Exterminatus
"Caedite eos. Novit enim Dominus qui sunt eius."
- – Arnoldus Amalricus, Inquisitor and Cistercian Abbot, 209M.2
"The world is what it is, which is to say, nothing much."
- – Albert Camus, reacting to the bombing of Hiroshima
"I say we take off and nuke the entire site from orbit. It's the only way to be sure."
- – Ellen Ripley in Aliens (986.M2)
- – Inquisitor Headsmash, ca. 990M.41
Exterminatus is the biggest middle finger the Imperium can give to xenos and Chaos infestations on their own planets. It basically involves UTTERLY DESTROYING THE PLANET SURFACE via heavy orbital bombardment if they decide that it would be impossible to retake the planet by drowning their enemies in corpses, like they usually do.
And, of course, there is no kill like overkill.
Before you go into some sort of sanctimonious tirade about the morality of blowing the fuck out of an entire planet, understand the context. A world deemed worthy of Exterminatus is one considered past the point where anything can be salvaged from it - whether because it's about to be lost to countless ravening giant insects that will zerg-rush and eat fucking everything or reality-warping fungi that reproduce into millions of spores every time one dies Breathes and will all kill you because they think it's fun...and then steal all of your shit because it looks shinny, hurty or fast. or because it will be turned into a fucking daemon-and-tentacle-rape-infested shit-pit where neither sanity nor time has any meaning, or simply due to an ineradicable belief that is deemed Heresy. The alternative is fucking glassing a planet and trying to deny it to the enemy or ensure SOMETHING can be saved. It's the last-ditch measure and it's there because the alternative sucks even worse. It is the Scorched Earth strategy on a planetary scale: if you can't have it, burn it.
...Then again, there's nothing stopping an Inquisitor from ordering it just because he wants one for his birthday. Oversight on Exterminatus orders is fairly nonexistent and it's easy to see why. The problem is the same one real life atomic weapons have, who do you want to have to be able to launch them? You want the most powerful, highly ranked people to have that authority, but if they're so highly ranked and with so much power, who watches them? Who second guesses an Inquisitor's judgement about if a world is to be blown up or not? Nobody. The Imperium's only solution is to just declare the trigger happy sod Excommunicate Traitoris afterwards if they don't agree. The over the top villainy of Warhammer 40k means that some fuckholes within the Imperium do get trigger happy with this, ordering an Exterminatus on worlds over things like a few of its people coming into contact with alien technology, or a small hint of heresy that would probably not require killing everything, or a loose pubic hair being in the Imperial's cereal this morning.
On the bright side these instances are few and far between, destroying an uncorrupted planet is seen as a gross waste of Imperial Resources, and anybody caught doing so will quickly be forced to explain the legitimate reasoning behind it by the full might of the Administratum... who will then proceed to hand out a light slap on the wrist, letting the offender get off scot-free unless they meet a giant, angry black dude in green (or the woefully understaffed Ordo Excorium). What, you're surprised that an Empire of "Space Nazis 2.0" can have actual legitimate excuses, common sense, reasoning and sensibility? You're in for a whole new series of surprises...
Deal with it. Bitching any further will rile the Commissariat. You have been warned.
Exterminatus has existed since the Great Crusade. Originally, the authority to authorize Exterminatus was reserved solely for the Emperor and the Primarchs. However, once the Horus Heresy erupted, the Inquisition was given the authority to speak for the Emperor. Ten thousand years later, the Inquisition, Chapter Masters and High Lords are the only ones left who can authorize it (along with Guilliman).
Joking aside, it's somewhat fluff dependent; in Seventh Retribution by Ben Counter, for instance, Exterminatus is never even mentioned as a 'solution' despite the fact that the planet got halfway to being a Daemon World (although part of this was because the Officio Assassinorum couldn't be 100% sure that even Exterminatus would do the trick and they needed to be fucking certain the antagonist of the book was a confirmed kill). Also in both the Space Wolves Omnibus and in the Ultramarines Nightbringer books, we get Inquisitors saying they have been at it for well over a century without calling down the Exterminatus even once. Even in Retribution, Lord General Castor admits that the world was lost anyway. As of current fluff, the Inquisition turns out to actually have an entire Ordo made exclusively to manage this sort of affair (dubbed the Ordo Exterminatus) and ensure that people aren't just committing mass genocides for shits and giggles.
Methods of Exterminatus
The Imperium has several means for dealing with hopeless infestations:
Just Shoot the Shit Out of It (Orbital Bombardment)
Saturating planets with over-sized cannons larger than apartment buildings is the stereotypical way of nuking the fuck out of something you don't like. Nuclear warheads, Space Marine Battle Barge bombardment batteries, Nova cannons, and/or banks of Lances are often used. Examples of this include the Dark Angels destroying their homeworld, Caliban, after it was lost to heretics within their chapter AFTER SOMEONE THOUGHT IT WAS A GOOD IDEA TO HUNT FOR DINNER FOR OUR TOTALLY NON-HERETICAL AND OBVIOUSLY LOYAL BROTHERS USING THE ORBITAL BOMBARDMENT CANNONS and the Night Lords' purge of Nostramo. If we take the purging of Typhon from Dawn of War II as canon, this method can also be used during the opening stages of the Exterminatus before you unleash one of the Inquisition's more thorough toys upon it.
