Subtle ways to render a planet uninhabitable

Question

A violent way of rendering a planet uninhabitable being the obvious: accelerate a large enough object to a sufficient velocity that everything on the planet goes the way of the dinosaurs.

Put briefly, an alien race secretly constructed a series of autonomous spacecraft that would go to other solar systems and snuff out the inhabitants without them ever realizing they were under attack. The way that comes to mind, broadly speaking, is something relatively slow acting and environmentally based, such as inducing a star to start CME-ing the crap out of the planet (not that I really know how exactly you'd go about doing that) and strip its atmosphere. At the same time, it preferably shouldn't take a million years; you want it to be slow enough to be chalked up to some nasty natural phenomena, but quick enough to kill everything on the planet (or merely drive them out).

The tech level I'm working with is pretty darn advanced. While the alien weaponmakers can't use the power of handwavium to crush everything before them, if it's physically possible, they can (probably) find a way to do it.

tl;dr, what are some plausible ways to strip a planet of its ability to support life without the locals figuring out they were being targeted?

Edit: Changed title to subtle. D'oh. The comment pointing that out is right; killing things is violent regardless of whether or not you're shooting them or poisoning them. As for the tech level of the target planet, they're interplanetary, but an order of magnitude less advanced than their attackers. The aim is to destroy the sentient life only, but if that's too tricky, the aliens are more than willing to just kill everything. They'd just prefer not to.

Answer 1

Ways to destroy sentient life on a planet:

• Upset the ecosystem. Introduce animals into places they shouldn't be (the poison frogs or cats of Australia), or destroy ones that should be there (see just about any extinction and the results). Move some plants into the wrong ecosystem (think the devouring capabilities of kudzu outside it's natural environment). When adding something, if it doesn't seem fast or destructive enough, give it a genetically modified boost.
• Destroy the key pollinators. All of them.
• If the populace is prone to mono-crops, introduce plant diseases targeted at the biggest ones--almost instant worldwide famine. Kick back and watch them destroy themselves.
• Drop unobtanium (aka, any resource so valuable that every country will want to own, hoard, or destroy it) in a highly public place, preferably in a country prone to trigger fingers and lots of weapons. Sit back and wait for the dust to settle.
• Insert a disease targeted at the sentient populace. Make sure it reaches above Black Plague levels of pandemic, then just mop up the survivors. Or pluck a curable condition or three and make them superviruses. Cackle evilly as they discover there is nothing they can do to cure them.
• If they're dependent on electronics but don't protect them very well, mimic a solar flare or CME and make it bad enough to fry everything.
• Make the water undrinkable. Bonus points if you can make sure animals can still drink it.
• Infiltrate them and introduce a new religion. Make sure it focuses on worshiping people from the stars, meekness, and never questioning the tenets of the religion. For good measure, add into it the illusion everyone is equal, but make sure there's a tiered system where some people are more valuable than others. Make sure it's extremely appealing. After successfully catching on, people will happily oppress each other and themselves, and will greet their alien overlords (and their inevitable submission to them) as natural and even euphoric. Slowly collect the willing populace to "elevate" them, but in reality sell them off as intergalactic slaves, and just mop up the holdouts.
• The above can also work if the correct political and/or social ideology is put in place. Combining the three could have seriously far reaching effects. (If you think this is silly, ask yourself why women will follow a religion that sees them as second-class citizens or even disposable, why a mixed-race black/Mexican man would vote for Trump and continue to support him, or why Democrats have swallowed the idea that it's better not to vote. A well placed idea can make a person not only irrational, but actively pursue a course of action that is against their own best interests or even their very survival [hence the emphasis of the honor of warriors--getting people to march to their death in war requires a certain amount of brainwashing, as does making them kill]. If we were as non-gullible as we like to think, everything from advertising to fake news would have a lot less effect on us.)
• Introduce climate change. Kick back and let the environment itself kill off most of the sentient people. Use terraforming technology to get it back under control and kill off anyone who remains.
• Of course, if you have a machine that can alter weather patterns, you don't have to kick back, just make sure the world has a very bad handful of years. Mop up what few survivors are left.
• If the sentient species is money-based and they use electronic means to keep track of that money, crash or erase all records of it. Make sure whatever was used will also destroy backups as they come online. Sit back and watch the worldwide pandemonium.
• Get a contaminant into their medical supplies, targeting their most effective antibiotics (or make them believe there is one). Enjoy the chaos as people die both from a lack of medical supplies and a lack of faith in medical professionals.
• Introduce new deadly diseases to various insect populations. Kick back and watch the sentient species have no way to escape the bites.
• Introduce a sterilizing element to the world targeting the sentient species. Wait a year or two for them to realize that no children are being born anywhere in the world. Watch the fun.
• Introduce a parasite that makes them docile as kittens. Watch them rejoice as all wars disappear from the world. Walk in and do anything you like to them.
• Introduce a plastic or metal-eating mold/fungus/or bacteria. Make sure it's near impossible to kill. Enjoy.
• Gas everyone. Basically change the atmosphere rapidly enough there's nothing they can do. Terraform it back, and kill anyone who managed to save themselves.
• Introduce something that changes everyone to a singular sex. Let them die off naturally. If you want entertainment, change everyone to the opposite sex and watch the madness that ensues as worldwide gender taboos, gendered power structures, and body dysphoria flip everything on it's head.
• Incite a world war. Make sure that every new country that enters the fray is equally well armed. Use propaganda and misinformation to keep it going. Invade only after they've inevitably reduced themselves to pre-medieval levels of society and mop up any survivors.
• Put the whole world in a coma. Kick back and let dehydration kill them. Send in robots to clean up the dead bodies. Take what remains.

