Apocalypse How/Class X

That's it! EVERYONE DIES!

—Vegeta, Dragon Ball Abridged

Planetary-scale Physical Annihilation. There used to be a planet here. There isn't anymore; it's gone.

Examples of Apocalypse How/Class X include:

Multiple Media

Various incarnations of The Hitchhiker's Guide to the Galaxy do this to Earth from the Vogons, ostensibly because of a hyperspacial express route, though it's really because Earth has the Ultimate Question being calculated, and that would be terrible for all the psychoanalysts.
- In the novel Mostly Harmless, the Vogons finally succeed in destroying not just Earth, but every version of Earth across all the infinite parallel realities. Douglas Adams was in a very bad mood when he wrote that one.
Superman's backstory is his planet Krypton exploding spectacularly, becoming the Last (or close enough) Son of Kypton. Occasionally, the planet's sun is what goes, which would push those versions into Class X-2, but the majority just had the planet explode. As of Infinite Crisis, Krypton's sun Rao still exists in the comics, but the entire planet was blown to smithereens, presumably naturally.
Transformers has had Unicron eat a planet or two.
- He's also eaten a universe or two. It's All There in the Manual.

Anime and Manga

Earth and (eventually) Namek in Dragonball Z. Earth was reset with the Dragon Balls, and part of Namek's population was resurrected and given a new home. Arlia was less lucky.
- Earth actually gets it twice, first from Super Buu, then again in GT thanks to the effects of the Black Star Dragon Balls. The Namekian Dragon Balls restored it both times.
- Planet Vegeta gets it before either of them, courtesy of Frieza himself in his definitive Complete Monster moment.
- And Frieza has a move called "Destroy the Planet."
- Broly does this countless times. Ironically, he survived Frieza blowing up his planet.
- Majin Buu does this so many times.
Futari wa Pretty Cure Splash Star, though it was reversed.
In Robotech: The Shadow Chronicles, humans nearly blasted Earth, averted only by Invid Regess destroying the missiles. Though they were aiming for Apocalypse Class 2-3 just to get rid of the Invid, but underestimated the power of the Neutron-S missiles.
In Infinite Ryvius, the Blue Impulse uses gravity manipulation to shatter Saturn's moon Hyperion, killing everyone on it. All six Vaia Ships working in unison would have enough power to alter the ecliptic plane of the entire solar system, which could potentially cause a Class X-2, although they were actually designed to use this ability to prevent a Class X-2.
Eureka Seven has a phenomenon known as the Limit of Questions. If there are too many intelligent life forms, a black hole or something will destroy the planet. Specifically, the Limit of Questions means that too much sapient life occupying a certain amount of space will create a rift that destroys damn near everything close by, like, say, the planet said life is on.
Attempted in Diebuster, using Earth as a projectile at significant fractions of the speed of light. The attempt is stopped by Nono/Buster Machine 7 as the Diebuster
RahXephon. Temporarily.
Sailor Galaxia is seen casually destroying planets in Sailor Moon StarS way early on in the series.
The Dimension Eater in Macross Frontier. The one we saw took a very large (like a quarter of the planet) out of Gallia IV. A couple more would probably finish the job completely.

Comic Books

Too many Marvel and DC Comics plotlines to count.
Narrowly averted in a Donald Duck story about the Universal Solvent, which dissolves everything except diamonds and does not dissipate. When it's poured to the ground, it's going to keep dissolving the planet from within unless the heroes recapture it with a diamond-linen container.

Films -- Live-Action

Star Wars: The Death Star has the power to destroy entire planets, and in A New Hope, it is used on Alderaan, Leia's home planet.
In Star Trek III the Search For Spock, the unstable Genesis planet falls apart. Special mention is however reserved for the Genesis Device in Star Trek II the Wrath of Khan, given that it formed a planet from a nebula, which would make it a class negative X. Or the class X equivalent if you happen to be a Nebula being.
In Star Trek VI the Undiscovered Country, the Klingon moon Praxis blows up.
And now appearing again in 2009's Star Trek film, this time Vulcan, by being sucked into a black hole.
The British 1961 film The Day The Earth Caught Fire has the Earth hurtling towards the Sun when it's thrown out of orbit by a series of large nuclear tests at both poles.
Arguably, Metaluna's fate in This Island Earth; it's hard to say it's really there if it's now a burning radioactive 'sun'.
The much larger planet Melancholia smashes into Earth and apparently absorbs all of its mass.