Virus Bombs
Virus Bombs are warheads loaded with the Life Eater virus, a biological payload that causes living tissue (plant or animal) to rot and decompose (which probably gives Nurgle a massive boner). The gist is that they release a virus that spreads by contact and causes necrosis of tissues and rapid decay of plant and animal tissues. This immediate rot causes a buildup of flammable gases, which in turn can be ignited by one of the lazors above (or any still smoldering Lho sticks, or any other source of flame), sweeping the area in firestorms. A relentless bombing of these fucking things is what reduced Tallarn from a verdant forest world to the desert hellhole it is now. They were also used by Warmaster Horus to kill off loyalists in the Traitor Legions during the Istvaan Campaign of the Horus Heresy [Life Eater virus eats through any filters and corrodes power armour ( because apparently "virus" means the same thing as "acid" to GW) till it gets to the gooey marine inside, though a Dreadnought can endure it easily]. Though popular during the Great Crusade and the Horus Heresy, according to Amberley Vail, virus bombings are rarely used in the 41st millennium, because the Inquisition has figured out that they feed the fucking Plaguefather every time they're used. Whoops. (They were falling out of favor even before that, since as demonstrated on Armageddon one virus bomb usually isn't enough to kill the whole planet and you might need to hit the planet several times all around)
Atmospheric Incinerator Torpedo
Atmospheric Incinerator Torpedoes are plasma torpedoes that burst in low planetary orbit and super-heat the atmosphere of a planet until all combustible material ignites. Basically a napalm airburst bomb on steroids and an additional dose of plasma. This method of Exterminatus was used on Medusa IV. Pretty much like the Virus Bomb, except it skips right to the firestorm part and directly turns the planet's surface into an endless expanse of raging hellfire. It is said that the aftermath of the planet's surface (Medusa IV's case) was melted to glass and that the entire world burned like a piece of amber in space even a month after the attack had been launched. They are only effective on planets with relatively stable atmospheres made of flammable gas, however, and plasma torpedos are both somewhat rare and expensive.
Modalis Atmospheric Missile
Another weapon that has similar results from the Atmospheric Incinerator Torpedo but its function is completely different. Regarded as the most powerful incendiary device accessible in the Imperium. The Modalis Atmospheric Missile is one ECKS BAWKS HUEG Phosphex weapons used to burn a planet into a crisp. Think White Phosphorous on steroids. A salvo of several Modalis Atmospheric Missiles from orbiting warships will blanket an entire world in deadly Phosphex. The resultant firestorm of green mist will eat away at every carbon-based element on the planet, rendering it uninhabitable. All that would be left would be dust and echoes.
Cyclonic Torpedoes
The primary method of Exterminatus used in the 41st millennium, these are basically the analog equivalent of the Death Star's main gun. These capital ship-fired warheads each generate a series of massive, self-sustaining nuclear reactions, which, when fired in bulk, fuels a much larger reaction that causes the devastation to spread and multiply, eventually glassing the entire world with a thermonuclear holocaust given a sufficient barrage. If you fire enough in the same spot it will break through the crust of a planet, causing part of the mantle to erupt out and royally buttfucking the entire planet in the process (see the Fire Warrior end cinematic). Krieg is an example of a radioactive perpetual-winter world that survived multiple cyclonic torpedo strikes, though in this case it was on a much smaller scale and in some sources are described as standard nukes. This was the method that probably killed Typhon, in combination with the above shoot-the-shit-out-of-it method. (Another theory holds that the bombardment is used to remove anything that might prevent the torpedo from reaching the surface or to weaken planets crust.) Only the Inquisition and the Space Marines are authorized to carry cyclonic torpedoes in their warships, the former because the Inquisition has the authority to do anything, and the latter because the Imperium figures that if the Space Marines can't beat it, nothing else will.
Cyclonic Torpedoes are pretty variable in their strength, either due to there being different classes of torpedoes or the fact that the strength of a single cyclonic torpedo has never been nailed down in official materials. In one case, ol' Abby dropped a dozen to fry a single hive, in another a single torpedo is a qualified planet cracker. Similarly, this method is the easiest to thwart with shields, as they disrupt the stacked efficiency needed for ongoing detonation. Presumably this variability in strength is due to "Cyclonic Torpedo" being as broad a description as "atomic bomb", which can refer to both a Davy Crockett and a Tsar Bomba.