That's just off the top of my head. Given time I'm sure I could think of some more. Society is actually pretty danged fragile. It's why how to keep everything running has always been such a problematic question, while how to kill everything is always so simple.

Answer 2

Infiltrate the planet's early societies and teach them about money. Steer the societies into loving it. Change their genes to make them hoarders and cheaters.

As soon as those societies reach the industrial age they will start destroying their own ecossystems in search of profit. Since there is more money to be gained via destruction than conservation, the process will be irreversible and at some point the planet won't be able to support complex life on its surface anymore. Just chillax and watch for a handful centuries. The inhabitants of the planet will live their last decades like:

Answer 3

Destroying all life is easy - slowly crank up the sun's temperature until you bake the planet.

Destroying complex life, while allowing bacteria to survive, is also doable.

Destroying intelligent life, while leaving other life alone? That's hard. Really hard.

Adapting to new circumstances is what intelligence excels at - it's pretty much the whole point of intelligence. While it's pretty easy to disrupt a society over the short term (revolution, war, famine, plague, economic collapse) the survivors will find a way to bounce back unless you hit them with enough force to kill all less-adaptable complex life as well.

In fact, there is only one way I can imagine destroying all intelligent life on a planet without destroying everything else as well, and that is to make them voluntarily take the means of destruction upon themselves.

So how might an advanced alien race accomplish this? By giving them seemingly beneficial, ubiquitous technology that will later be used to kill them. Medical implants that will administer a poison to everyone when exposed to a worldwide signal, for instance.

Of course, even this is a dubious method. There will always be traditionalists who refuse the technology, but the more beneficial the technology seems, the greater the percentage of people who take it in will be. Once the vast majority are dead, you can send drones in to mop up the survivors (who will probably be less technologically inclined than the ones who died, since they were the ones who refused the beneficial technology in the first place).

Answer 4

Here's a new and really insidious one:

Approach them with big smile and offer them a "happy-machine". It hooks up directly to their brains and stimulates all the pleasure centers and whatnot. Along the way, cure mental diseases with the same tech. Instant happiness for everyone who wants it, for as long as they want it. Purely voluntarily, of course. And no negative side effects!

Of course such immense pleasure is by its very nature immensely addictive and people will demand more and more of it. Anyone who has ever tried it will want it again, because nothing else in life can quite compare to it.

Then offer the next level - immortal pleasure pod. Your brain is removed from the body and is placed in a pod where the life support systems basically guarantee your immortality, plus infinite pleasure, of course. You can still stop the trance and communicate via the pod's eyes and ears if you want to, thus convincing your friends and family that it really works and is worth it.

In a while, it will become increasingly silly not to get into a IPP, so everyone will get one.

At that point - checkmate. Although you really won't need to do anything, because everyone will be in an eternal happy trance anyway and won't care what happens to their planet.

Answer 5

Global Warming

Pump extra CO₂ into the atmosphere, and seed some ideas into anti-capitalist intellectuals that man is causing all the global warming.

No matter what we do, the temperature keeps on warming and blaming each other for the problem. Sooner than later, food production will start to fail, wars for resources will happen, nukes go off and everything dies.

Answer 6

Global Cooling

Reduce the amount of carbon dioxide in the atmosphere. The planet goes into a "snowball earth" global cooling episode where the whole planet is covered in a miles thick layer of ice. Given the current level of hysteria over global warming it would not be difficult to convince humanity to participate in this. "Hey our carbon sequestration scheme is going really well".

The only life that will survive that is some cryophile bacteria and life around the ocean volcanic vents. This has the advantage that it does minimum long term damage to the oceans and atmosphere.

When the planet has been sterilized reintroduce Carbon Dioxide and the planet warms up again ready for occupation.