Literature

The Cthulhu Mythos has Dholes, enormous worms that destroy planets by burrowing through them until they collapse. In the Tabletop RPG based on the books, an investigator unfortunate enough to be attacked directly by a Dhole will get a single roll -- to determine if there's enough bits left for a funeral.
The Lensman books end up using planets as billiard balls against other planets as well as planet-sized masses of antimatter.
Greg Bear's The Forge of God reaches its climax as a neutronium-antineutronium bomb, deposited in the core of the Earth by malevolent von Neumann machines, detonates, disrupting the entire planet over the course of a few hours with enough force as to put significant portions of it in orbit around the dead, molten remains. A small selection of humanity is rescued before the disaster by another group of friendly von Neumanns who had failed to stop the attack.
Arthur C. Clarke's Firstborn (in Time Odyssey saga), has Class X-lite on, with Q-bomb hitting planet which will create strange quantum effect on space-time that will basically rip the upper layer of the planet off. This happen to Mars, when the Martians were eliminated by Firstborns. When Mars get hit second time after the Martian from the pocket dimension was contacted by humans managed to make them divert the Q-bomb, Mars was destroyed completely. The planet redirection into the sun by the Firstborn (listed in Sunstorm example above in Class-5) is also Class X.
In Clarke's Odyssey series, "something wonderful" happens to the planet Jupiter. Though it isn't mentioned in the films, Dave Bowman detected life on both Jupiter and Europa; the beings who control the Monoliths deliberately sacrifice Jupiter and its inhabitants to support the Europan life, which they considered to have more potential as intelligent beings.
In Eric Nylund's Signal to Noise, not-so-nice aliens provide humans with a teleportation system which is powered by the rotational inertia of the planet. The aliens then abuse this system to blow up the Earth, as well as an unrelated detonation of Mars by some post-humans trying to get away from Earth as fast as possible.
In The Saga of Seven Suns, the Faeros do this to the moon. Yes, chunks do threaten earth, but a coordinated response is possible. Additionally, humans destroy a brown dwarf more than 13 times the mass of Jupiter inhabited by Hydrogues, who then obliterate four moons around that once-planet-now-star (Class-X destruction), declare war on humanity and the Ildirans, attempt to destroy two human planets, manage to destroy one Ildiran (two would-be class 3s except other parts of humanity help them, and one class 4-5 forcing an evacuation of the few who survive up to that point), had attempted to obliterate three ancient races previously, make a second (or possibly even further in) attempt on two of them, destroy a number of skymines, which given the size of the machines and their crew could count as class 0s, to name a few things that happen.
In Orson Scott Card's Ender's Game the title character unknowingly destroys the Bugger home world using the Molecular Disruption/Detachment Device (alt. MD. Device, "Dr. Device", "Little Doctor").
Michael Reaves' The Shattered World is a fantasy novel about a world that already suffered a Class X, but without losing its civilizations or ecosystems. The planet broke apart, but some fast action by the world's wizards kept its many fragments orbiting one another in an envelope of breathable air. The sequal, The Burning Realm, is about attempts to avert the belated Class 3 and Class 5 consequences of this Apocalypse before these orbit-sustaining spells wear out.
This is Ruin's immediate goal in Mistborn, and he comes uncomfortably close to succeeding. Because he's a literal god of entropy and decay, it's implied that having accomplished this he'd then keep marching up the scale if left unchecked.
In When Worlds Collide by Philip Wylie and Edwin Balmer, the Earth is about to be hit and destroyed by the larger of a twin planet. Fortunately the smaller planet is drop-in replacement for Earth. The story (which arguably is the ur-creation that gave rise to stories like Flash Gordon and the many similar tales that followed), revolves around the attempt of a group of scientists to build a pair of rocketships to escape a world facing destruction.