Two-Stage Cyclonic Torpedoes
In the two-stage torpedo, a melta charge activates first to allow the weapon to burrow into the planet's crust and down to the core. The second stage thermonuclear charge then goes off, causing the planet to break apart Death Star style. This is really the only way to deal with Necron Tomb Worlds since, due to their tendency to make everything subterranean, they aren't overly bothered by the other methods which devastate the surface but leave the planet as a whole mostly intact. Talos of the Night Lords used a smaller version of these when a Genesis Chapter strike cruiser tried to hide behind a moon. So he blew a continent-sized hole through the moon, and watched the loyalist ship get torn apart as a new asteroid field got shotgunned into space.
Smashing It with a Fucking Moon
This method involves radically altering the orbit of a nearby moon or large asteroid and placing it on a collision course with the planet, and therefore requires the use of several Mechanicus voidships. This method was used to destroy Phaenon Prime when the Virus Bomb failed to wipe out the planet's corruptive influence. It was also used during the Horus Heresy by renegade Iron Hands commander Autek Mor to destroy the World Eaters recruitment world of Bodt and during the Badab War to finally smash through Huron Blackheart's defensive Ring of Steel around Badab. Needless to say, this pretty much fucking annihilates the planet in question (or whatever else it's thrown at like Huron's defensive systems). Despite its flair and effectiveness, the Administratum vehemently requests that Imperial commanders avoid this method whenever possible because it's stupidly expensive -- it can take weeks or even months for the moon or asteroid in question to actually strike the planet, which costs rations and sublight fuel while the ships sit around doing fuck-all, while orbital bombardments only cost one day's worth of rations and fuel, plus ammunition.
Release the Krourk
Krourk Ogryn are known as the most brutal, powerful, and primitive tribe of Ogryn in the Imperium (and that's saying something). They are so well-known for their frightening savagery in close combat that they're considered a solid match for Orks, and are also known for being so primitively stupid that the Imperial Guard can't even teach them to use traditional Ogryn weapons like ripper guns. Their reputation is so fearsome that it has gotten to the point where deploying thousands of these things is considered a crude method of Exterminatus amongst Imperial commanders since they can't be taught to discriminate between friend and foe.
Send it to Hell The Warp
Sending your problems somewhere else is a rather simple solution to most problems. Doing so with an entire planet is possible, though difficult. As a result, various factions have simply thought if we send a planet into the Warp, it’s no longer in the materium, therefore it’s not our problem. There are many problems with this, but that has not stopped some particularly idiotic individuals from doing it anyway. The reason it is listed here (and not in the non-Imperium section) is because the Imperium believes that this is actually a valid method to dispose of Tyranids...and only Tyranids. (anything else would be ether redundant, or basically what they wanted in the first place, so stupid). The Warp is mostly foreign to the Tyranids as a result it is one of the few things that they cannot truly combat, not to mention the fact that Khorne hates them. Kill two birds with one stone. The shadow in the Warp however makes it difficult, so it has not been tested yet.
Non-Imperium Exterminatus
Several factions outside the Imperium do things similar to the Imperial Exterminatus (adding any examples from the lore would be greatly appreciated). However, most of them don't use these methods often. Some examples:
Craftworld Eldar
The Craftworld Eldar have some respect for life (and not nearly as many weapons of mass destruction as they had before the Fall) so they don't do it often. Didn't stop them from purging all life in the Octarius system to clean up Kryptman's mess, though (in that case the Dark Eldar provided the WMD). The most well-known Eldar engines of planetary destruction are called Blackstone Fortresses, which are ancient weapons they designed to fight the C'tan. To put it simply, think of a floating citadel with a distort weapon (like the ones the Wraithguard have) the size of an Emperor-class battleship. During the Gothic Wars, three Blackstone Fortresses combined their power to cause a star to go supernova, destroying an entire solar system.
In Apocalypse War Zone: Valedor, the Craftworld Eldar from Iyanden procured another type of ancient WMD, the Fireheart: a complex nodal resonator capable of causing a planet's molten core to enter violent death throes and send lakes of lava to the surface (or just explode two-stage torpedo style, the tie-in novel plays the fireworks up to a big degree). The Dark Eldar originally had this, but they gave it away because they didn't have the psychic power to activate the weapon. The Fireheart was used successfully on Valedor and prevented Hive Fleets Kraken and Behemoth from joining forces. If they had, the Tyranids would have had all of the genetic data of the Orks and the Eldar, enabling them to fashion unthinkable monstrosities.
Before the fall (and possibly still kicking around somewhere) they had devices that fired entire suns or black holes at their enemies.
Dark Eldar
The Dark Eldar as a whole lack a good stockpile of planet-killing weaponry and prefer to keep planets intact for slaves, although they are still capable of exterminating the populace of entire planets if they wanted to. One method is pillaging the shit out of it. It has been proven time and time again that an entire major kabal or dozens of separate, smaller kabals, is more than capable of kidnapping an entire planet of its populace, faster than that local PDF trooper can finish his scream of agony. Granted, being taken captive isn't part of an exterminatus' MO, but if you know the fate of a hapless mortal in Commoragh; they are already dead (at best).
