos for each 4 GW of power, and of those, only about 6 MW are "sterilizing" neutrinos, for a total efficiency of 0.15%. The planet would be crisped by the fission or fusion radiation long before the neutrinos did their job.

Centuries would still be needed before the radioactive poisons from the inverse beta decay would disappear.


Drop a large asteroid into the planet.

This should effectively turn the entire planet back into a molten mass of stone which will kill of all and any life on the planet from deep underground, under water, in the air and anything on the surface.

All your water will evaporate away, but it should still stay within the atmosphere of the planet, so once the surface cools down into a rocky stony surface, the water should also return. Your aliens are then free to seed the planet however they want because they would of eliminated all forms of life on the planet.

  • 1
    $\begingroup$ Or if one big one doesn't do the trick, go for a massive bombardment of smaller asteroids. One of the prevailing theories for how Earth got its water was from the Late Heavy Bombardment. The biggest issue with this is the time constraint, melting the entire crust will take a LONG time to cool back down. Not melting the whole crust will leave extremophiles alive. $\endgroup$
    – abestrange
    Commented Dec 12, 2019 at 0:04
  • $\begingroup$ I'm Not even sure that would eliminate all life unless you blow the atmosphere off the planet too, a lot of stuff gets suspended in the atmosphere $\endgroup$
    – John
    Commented Dec 12, 2019 at 1:04
  • 1
    $\begingroup$ Surely that wouldn't cool in only a few thousand years? Also I don't think we really know how far underground life persists. How deep do you really have to go? $\endgroup$
    – Mike Wise
    Commented Dec 12, 2019 at 8:15
  • 3
    $\begingroup$ @John Large enough asteroids cause all sorts of bad stuff to happen to the atmosphere. At the very least, you're dumping ridiculous amounts of energy in, which should be quite enough to sterilize most life. It's probably a lot harder to eliminate every single viable cell on the planet, though. $\endgroup$
    – Luaan
    Commented Dec 12, 2019 at 8:18
  • $\begingroup$ "they would have eliminated" $\endgroup$ Commented Dec 12, 2019 at 10:05

I wouldn't write off gamma ray bombs and beam killer weapons due to hard science. You do realize that in hard science, any starship with engines powerful enough to travel between the stars can just use those engines to irradiate the planet. There is no such thing as an unarmed interstellar starship in hard science. I would be more worried about not destroying the atmosphere while you're doing it. In a sense, that might be the most convenient and expedited method since the starship is already there.

But that aside, a self-replicating weapon is probably the laziest. Such as an engineered virus, either biological (whatever that may entail for the planet) or machine. One step further is just to gray goo everything which is how you are probably going to set up a new ecosystem anyways. Why wait to seed in a few thousand years when you can start today? I don't think they would be any more effective at getting underground into isolated ecosystems than irradiating the planet with your starship engines though.

I'm sure throwing an asteroid could do it too. But one big enough to literally sterilize the planet, including deep underground life might need to liquify the surface and might blow the atmosphere off and turn into a rock that you could find anywhere else. You're going to need a specific reason to want that location in particular. But if it's in their territory, maybe that's good reason enough. No idea how long it would take for things to cool down though...could hundreds of thousands of years or more, for all I know.

  • $\begingroup$ |You do realize that in hard science, any starship...| Wait what? Lmao - Sorry for the total meme comment $\endgroup$ Commented Dec 12, 2019 at 18:03
  • $\begingroup$ @AndrewMellor ...I don't get it... $\endgroup$
    – DKNguyen
    Commented Dec 12, 2019 at 20:07

Need 2 steps

1) let the star or some other process "blow away" all atmospheric gases (the complete atmosphere and all vapor from step 2 ... are completely removed - takes care of lifeforms in the atmosphere and most life on the surface

2) heat up the whole planet to make it a giant "drop" of lava again - this takes care of remaining microbes on the surface that somehow survived space's vacuum and eliminates all life "IN" the soil / ground caves and so on ...

ta da .. a cleaned planet .. afterwards you can start by switching off process 1 - then 2 let volcanic gases build a new proto-atmosphere

  • $\begingroup$ Heating up is enough. At four-digit ground temperatures, the lower atmosphere will be just as hot, sterilizing everything, and the higher atmosphere will be pushed so far out into space that gravity cannot hold it any longer. But yeah you'll have to heat it up to get rid of those deep-crust bacteria. $\endgroup$
    – toolforger
    Commented Dec 12, 2019 at 8:57
  • 1
    $\begingroup$ Unfortunately this "disturbs" ground (mineral) deposits (the heavier [ores] tend to sink towards the center ) And everything burnable (coal / petrol / natural gas) would either evaporate or right out burn .. thus making the planet worthless to be settled - and I don't know if the time frame of a thousand years would suffice $\endgroup$
    – eagle275
    Commented Dec 12, 2019 at 9:11

If a planet had enough of an air supply to allow a deep resonance of species in livable conditions, to escape that planet the whole species would need to harvest something that by any greater need would allow the species to also endanger itself well enough to need to escape to that space.

When the space is no longer faring in its base-for-itself form, we are carbon, that base of life will render to what the most occupied spaces are used for and the outlying and living in dangerous conditions of formidable space routine beings there would need to extract from something a new version of some greater or lesser form of the same thing, for secondary purposes and for not saturating the exact species to anything a purer breed of being might rely on for separating existing lifeforms with past ones.

If the past ones were to navigate to space and require the existing forms' current environment to harvest, which is what it sounds like (Aliens Resurrection) the living condition would require a living form of the existing past form, but not the requirements for it to meet any kind of doom. That might be a sort of rapid expansion idea that does not linger in outlying dangerous conditions as they exist in space the way we know them to (long distance, long time).