Live-Action TV

Doctor Who has a number.
- The home planet of the classic series Cybermen, Mondas.
- The eponymous "Impossible Planet" (causing the deaths of hundreds of Ood).
- Martha planned to use the Oster Hagen Key to blow the Earth up with strategically-placed nukes so that the Daleks couldn't go through with the death of reality itself.
- The constant jumping between realities in the series 2 finale also risks the two Earths to crash into each other.
- In "Boom Town", Blon Fel-fotch Slitheen plans to use the force of the rift to tear the Earth apart, riding her tribophysical waveform extrapolator to safety.
- Another example from the classic series, there's a 'planet' (more a space craft, but the inhabitants are unaware of this) which eats other planets. When it is finished eating, it makes a hyperspace jump to another planet and starts eating again. It's better than it sounds.
Babylon 5's Class X member is the Vorlon Planet Killer, which, in accordance with a philosophy of swift and terrible justice, simply obliterates the entire planet being targeted, unlike its Shadow counterpart which leaves the planet intact but lifeless and is therefore only a Class 6. There is some contention over the Vorlon Planet Killer's effectiveness as a Class X weapon, as some of the planets attacked had survivors and atmosphere (as opposed to the planet Arcata 7, which post VPK contact is described as "not there anymore"), though one can get around that by assuming the Shadow bases for said planets were on their moons. The Vorlons would have blown those up, raining down devastation on the planet but not instantly killing absolutely everyone.
Odyssey 5
Star Trek gave us the Doomsday Machine, which would break up a planet into smaller chunks and then eat them, and the Xindi Weapon from Star Trek: Enterprise.
In Star Trek: Voyager, the episode "Scorpion" features speciae 8472, which blows a Borg planet to pieces with just 9 small ships.
The titular ship of Lexx had only one weapon: a Wave Motion Gun that destroyed planets in glorious fashion. In point of fact, any place the Lexx visited was either destroyed by it or something worse (see Mantrid in Class X-2) including the afterlife and in the finale, Earth itself.
In Andromeda, a planet can be destroyed by either firing a slipstream core at the planet, or by chucking a Nova Bomb at it (though planets hit by Nova Bombs more fall apart than explode).
While more notable for their Class X-2s, the Stargate Verse has seen its share of destroyed planets. In fact, when Stargate Atlantis had its Gondor Calls for Aid in the fourth season, the Traveler leader is dumbstruck when the Atlantis team casually suggest blowing up the Asuran homeworld.
In Lost, not entering the numbers into the hatch every 108 minutes and removing the plug from the Island's source is thought to destroy the Earth.
Season two of the Power Rangers sees Lord Zedd building his own zord Serpentera, a giant craft specifically stated to be capable of wiping out entire planets. In the episode of its introduction, Zedd takes Serpentera, follows the Rangers to a deserted planet, and destroys it. Luckily, with the sheer amount of power it uses, Serpentera runs out of power really quickly, making it a very ineffective weapon in the long-term.

Tabletop Games

Warhammer 40,000's aptly-named Planet Killer and Blackstone Fortresses.
- The Blackstone Fortresses scale up the more combing their beams. One is a Class X, disintegrating the planet (compared to the Planet Killer, which leaves enormous quantities of debris), but three or more could qualify as a X-2, since it is capable of destroying a star, wiping out the entire system with it. Technically, Blackstone Fortresses don't destroy planets outright. They open a split-second rift into the in-universe equivalent of hell that overlaps their target.
Old Alphatia met this end, forcing the Alphatians' ancestors to emigrate to the Mystara setting. It was an academic dispute between rival factions of archmages that kicked off this particular Earthshattering Kaboom.