There has also been one instance of an entire Hive World being poisoned by the Dark Eldar, smashing a Space Hulk at a realm and havings its warp drives detonate to release hordes of daemons, but that's not exactly a 'repeatable' means of planet killing, and there's also counting the DE's ability to steal entire suns; allowing them to turn entire habitable planets into ice worlds if need be. Like the Craftworld Eldar, they also possess a psychic doomsday device called The Fireheart to implode a planet's core. However, the Kabal of the Dying Sun actually does have a stockpile of WMDs - some of which are powerful enough to destroy stars - and Vect keeps black holes in his back pocket to control people. The former is kept in check by several things; they don't know how all of them work, many are psychically activated - psychic powers being forbidden in Commorragh on direct orders from Vect, if other Kabals found out about this stockpile they'd gang up on them and it's usually more lucrative for them to conduct raids. For the latter, portable black holes are a special occasion weapon. Like party favors only with event horizons.
The Dark Eldar can also just have other races kill planets for them. Through manipulation from the sides; they could convince (and managed to do so at one point) the Imperium to declare Exterminatus on a planet. And of course as part of an ancient race artificially engineered designed for war from before the rise of man they can just assemble a nuclear weapon if they need a really big, if crude, bomb, but again they like their victims alive.
Necrons
The Necrons have lost many WMDs, but may have several more just waiting to be awakened. (Maybe the Necrons are more trigger-happy with Exterminatus than the Imperium, but they're better at ensuring there're no witnesses). One of their most notorious Exterminatus-tier machines was the World Engine, which was a planet-sized vessel equipped with the largest gauss weapon known to man. It looked like the combination of a Death Star, Unicron and a Forerunner Shield-World all rolled into one. A flying rape-machine of ungodly proportions, it took a coalition of several Space Marine chapters and the entire Imperial fleet of the Vidar Subsector to destroy it. For some reason, it had shields that could withstand the bombardment of an entire navy, yet it was vulnerable to a ship impacting at sufficient velocity.
The Maynarkh dynasty deploys a peculiar device that causes supercharged solar flares that incinerate the daylight-facing sides of ALL planets in a system. Unfortunately for the population on the side of the planet facing away from the sun, incinerating half a planet's surface would also incinerate its atmosphere, stripping the whole planet bare of its life giving biosphere, or whatever gases it had trapped. Even more unfortunately, Maynarkh Necrons are even more interested in making a planetfall and skinning them alive.
Maybe under certain circumstances, if the Necrons wanted to destroy a world, they could just unleash a particularly powerful Transcendent C'tan shard on it without a Tesseract Vault. Though it would most likely escape and be nearly impossible to return to Necron control, it would achieve the same effects. Also, the Tomb World of Thanatos has a giant hologram map of the galaxy known as the Celestial Orrery, and if you were to destroy a star on it, the real life counterpart would go supernova. While this makes the Necrons seem like the most powerful faction in the entire galaxy by far who could instantly kill everyone else in the galaxy without any risk, the lore also states that a star detonated this way could set off a catastrophic and unpredictable chain-reaction of dying stars which in turn could destroy the whole galaxy. It could also destroy the Necrons of Thanatos, which would destroy the Celestial Orrery and guarantee the death of the entire galaxy, something even the Necrons are not willing to risk.
The Necrons can also employ an Abattoir when directly terraforming a planet. They are large, monolith-like devices except that they physically carry what they're transporting, are the size of a small city, and are covered in tentacles that disintegrate organic material while harvesting its anguish. Also, given that their standard guns can disintegrate adamantium, and they don't mind waiting a few million years to achieve their goals, an enterprising (or bored) Lord could just order his legions to start shooting rocks, making for a thorough but hilariously slow exterminatus.
Tyranids
No, Tyranids, you are the exterminatus.
A Tyranid fleet's primary objective is to devour entire planets and systems for biomass. After they're done, the world they invaded is left a lifeless rock, utterly devoid of life.
The Tyranids also travel through sublight via gravity manipulations, and these can rip apart asteroids, voidships, space stations and small moons entirely and cause a massive series of earthquakes on anything bigger before the 'nids make planetfall.
In one special case however. It is proven that even Tyranids can accidentally cause an Exterminatus that doesn't involved being devoured. In the Doom of Hesp where an escalating Chemical/Biological war between the Death Guard and the Tyranids using Venomthropes and Toxicrenes led to the planet being so toxic that the biomass on the planet was inedible and the bioship got destroyed out of fear by fellow hiveships when it tried to devour the biomass to replenish itself.
Orks
In theory, the Orks could develop an Exterminatus-size weapon (as much by accident and luck as by design); they grab an asteroid, put engines and weapons and armor on it, fill it with Orks, and then ram it full speed into a planet. It wouldn't matter if it turned out to function as a giant transport or just a suicide missile; it generates tremendous amounts of lulz and serves its purpose of making a big boom, which is all the Orks are concerned with. This haphazard design and construction process would limit the amount of these contraptions the Orks could build (if any). In general, however, Orks want to avoid wiping out everything on the planet from orbit, as it would leave them with nothing to fight on the ground. Although a Big Mek in need of roks once smashed a moon into a planet and took his pick from the best bits.
