"Planet" is a term whose etymology originates from the Greek word for "wanderer" — and while most planets in fiction can be found 'wandering' in orbits around a (or multiple) star(s), this one truly is wandering — completely alone.
It wanders across the universe silently, without the light of a sun to provide heat. For obvious reasons these planets tend to be Single Biome Planets of the frozen and dark varieties, although if they're covered in a thick ice sheet, life might flourish in liquid oceans heated by the planet's core — a moon orbiting one of these planets might also get enough energy from tidal forces to have a liquid interior.
If the planet was not always a lone wanderer, ruins of civilisation might be found on it — dry cold is one of the best means of preservation we know of today, and ruins will thus be preserved for very long time. Typically, the fact that the planet was flung away doesn't bode well for the civilisation once found on it, though examples do exist where the original inhabitants managed to perform a Homeworld Evacuation.
If one of these is deliberately built by a civilization, usually a Higher-Tech Species, possibly because their original sun was killed, or died and they wanted to Outrun the Fireball, it's a Planet Spaceship. See also Big Dumb Object, which may be mistaken for one of these at first glance. If the planet moved itself out of orbit, you might be dealing with a Genius Loci.
- Hellstar Remina: The titular planet emerged from a wormhole sixteen light-years away from Earth in a corner of the Hydra constellation and was observed to have an erratic, zigzagging orbit where stars around it vanish, until a scientist noticed that it turned around to the solar system's direction and made its way in a direct path. That's because it's alive and both an Eldritch Location and Abomination.
- The Green Slime: The plot occurs because a rogue planet called Flora wanders into the Solar System in a direct collision course with Earth, forcing the protagonists to set foot on it and blow it to smithereens and as a result getting their suits infected with bits of the titular Blob Monster.
- Melancholia: The titular blue gas giant is a rogue planet that collides with Earth at the end (and the beginning) of the film.
- The Phantom Planet has a goofy B-Movie take on the subject. Rheton, the wandering planet of the title, has a population of tiny Human Aliens living on it—and when astronaut Frank Chapman lands, breathing the atmosphere makes him shrink to the size of the locals as well.
- The Wandering Earth: The backstory involves humanity turning Earth into one of these, by way of adapting it into a Planet Spaceship to escape the looming death of our sun. The plan is to one day reach Alpha Centauri, but the voyage will take thousands of years.
- In Foundation and Empire, Bel Riose sets his headquarters on a rogue planet during his war with the Foundation.
- Red Dwarf: In Better Than Life, the Earth itself ends up as one of these: after being reduced to a dumping ground for the rest of the solar system, a catastrophic chain reaction of methane storms, a crashing garbage scow and several abandoned nuclear power plants farts the entire planet out of orbit and into deep space.
- Star Trek: The Original Series: In Planet of Judgment, the U.S.S. Enterprise encounters a rogue planet which has a small black hole in orbit around it.
- When Worlds Collide involves the discovery of two planets, Bronson Alpha and Beta, wandering into the Solar System, and the increasing worldwide panic when it's figured out that the bigger of the two will impact Earth directly.
- National Geographic: The special "Evacuate Earth" has a rogue neutron star on a collision course with our solar system, which will destroy Earth and presumably all of the other planets as well. Humanity has 75 years to build The Ark and get away before it arrives.
- Space: 1999 deals with the perils the people of Moonbase Alpha endure when a massive nuclear explosion kicks the Moon out of orbit and wanders thru the galaxy on its own volition, courtesy of running into wormholes and other phenomena. Played With in that while it's not outright a planet, it still hits the trope's notes of civilisation facing adversity (Not to mention that there are planets smaller than the Moon).
- Star Trek:
- Star Trek: Deep Space Nine: The Changeling homeworld is an M-class rogue planet inside the Omarion nebula. How the planet supports life is never explained.
- Star Trek: Enterprise: "Rogue Planet" features such a world, called Dakala. It's a Minshara-class planet with a breathable atmosphere and a temperate climate, provided by hot gasses venting from its interior. It somehow has plant life despite not having anything to fuel photosynthesis.
- Dungeons & Dragons:
- Atropus, the World Born Dead, is an Undead Abomination in the shape of a planetoid, which roams the universe in search of inhabited planets. As it approaches a world and installs itself as a new moon, it causes an escalating Zombie Apocalypse.
- Stars infested by the power of the Far Realms dance and waver across the sky at will, driven by a malign intelligence. They're also prone to reach out to mortal minds, offering them pacts to become Warlocks.
- Transformers: Cybertron wanders through space without an orbit, which is why the characters tend to rely on a teleportation device to make it back and forth. Similarly, Unicron's vehicle mode, itself a planet, flies through space of its own volition.
- Metroid Prime 2: Echoes: Aether is a rogue planet that does not orbit a star. Instead, light and heat are provided to its surface inhabitants by a mysterious native energy referred to as "the Light of Aether".
- Star Control 3: The Owa are one of very few alien races in the franchise whose homeworld can't simply be happened across as you're exploring. That's because your ship's navigation is based on travel from star system to star system, and the Owa inhabit a rogue planet. You can't travel there without coordinates straight from the Owa themselves.
- Dad has an entire song named after real-life rouge planet CFBDSIR_2149-0403. Dad discusses that something went down there, but what is never revealed, because not even he knows.
- Schlock Mercenary: Random Access Memorabilia is largely set on a wandering planetoid, or rather a Precursor artifact that accumulated enough debris to look like a planetoid.
- Shadow Raiders: The Beast Planet is a rogue planet of a predatory type, constantly seeking out other planets to devour. It starts the series making a meal out of Planet Tek.
- Truth in Television. Though they're understandably difficult to detect, a handful of these planets have been discovered over the years and it's believed there may be billions out there. Most are believed to have originated in a solar system before being ejected by a close encounter with another massive object. Hopefully they weren't inhabited at the time.
- 'Oumuamua was the first interstellar visitor we discovered in our solar system, but our measurements of it are notoriously spotty; by the time we noticed it, it was already on its way back out of the solar system, and at such a speed that it is beyond our reach to send up probes. Thus these spotty measurements are all we have, and while the general consensus is that it is too small to be a planet, it cannot be ruled out entirelynote .