Video Games

Planet annihilation is possible in Star Ruler. Simply park a big enough ship nearby, and order it to attack, and the ship will bomb the planet to the point where the planet will break up.
In Kingdom Hearts, The Heartless will do this to a world if they rampage across it unchecked.
Super Metroid: Mother Brain's self-destruct sequence destroys Planet Zebes. And of course Phaaze in Metroid Prime 3 goes boom, thanks to Samus. Hey, it was aiming for multiple Class 2 catastrophes anyway due to The Corruption, so she beat it to the punch!
Commander Keen episode two (entitled The Earth Explodes!) has the villains of the first episode position a planetary destruction ship in orbit over Earth. If you screw up, you get to activate it. Guess what happens.
High level overlords in Disgaea are usually capable of destroying entire worlds in giant Earth Shattering Kabooms. It's best not to get them mad enough to do this.
In Master of Orion 2, the player could research, build, and mount a stellar converter on a large enough ship and wipe out entire planets with a nifty movie showing the massive explosion. This typically doesn't eliminate the empire that occupies it, but it does remove the planet from play permenantly. Though if the system still has a colony, the resulting asteroid belt can be used to build new planet. Even better, all planets constructed this way have the same size, which may be larger than the original one. Also, it is possible to avoid the 'no terraforming on toxic planets' rule this way, making this a very rare example of using a Class X as a reasonable (if abusive) step in terraforming.
- Also, the newly reconstructed planet can easily be bigger than the original one since all planets built from asteroids are the same size, making this actually a class 0, unless you happen to lose your last planet in this fashion.
Master of Orion 3 also has the Stellar Converter, but it was sadly de-powered to Class 5 level, leaving the planet an un-colonizable, lifeless cinder, but otherwise intact.
Final Fantasy VII. Sephiroth's plan for his planet was to crash Meteor into it, thus forcing the Lifestream to stream out of the impact point in the planet's death throes. Where Sephiroth will absorb the entire Lifestream, become a god, and use the planet's lifeless husk as a giant spaceship to explore other worlds.
Cyborg Smoke's gloriously OTT (and immediately retconned) fatality from Mortal Kombat 3.
In the cult DOS game Captain Blood, YOU could nuke a planet into tiny pieces of VGA debris, though sometimes this would make the game Unwinnable.
Though possibly not a perfect example, World of Warcraft has Outland, which is the result of portals ripping the planet to bits, leaving only a large continent sized chunk and several hundred minor pieces of debris. Yet the place stillsupports a few thousand inhabitants despite by logic lacking an atmosphere and dangerously close to a planet that would drag the left over planet chunks into firey death...
Galaxian3's bad ending (whether you fail to destroy all of the power relays or die in the middle of your journey) has Cannon Seed using its planet-fueled Wave Motion Gun to blow up the Earth into "fragments smaller than California."
Take a guess at what the Planet Buster in Spore does?There's a reason if its use is forbidden by the Galactic federation...
This happens to Palma in Phantasy Star 2, while Motavia suffers a type 2 shortly after the end of the game.
The entire story of Shin Megami Tensei Nocturne is based around the premise of rebuilding the Earth after it is transformed into the embryonic, demon-infested Vortex World as the result of an event called the Conception. The Earth as you once knew it, and any living thing outside of a small hospital in the middle of Tokyo, is utterly destroyed. This happens in the first ten minutes of the game, by the way.
In Ratchet and Clank, Chairman Drek tries to do this to Veldin twice- first with the Planet Buster Maximus, and again with the Deplanetizer- the latter being used to blow up his new planet (and him).
The Planet Eaters in Lunar Knights are able to trigger one of these, especially when its governing Immortal decides keeping it around isn't worth it any longer. This can be because the Immortal is bored, because they just hate the people on the world, or because their plot to impose eternity upon the cosmos goes off the rails. Alice is a lagomid survivor or a descendant thereof (Sheridan explicitly says a Planet Eater visited her homeworld), while the titular Lunar Knights cause Polidori to trigger its self-destruct before it can do the same to theirs (and shoot him down before he can attempt a Class 4/5 himself). When he's revived in the optional epilogue, Dumas states that this act of rebellion has not gone unnoticed by the rest of the Immortals, but we haven't seen this play out so far...
While not directly in the game it is revealed from an Aperture Investment Oppertunity video that the Portal Gun is capable of creating a black hole that could destroy the Earth.
An unusual version happens in Asura's Wrath, where it is a planetary sied Beign that gets destroyed this way, Wyzen get's punched to death and explodes after turning into the planet sized Gongen Wyzen.

Web Comics

Turned on its head with the Planet Eater of Casey and Andy. He's designed to eat the planet Earth but, being about the size of a hamster, can only do it one bite at a time.
Homestuck: Jack Noir does this to Prospit in the kids' session, and later on (sorta) he destroys Prospit and Derse in the Trolls' session. He does this with a magic ring.
This is what will happen to the world of Order of the Stick if TheSnarl gets released - and it IS what happened to the previous world before the existing one was made. (Probably.)

Web Originals

Tech Infantry blows up Earth's moon in this way, which does not do good things for Earth.
in the Whateley Universe there was a sundering, which effectively destroyed the elves, their home, and their civilization. their souls survived to arrive on earth in a few instances, and their servants, the weres, were sent to earth before the end, so this may be a subversion of it.
In The Demented Cartoon Movie, the world is continually either blowing up or crashing into the sun.
In Fine Structure, this happens to the Earth as a side note during the Final Battle. Barely any human casualties though, thanks to the intervention of some really advanced alternate universe humans with copious experience in large-scale split-second rescues. They even build a new planet afterward.
The Chaos Timeline faces this threat at its end. (Cause: Nanomachines.)
One article at Things of Interest discusses why and how to destroy the Earth. Destroying humanity is not good enough, and ideally the destruction would be permanent.