The War of the Beast proves that this wasn't the furthest extent either. The Orks under the Beast's control were weaponizing entire moons and used their gravitational fields to rip apart planets.
Tau Empire
While the Tau almost certainly have the technological capability to destroy entire planets (if the fucking Orks can figure it out without having to build back up to Krork, then the Tau probably at least gave it some thought), there are a number of philosophical, political, and strategic reasons that they would avoid doing this in all but the most extreme circumstances. For one, the Tau Empire is in the process of expanding, and it isn't exactly conducive to your expansion efforts to blow up perfectly colonizable worlds; thus the Tau would likely see Imperial Exterminatus orders as an egregious waste of resources. Also, the Tau are arguably the only race in the 40k universe who operate by something parodying a moral compass that is beyond survive at any cost, so the idea of obliterating a planet and its inhabitants is likely appalling to their naive wittle sensibilities.
On the other hand, the Tau have officially declared some races (Orks, Tyranids, Dark Eldar, and Necrons) "lost causes" to be destroyed wherever encountered, so one could plausibly imagine a situation hopeless enough that they would sacrifice a planet to be rid of them. Still, they would probably try to at least leave the world itself salvageable and only exterminate the infesting species. This might not be a concern on a lifeless rock that happened to be a Necron tomb world, however. There are stories of populations being sterilized or generally dispatched, which is about as mean as the Tau get; one such case was the Poctroon, who were the first sapient species they ever encountered. Their planet was ripe for colonization, and when the Tau arrived, the Poctroon all died of a 'mysterious' contagion, though the Tau obviously have admitted no diabolical fuckery.
As their expansion accelerated deeper into Imperial space, the Tau started to deploy more and more experimental technologies to both battlefields and production lines, some of which weren't properly tested. As a result, quite a few moons, planets and even stars have been accidentally destroyed by various mishaps. While such destruction sometimes happened to be advantageous to Tau forces (for example, by shattering Imperial defenses with massive tidal waves and earthquakes after the destruction of a planet's moon), they have shown no attempts to weaponize it.
The Tau also have orbital high-yield nuclear warhead options, but they generally use them to generate EMP pulses to blackout a wide area. They can also use these warheads to scatter toxic radiation over an area instead, though, burning through flesh and killing those below.
The Tau are also one of the few factions in 40k who still possess functioning terraforming technology (the Eldar lost theirs during the Fall, Tyranid "terraforming" is more just them going about eating everything, and Necron terraforming is an Exterminatus on its own), so they can restore exterminated planets to habitability again, provided they haven't been utterly destroyed Deathstar-style. So yes, in the Tau Empire, Exterminatus get purged by YOU!
One of the notable examples would be Commander Or'es'Ka from Dawn of War Soulstorm, where he had this huge ass gun called "Ar'Ka Cannon" installed on the moon of Kaurava II. The cannon can fire anywhere in the Kaurava system (including the moon where the cannon is), obliterating any enemies before the main force moves in. The said BIGGIZT GUNZ is also the most Eco-friendly WMD ever built in the grimdark future, as it is capable of damaging only advanced life forms while incapable of harming plants and buildings.
Forces of Chaos
Being former servants of the Imperium, fleets of Chaos Space Marines often still possess the good old Imperial Exterminatus weapons, like virus bombs for the old legions, cyclonic torpedoes for more recently turned traitors, or Just Shoot The Shit Out Of It for any warband with ships in their fleet big enough to carry the guns. Occasionally they will pillage Imperial Exteminatus weapons, or else invent some of their own with technology, sorcery, daemonic shit or some combination of the three. Some Chaos guys tend to be quite inventive in finding ways to kill planets.
During one of his Black Crusades, Abaddon managed to steal or destroy all of the Blackstone Fortresses that the Imperium had in their possession. Naturally, they work just as well for Chaos as they did for the Eldar (and far better than they ever did for the Imperium). He also commissioned an incredibly huge destroyer of a spaceship, the front half of which is basically a battery of miles-long energy cannons. This "Armageddon Gun" can split a planet in half with one shot.
Then there is World Eaters, who live up to their name when they are united. 50,000 of these motherfuckers slaughtered 70 Sectors in Angron's Dominion of Fire campaign. Then all the planets they conquered were retaken. It seems like they forgot to salt the earth. To be fair though, Imperium needed four Chapters, two Titan Legions and more than thirty Guard Regiments (However, WAAAGH Skargor took on fifty Guard regiments and SIX space murheen chapters. Perhaps World Eaters lack the power of dakka.). Back in Great Crusade, these butchers manually killed everything on the planets they went to conquer. Most of the time, it took them one day. This gave birth to another problem: There were no subjects on these planets to rule over. So the Emprah had to sent fleets to colonize planets left over by World Eaters, which was a pain in the arse for him.