When these conditions don't meet to exact real or alternate forms of what might be bridging the current need to what these beings might need for us to extract what is at the deepest of it, the aliens in the situation could offer their "young" to rival the current state of what they are doing to have the existing species offer itself as a living form of that extraction to allow itself to gain that outer peace.

So we might be able to alienate what offers women in this life to men, their ability to feed an entire family, as like what these species might do to the extraction process as it turns the civilization of prowess, to enlarge a colony is to offer it more resources, the shortest hand of that ability to extract would be to make them a powerful workforce that then relies on the same power of extraction as the powerful and principled issue of what an extraction process might create for something secretly looking to destroy it, so that the existing colony, and not the space faring colony, to be able to actually navigate a modern form of the same thing without extracting its own youth to offering it. That when that youth is offered they create intelligible programs that design a method to remove that youth, the space faring colony need only devise what the instructions of that living and in fact civilized colony may create to extract what the space faring colony may need. Even if only as the surmount of fuel to lead it to a new colony and leave enough behind to grant itself another extraction point.

The extraction could be, by our standards, water. It otherwise could be blood. It could be the remnants of bone and or the existing tissue as it regenerates over the processes that continue to understand our bodies well enough to protect them as in the fingernails. So when that process isn't busy enough to make the richest of the soul what is busy to making the not so rich parts a not good zone, the richness in the soul is what grants us a colony that can handle the task of excavation. That gives it colony for proof and manages that in distinct parts, the colony can barely understand itself well enough to cater to the whims of any underlying youth as the process of development, essentially enslaving them, but then also originating that fact within another origin in case the understanding comes from the greatness of what richness is in the preposterous outcome of the underlying youth, which would be slavery if the work were not intended to offer them the greater of the richness as of those in inheritance. So to speak.

The generation here is not in name but in trade and that would by extraction also offer what the blood in the bones can't generate to the occupying lands so that the existing pair of parent and child cannot originate the pair without the help of the existing movement of process into space (so no one actually brings their children with them but that isn't some sort of offer for peace, it's their actual way of life). That would make it dangerous to speak on the generation of youth for it would be foreboding but also offers work when the young one cannot contemplate the absence of work, as that is not the colony of the worthy extract.

It is just a bold reliance on the same exact nutrients, to gather nutrients these are the strongest parts of the body, the literal element of that disguise, none of the space faring would ever require themselves to destroy what is in existence in the zone of their planets, then, the exact anatomy of that physics would only be required to someone that is in the original colony by the space faring and the existing points for that extract, like Tatooine, would be the colony for its own entertainment, and not for the busy parts of its plot to originate that it has an entertainment for the body of power that the space faring have in their most valid insights.

The creator of this situation would be within origin and their most valued trait, possibly intelligence, would be the exacted and minimal exact point of the conversion to between planets and for to be used in their attempt to mind a non-criminal escape. As they astound themselves to find that they have indeed extracted only the most common points of insight to actually gather the same points as prior to finding that perhaps the formula may be pointing at them. And this way, the traps in leasing to that land a fortune of its own youth, that extraction is not in principle and the only regained part of that exchange is the conversion process and that would be to alienate what of the space faring colony was not able to ascertain in the conjunction between what is here and what might be the dividing factor, a youth of work and a youth of rebellion, to existing between what is in work and what is, in fact, simply a way of life that does not escape into space.

So that no-one sacrifices anything and the destroyed and simply, and only, in remnant, a disguise of what warfare might reveal to those of any space faring, minding, and workful exercise of what the criminal mind to create a genocide might create in the possible extraction of fact to the extraction of faith in the use of regular expansion and not rapid, to allow that the extraction be a deep and rich one WHILE the removal process is kept in place to create a free workforce, that will in turn also allow the exercise of food and fuel to remain the only piloting factor to allow extraction as possibility. Like in Gurren Laagan. Except only for minding their men as opposed to their women as they have it in Gurren Laagan.

  • 4
    $\begingroup$ Welcome to WorldBuilding.SE! This is a gigantic wall of text right now and it's extremely hard to read, would you mind breaking this up into paragraphs so it's easier to parse? Thanks! $\endgroup$
    – F1Krazy
    Commented Dec 12, 2019 at 11:05
  • 1
    $\begingroup$ Well, I hope you have a good day too and are able to work through whatever is really bothering you. $\endgroup$
    – DKNguyen
    Commented Dec 13, 2019 at 0:48
  • 1
    $\begingroup$ @DewiMorgan At first, it had only the slightest hints of coherence that it was not generated by an online jargon generator. But after revisiting it occasionally over the past week, I've concluded it actually does hold meaning. However, it's needlessly convoluted. I think the author just has just become so used to writing defensively as a lawyer that he has forgotten how to write with the purpose of communicating an idea clearly and concisely. $\endgroup$
    – DKNguyen
    Commented Dec 17, 2019 at 21:29
  • 1
    $\begingroup$ If the archaic language were removed, it would resemble my writing if I was explaining an advanced technical principle to a beginner, but made every possible tangent along the way to remove ambiguity, at the expense of legibility. $\endgroup$
    – DKNguyen
    Commented Dec 17, 2019 at 21:32
  • 1
    $\begingroup$ @DewiMorgan Well, no. The tangents here do not disambiguate. Their sole purpose seem to only be obfuscation which is a strange thing to do if your goal is to communicate an idea to someone. He, himself, admitted that was the purpose of his writing style. However, why he felt it was appropriate to apply it here is perplexing unless the reason is he simply can no longer write any other way, which is a shame since if his writing did not go out of its way to confuse people, he would sound like Tolkien whenever he wrote. That's why I think it is an inescapablely habitual defensive legalese. $\endgroup$
    – DKNguyen
    Commented Dec 18, 2019 at 20:55

Not the answer you're looking for? Browse other questions tagged .