Western Animation

Titan A.E.: Though a handful of humans evacuate.
Marvin the Martian tried to accomplish this, but was repeatedly thwarted by a dangerous Earth creature. In "Duck Dodgers in the 24½th Century", Planet X, the planet of the shaving cream atom, is blown to pieces by Duck Dodgers and the Martian one-upping one another.
What the alien installation did to Mars in Exosquad and what Phaeton plans to do to Earth in revenge.
In Shadow Raiders, the Beast Planet eats other planets, as demonstrated with planets Water, Tek, Fire, the Prison Planet, and Reptizar. Another planet was blown up by setting a crap load of explosives next to its (nonfunctional) world engines. The Beast ate the remaining chunks. Given that the Beast Planet is not only capable of self-propelled intergalatic flight, but actively seeking out new planets to consume, it could be taken as an example of X-2.
In an episode of Gargoyles, Fox and her mother are working with some nanomachines which escape and begin a Grey Goo scenario (below), where everything it doesn't turn into more nanomachines is restructured into unchanging order. Since it's meant to grow and evolve it will eventually engulf the entire earth (the heroes only have as much time to stop it as they do because it's reached it's current bounds and will take some time to gain the capacity to come further). They eventually stop it by sending Goliath and Dingo into the Dreaming, where they discover they can communicate with it. Dingo manages to convince it to protect the earth rather than destroying it by suggesting that instead of mathematical order it try law and order, which is similar, but requires less extinction.
Pinky and The Brain. Episode: 'It's Only a Paper World'. Pinky and the Brain move everyone to a duplicate Earth next door so that Brain can conquer the empty Earth. Then it gets smooshed by meterorite.
In Futurama, one of Fry's lifelong desires was to blow up a planet. They do it one afternoon.
- After being mined hollow due to containing starship fuel, the planet Vergon VI implodes on itself. Nibbler manages to eat most of the rescued wildlife too.
Green Lantern John Stewart is accused of causing a chain reaction leading to this in Justice League. It turns out that it was actually a hologram.
In Dexter's Laboratory, Dexter and Mandark bicker over each other while forgetting about the meteor shower that destroys the Earth. This is ignored the very next episode.
The planet of Thundera suffered this fate in the beginning of Thundercats, forcing what was left of the Thunderans to get off the planet and find a new world to call home.

Real Life

The (in)famous and controversial Grey Goo scenario. Poorly programmed nanomachines go all von Neumann on all matter with which they come in contact, turning it into more poorly programmed nanomachines that do the same thing until every bit of matter within reach has been turned into poorly programmed nanomachines.
Some scenarios for the origin of the Earth-Moon planetary pairing suggest that a somewhat larger "Earth 1" was hit by another, Mars-sized planetary body. This impact tore off a huge section of its surface, leaving behind our present "Earth 2". The dislodged debris settled into orbit as a ring, then gradually congealed into the Moon over millions of years.
Continuing the fate of the Solar System, some five billion years from now the Sun will turn into a red giant, engulfing and destroying Mercury, Venus, and probably Earth as well.
- Models also show that not long before the sun does that, Mars is all but confirmed to pass too close for comfort from Earth ^[1], with a small chance of frontal impact.
Gamma ray bursts.
- Only if it happens really close to the planet, like in the same solar system or a few lightyears away. A more likely scenario for a gamma ray burst-based apocalypse would be "merely" having the side of the planet facing the blast completely sterilised while the other side burns as the atmosphere catches fire.
  - The distance for mass extinction (this one would dwarf anything Earth has seen before) would be about 1,000 parsecs. The distance for "Farewell, atmosphere." would be 500 parsecs. A parsec being about 3.26 light-years. GR Bs emit obscene amounts of energy, ejecting matter from the star at 99.995% the speed of light. Still, there'd still be a planet. Granted, there would never be life on it again...
    - Before you panic, the galaxy is around thirty thousand parsecs across.

↑ somewhere in the lines of 700km

[1] somewhere in the lines of 700km

[1]