Uniquely amongst 40k factions, the armies of Chaos can make planets Exterminatus-proof by turning them into Daemon Worlds, where the laws of physics are fucked up so hard by the power of the Warp that all weapons just cease to function on and around it, or even achieve the opposite effect by nourishing the daemon patron of the world and making him even stronger (don't even think about virus bombing a Nurgle Daemon World). Though admittedly, from literally any point of view besides that of Chaos, Exterminatus is a preferable option to Daemon World transformation, as it would just kill you, rather than damning you to the eternity of torment.
Of course, there's also the act of summoning Aetaos'Rau'Keres. Keres will turn any planet he's summoned on into a lifeless husk. He doesn't care what side you are on or even if you're the cult that summoned him; he will murder everyone unlucky enough to be on the planet he's currently on. Such is his methods that he's the closest thing the Chaos Daemons have to a true planetary exterminatus.
Exterminatus in other settings
Star Trek
In Star Trek the Federation's General Order 24 calls for the extermination of all life on a planet. It is threatened multiple times, but never seen on screen. One novel and (very weird) comic do show it, however. Just the original Enterprise on its own is supposed to be able to accomplish this task. While this would take time, the fact that it's theoretically possible without preparing the ship at a shipyard indicates Federation ships are quite a bit more powerful than the Federation's peaceful goals suggests.
More famously, Star Trek II: the Wrath of Khan introduces the Genesis Device. When used properly it's a planet-seeding device that can instigate the formation of life on dead worlds. In practice the massive amount of energy released means it also functions as a planet-glassing bomb, killing everything that already lived there.
Star Wars
While it is known to the Clone Wars era (possibly earlier) Republic and the Galactic Empire as Base Delta Zero, the Galaxy Far Far Away has known of orbital attack causing the destruction of all life on a planet for much longer. Knights of the Old Republic implies Tatooine is the giant ball of sand that it is because it pissed off the Rakatan Infinite Empire precursors long before the formation of the Republic, and shows the city-planet of Taris destroyed with such an attack on orders of Darth Malak.
The Empire regularly made use of Exterminatus as part of its campaign to rule through terror. The peaceful planet of Caamas was destroyed early into the Empire merely for the suspicion they would one day oppose the Empire and (due to the widespread respect they had for maintaining a stance of actual pacifism) encourage others to do so. BDZ is implied to have been relatively common under the Empire. This was taken to the extreme with the Death Star, a weapon designed to make exterminatus so easy nobody would oppose it out of fear. This backfired horribly when the station was destroyed after its second use at Alderaan, causing the galaxy to lash out in revolt (the first use was the slave labor planet it was being built at, it took three mid-power shots to kill everything, crack the mantle and blow it up in that order. Oh also it fired one other time minimal power at a Rebel task force who tried to blow it up under construction at the aforementioned slave planet, one-shotting the sole capital ship in the attack).
While the New Republic initially rejected Exterminatus, the war with extra-galactic invaders the Yuuzhan Vong (who had their own Tyranid-esque Exterminatus methods) eventually pushed them into using it. BDZ was frequently used in later years as well, most notably at Ossus in 137 ABY.
Halo
In the Halo universe wiping out all life on a planet happens surprisingly often and has many interesting ways of doing so. It is the only way to be 100% certain you have dealt with a Flood outbreak
The UNSC has the options of nuking the planet in the box standard Nuclear Holocaust situation that's very much what you'd expect. However, their most powerful weapon is the NOVA Bomb. The NOVA Bomb is created by strapping together nine Standard Nuclear Fusion warheads and encasing them in a casing of Lithium triteride which amplifies the warheads destructive output that is unfortunately never mentioned but has the power to crack open a planet, earning the nickname "Planet Crackers". The first real use of this weapon was when it detonated between a planet and its moon destroying the moon and scorching half the surface of the planet, as well as destroying three-fifths of a 300 strong ship fleet.
The Covenant is well known for their orbital bombardment technique known as Glassing, where through high-energy plasma bombardment, a planet's surface is reduced to a glass-like material resembling Obsidian. This was the fate of many human planets during the Human-Covenant war including but not limited to Harvest, Madrigal, Eridanus II, Miridien, Paris IV, and Reach.
The Forerunners, of course, have arguably the most powerful method of wiping out all life on a planet in HALO Cannon. The titular Halo rings, which can cleanse either a single planet of life on lower more directed settings, to Massive regions of space at 25,000 light-years. When all seven are fired in concert, they can wipe out all life in the galaxy, which they have done before. The Forerunners have also created a device known as the composer, which takes an organic being, destroys their body, and uploads their consciousness into a digital format. The process is extremely painful and those composed aren't always sane by the end of it. Of course, there could be who knows what kind of life killing weapons in the Forerunner arsenal that haven't been revealed yet
IRL Exterminatus
Thermo-Nuclear Holocaust
Apparently, even when we aren't in the 41st millennium we still mastered the art of royally buttfucking a planet. In this case, it's ours, and a full-scale thermonuclear war between the United States and the Soviet Union (who each have thousands and thousands of nukes) would be enough to kill off humanity multiple times over. This is how Mutually Assured Destruction works, threatening each other and our own planet with Exterminatus with zero chance of survival, just so we won't begin another World War. Because we're bastards like that. The Cobalt Bombs described by Dr. Strangelove above are actually possible, though currently theoretical. Nuclear weapons designed to be deployed as bombs or missiles aren't strong enough to destroy the world with only 50 warheads, but if you don't mind moving the weapon once it's built, the only limit on how big your nuke can get is how much material you're willing to use on it. In theory, the doomsday device of Dr. Strangelove could be achieved with a single massive bomb.
This is further worth mentioning because automated retaliation systems that could activate nuclear weapons in response to a detected threat actually existed. The Soviet Union had the "Dead Hand" system, based off of seismic, air pressure, and EM sensors. The system was normally kept inactive and was only supposed to be turned on during a crisis to guarantee that the Soviets would still be able to use their weapons even if their leadership was taken out by a first strike.
Some believe that elements of the Dead Hand system may have been lost or buried, and are active to this day. A ticking automated Exterminatus waiting for a signal from aging cold-war era sensors.
An old quote from the film WarGames summarizes the game of Global Thermonuclear War/Exterminatus: The only winning move is not to play..."
Side note: simply saturating the surface of a planet with nukes is actually exponentially more energy-efficient than any method of destroying the actual planet itself, so if all you want gone is the people or creatures inhabiting it, then realistically this is what you'd go with.
An Asteroid
Really, all it takes to kill everything on a planet is a big enough rock traveling fast enough. Normally it's the cloud of dust that is kicked up into the atmosphere and blocks out the sun that does most of the work. Dinosaurs learned this the hard way. Of course, this doesn't really work too well on a forge or hive world which is already like that. For raw destructive force, however, the damage is a function of the speed and size of the asteroid. The former has some practical limits (though a civilization looking to weaponize this sort of exterminatus could possibly bring the rock up to relativistic speeds), but the latter can be nearly unlimited. A collision with a near planet-sized object would be more devastating than most fictional exterminatus weapons, obliterating the target world entirely. There could be any number of so-called 'rogue' planets floating in the empty spaces between stars, ready to slide into the solar system and crash into Earth, assuming humanity fails its collective Save or Die roll for the week. They'd have to fail incredibly hard because the overwhelming chance is that the rogue body will end up into the Sun (or Jupiter as a distant second choice), but yeah. Shit happens, yo!
Also, to clarify something: once the asteroid rises above a certain size threshold --many times larger than the one that killed the dinosaurs, in fact-- then a collision with Earth would actually result in completely sterilizing the planet, as in everything down to the last microbe would die. So... sweet dreams.
Super Volcano
Works on the same principal as the asteroid, that if you get enough shit into the atmosphere you've royally fucked all life bigger than a mouse. This may not be very likely though on Earth as one of the biggest volcanoes (see yellowstone park) wouldn't wipe out humanity, probably. Maybe.
Unless, of course, seismic activity from that eruption managed to trigger the OTHER NINETEEN super volcanoes.
Artificial Plague
The Black Plague killed up to 60% of Europe's population and the Spanish Flu over doubled the death toll of what was then most devastating war in history. With SCIENCE! you could do even better! You could even render your own people immune to the effects before hand and only kill the enemy. Sane people dismiss this as possible but a fantastically stupid idea because viruses are impossible to contain and like to mutate, rendering any vaccine you used worthless. Still, people interested in causing the end of the world have minimal overlap with sane people, so terrorists causing one is a popular plot. Another possibility is a virus in a research lab breaking containment rather than being released intentionally.
However before you start panicky a plague basically has no chance at killing humanity off. It can do a lot of damage yes, but once it kills enough people it will, like a fire, run out of fuel and burn it self out. This is why the common cold is so, well, common, It doesn't kill you so you can keep spreading it. Any sickness lethal enough to even have a shot at killing humanity off, will kill people so fast that it can't spread, even assuming humans can't treat it.
Other Methods
Some dude actually collected a list of ways that the Earth itself could be physically destroyed here: https://qntm.org/destroy But note that unlike with Exterminatus, the working goal there was to physically destroy the planet itself, not just the population.
It's a really clever and thorough list. One listed method, for example, is basically to just yeet rocks into space until you run out of rocks.
The Fall of Typhon
Good to know there's a ceremony for blowing up a planet.
"We have arrived, and it is now that we perform our charge.
In fealty to the God-Emperor (our undying Lord) and by the grace of the Golden Throne, I declare Exterminatus upon the Imperial world of Typhon Primaris.
I hereby sign the death warrant of an entire world and consign a million souls to oblivion.
May Imperial Justice account in all balance.
The Emperor Protects."
The Words of Gabriel Angelos
"It is human nature to seek culpability in a time of tragedy. It is a sign of strength to cry out against fate, rather than to bow one's head and succumb. Inevitably many shall fault the hands upon the sword which felled Typhon, the Ordo Malleus. But the Inquisition merely performs the duty of its office. To further fear them is redundant; to hate them, heretical. Those more sensible will place responsibility with those who forced the hands of the Inquisition. With some fortune, they may foster this hatred into purpose, and further rule their own fate by coming to the Emperor's service.
Yet ultimately, it was I who set these events into motion, with a single blow from my hammer, God Splitter."
- – Gabriel Angelos of the Blood Ravens
Fuck, that's deep. The use of a properly modified version of this quote from Dawn of War Retribution has proved highly effective in sageing furfag troll threads and thus has been sanctioned by the holy /tg/ Inquisition for public use (keep it on /tg/).
Battlefleet Gothic: Armada
"Admiral Spire, it is said that heresy is like a tree.
Its roots lie in darkness while its leaves wave in the sun.
You can prune away its branches, even cut the tree to the ground.
But it will grow again, ever stronger.
Such is the nature of heresy and why it is so difficult to destroy.
Some may question my right to destroy a world of ten billion souls.
But those who truly understand, realize I have no right to let them live.
No sacrifice is too great. No treachery too small."
- – Inquisitor Horst
Exterminatus on the Tabletop
Though not the most effective of lists, it is particularly hilarious and surprisingly fluffy to declare Exterminatus on large table games of 40k. In general Exterminatus is used when one player with a large force of 3500+ points of space marines and is in danger of losing the field of battle. In which case the player grabs the closest heavy object and begins to smash the opponents models screaming “EXTERMINATUS IN THE NAME OF THE EMPEROR EXTERMINATUS” until the opponent’s army is destroyed and (or) he is forcibly removed from the table . Another relatively simple way is to simply take a Grey Knights army, field a single Bro-Capt. or Grand Master with an orbital Strike Relay, Karamazov (who also has one) and two troop choices (if you're playing a regular game -- if you're playing Apocalypse, you can skip the troops) Then cram in as many Techmarines as you can, give them all Orbital Strike Relays and watch the bombs drop. For the average 3000 point game, you can get Krazypants off and 20 bare-bones techies with the relays. that's 21 Strength 10 AP1 pie-plates smashing down on your opponents Baneblades, Warhounds and other special hard-as-balls to kill shit your opponents have! Also great for swarm-busting (the relays can fire D3 pieplates each per guy but at Strength 6). Picture Krazypantsoff standing on a hilltop, pointing at buildings and going "Bang.", then watching them all blow up. Of course, if the Inquisitor dies, you're fucked. So maybe just camp him in cover. But that's only if you're lame.
For those of you with enough money to field the Horus Heresy army list from Forge World, Horus can call down an orbital strike with infinite range and S10 AP1 from anywhere on the map. Now you can reenact the Istvaan III atrocities yourself!
For an Apocalypse game you can also field an exterminatus guard force. All you need is: n * 6 guardsmen (one with a vox). The list is fairly simple - Just field as many Company Command Squads with nothing but Master of Ordinance and fire away (for a 3k game its almost 38 s9 ap3 blasts a turn) Don't forget to field some epic(troll) music to laugh at your opponents face, and after the battle proceed with knocking the table down to finish with a speech gritty nuff to make Sturnn himself proud.
Another option is to us the Exterminatus rules for your Apocalypse game (In the unnatural disasters table (by rolling a 6))
As of 7th edition, it's now possible to forego the FoC chart and take whatever models you want. This means you can take 15 Chapter Masters in a 2k list.
In 40k terms, you can get some SERIOUS Exterminatus going with the 'Crons and their Doomsday Arks. In one Primary Detachment, for example, you can take a fully viable 1500-point Necron army as so: Overlord with Warscythe, 5 Immortals, 10 Warriors and 3 Doomsday Arks. If the Doomsday Arks don't move, they can provide one 72" Strength 10 AP 1 Primary Weapon Large Blast each, allowing for some serious butthurt from your opponents (and this may make you That Guy if done well because this is a level of cheese on the table that France would be proud of). If you're trying to break into a bunker-sized fortification, use these three things on the doors. Then you can re-enact the dying moments of The Conquest of Uttu Prime sans the Megalith!
TL;DR
You fuckers just backed Chaos and now you have a daemon infestation? Your planet 'gon git raaaaaaaaaaaaaaped.
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Suffer not the Tau Player to live
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The E-quisition vigilantly purges the Emperor's internets of chaos taint.
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If the inhabitants of a planet remotely resembles this creature, it's guaranteed to be exterminatused upon discovery. If pictures like this are found on a thread in /tg/, it's guaranteed to be saged and trolled upon discovery
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If said inhabitants started space-faring like a certain Chakat, then you could call your local Inquisition or any Xeno manly enough to get shit done, just like the Covenant shown in the image.
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See? The Internet can even make the end of the world look sexy!
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Don't suggest bolides as a method of exterminatus.
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War is Hell, and Hell is beautiful.
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Although this is not a planet, it's just too B-E-A-weeaboo-TIFUL.
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Not actually how the Inquisition works...mostly.
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The original Magic: the Gathering Exterminatus
Vidya: