Tidally Locked Planet

Honestly, it gets a little boring looking at the same side of the Moon all the time. Source

"What I meant, sir, is that Daled IV rotates only once per revolution. Therefore one side is constantly dark, and the other side constantly light. One might surmise that the two hemispheres have developed disparate cultures, which is a major cause of most wars."

Tidal locking is the result of a body (a planet around a star or a moon around a planet) being close enough to its parent that the pull of gravity on the satellite is stronger on the facing side than on the other. Over astronomical timescales the parent body's gravity will slow the satellite's rotation until one side always faces the parent and the other always faces away.

Because of this mechanism, a planet orbiting a star in this fashion will always be daytime on one side of the planet and always night on the other. Originally it was thought that the starward side would always be a blazing hot desert and the night side freezing cold. More recent computer models indicate that, assuming the planet has an atmosphere, convection currents will transfer hot air from the day side to the night side and bring cold air to the day side, alleviating the extremes somewhat.

Also known as a Twilight Planet, in reference to the perpetual twilight experienced by the narrow band between the starward side and dark-side. It is guessed that this narrow band may be capable of supporting life, and is a popular way to make a planet unique. In science fiction most of the population of a tidally locked world will inhabit this region, where the climate is fairly temperate.

Compare Single-Biome Planet. The main difference is that a tidally locked world tends to have single biomes over vast stretches of its surface, but not the whole thing. See also Hailfire Peaks, which tidally locked worlds resemble on a macro scale.


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  • Macross Frontier: The planet Galia IV is tidally locked, with a habitable strip around the twilight zone. A human colony was established there, but was later destroyed by the Vajra.

  • Mercury in DC One Million has apparently been engineered to be tidally locked, just as people used to think it was. "The planet that's too busy to spin" is the hub of the 853rd century's all-important data network, with solar panels on the light side powering supercooled processors on the dark side.
  • Another DC story states Krypton used to be tidally locked, with early civilisations from both light and dark sides warring over scarce resources from the habitable twilight zone. Eventually both decided to have one last battle with elected champions to determine who would have sole rights to the land. At one point both warriors became disarmed, their weapons of unique metals in contact with each other. They interacted to generate a force that rotated the boulder they rested upon. Duly inspired they reported this phenomenon back to their people, who then mined as much of if not all their respective metals as possible and dumped them together into a chasm. This had the desired effect of causing Krypton to rotate, shifting the ecosystem to something more hospitable for everyone.
  • The planet Taldain, where White Sand is set, is such a world, with Dayside always having sun and Darkside never seeing it - though Taldain's situation is more complicated, as there is another, UV-heavy star shining on the Darkside. Taldain itself is trapped between gravitational forces of the two stars - a blue-white supergiant and a white dwarf surrounded by a particle ring - probably as a Trojan planet in Lagrangian point.

  • Supplementary material for Star Trek: Nemesis indicates that the Remans evolved on the dark side of tidally locked Remus, explaining their photosensitivity.
  • A speculative documentary, What If The Earth STOPS Spinning?, examines what would befall human civilization if this trope began to be applied to Earth (answer: nothing good).

  • In Star Wars Legends the Twi'lek homeworld Ryloth has the sunward side an uninhabitable desert and the night side freezing cold. The Twi'leks mostly live on the terminator (and build their cities underground) and use exile to the sunward side as a form of capital punishment.
  • Adumbria in Ciaphas Cain: The Traitor's Hand is mostly inhabited in the twilight zone, and its inhabitants have 37 different words for degrees of twilight. (Amberley Vail cites a Fictional Document titled Sablist in Skitterfall whose title derives from this. Witty wordplay to an Adumbrian, nonsensical to an offworlder.) Cain's Valhallan 597th are from an ice world and are assigned to the perpetual winter of the night side, while the Tallarn 229th, from a desert world, cover the sunward side.
    • Warhammer 40,000 also has Mordia. It is a hive world and a death world, where the sun side is only inhabited by mutants and chaos cultists, while the rest of population lives on the night side of the planet. The place is homeworld of Mordian Iron Guard, regiments relying on line infantry tactics.
  • Jinx in Larry Niven's Known Space series. The colonists live along a narrow band encompassing the prime meridian and have completely black skin from the radiation. It also has very high gravity.
  • Isaac Asimov:
    • The 1956 sci-fi murder mystery "The Dying Night" [1] used the then-current scientific knowledge that Mercury was tidally locked to the Sun as a major plot point. The killer had lived on Mercury's dark side and forgot that Earth had a normal night and day cycle. After astronomers found out that Mercury did have a conventional day and night (albeit very long), Asimov mentioned in the author's notes of later printings that he'd wanted to fix the problem but couldn't figure out how to do it without rewriting the whole plot.
    • Radole in the Foundation universe: most of it is uninhabitable apart from a few areas on the terminator. The capital city is in the largest such area, where conditions resemble a warm June morning on Earth. Possibly one of many, as planets with such a narrow habitable strip are commonplace enough to have earned the nickname of "ribbon worlds".
  • In the Draco Tavern 'verse, the Chirpsithra are probably the most powerful race out there, but aren't seen much because they only like tide-locked planets orbiting red dwarfs.
  • In Proxima, Per Ardua is tidally locked to Proxima Centauri.
  • Star Trek Expanded Universe:
    • In Star Trek Twilights End, the planet Rimilla is one such world, until a daring plan comes up with a way to give it a standard rotation, thus enabling colonization over the entire planet.
    • In The Romulan Way the planet ch'Havran (a.k.a. Remus) is tidally locked to neighboring ch'Rihan (Romulus). Though oddly there's no indication that the reverse is true.
    • In the Section 31 novel Rogue, Chairos IV is tidally locked, giving rise to a species that is noted for being extremely robust.
  • In the science-fiction short story "Hothouse", the Moon's tidal locking has progressed further over millions of years, to the point that its orbit now perfectly keeps pace with Earth's day/night cycle. As a result, the Moon floats over one sole area of Earth's surface, making travel to it much easier by "traversers", enormous spiders capable of passing through space on silk strands miles long connecting the Earth to the Moon.
  • In Peter F. Hamilton's Night's Dawn Trilogy, the Ly-cilph home planet is a hybrid case: it's a moon tidally locked to the planet it orbits; it thus experiences one solar day with respect to the primary star for every orbit around the planet. But it orbits a young, hot "super-Jupiter" (bordering on being a brown dwarf) which glows in the near infrared and red. This gives rise to a less extreme version of the climate duality experienced by planets that are tidally locked to their stars. The nearside biome is dominated by plants that exploit the always present red light of the planet; the farside has plants adapted to use just the yellow light of the primary star, with long nights.
  • The Slan in Star Carrier: Deep Space evolved in caverns underneath the day side of a tidally locked world. They "see" by echolocation, with the closest thing they have to actual eyes being light-sensitive organs on stalks to keep them from accidentally wandering out onto the surface. Their version of capital punishment is to be "sent into the light", which the humans use as a Badass Boast during the final confrontation, helping convince the Slan commander to turn tail in defiance of his orders from the Sh'daar.
  • Clark Ashton Smith's short story "The Immortals of Mercury" describes the planet as tidally locked and inhabited by utterly insufferable Space Elves. By coincidence, Mercury's correct day-night cycle was discovered the year after the story was published.
  • Earth is on its way to becoming one in the novel The Age Of Miracles by Karen Thompson Walker. An event called the Slowing begins, and days and nights become longer. At first, it's only barely noticeable, a few extra minutes. But then it goes to hours and then days and by the later part of the book, each day and night is lasting weeks. It's indicated that eventually, the rotation will stop altogether and Earth will effectively be tidally locked with the sun. Some people try to adapt to the new cycles, while others try to keep going on regular 24 hour cycles. The protagonist is sure by the end of the book that civilization is nearing its end and humanity will eventually die.
  • Arthur C. Clarke's short story "The Wall of Darkness" is set on a planet whose one side is permanently oriented towards the sun while the other is in permanent darkness and seperated by a mysterious Wall Around the World as well, with the planet's orbital inclination being the only thing that give its inhabitants a sense of time passage. The protagonist's lifelong ambition is to find out what secrets lie on the dark side.

    Live-Action TV 
  • Star Trek: The Next Generation never uses the actual term, but based on its descriptions of two Planets of the Week the trope applies.
    • Dytallix B in "Conspiracy" was a world inhabited only by the Dytallix Mining Company. Due to the temperature extremes on either facing of the planet the company placed its facilities in the twilight region.
    • "The Dauphin" had one distinct culture develop on the day side of Daled IV, and a different one on the night side. Their differences led to a world war that the Enterprise is trying to put an end to.
  • Stargate SG-1 had an example in a first-season episode called "The Broca Divide." A planet was tidally locked with its sun so one side was always light, the other always in darkness. The civilization lived in the light side near the terminator, where it was temperate. A plague that made humans devolve into Neandertalesque creatures had broken out, and the infected were banished to the dark side of the planet. Unrealistically, the border region was not in twilight, but had a sharp edge where day instantly turned into night.

    Tabletop Games 
  • In 2300 AD several planets in human space are tidally-locked, probably most notably Aurore, which is featured in the introductory module.
  • Space 1889 Mercury is tidally locked, in keeping with the game's theme of "the planets are as they were once believed to be", giving rise to remarkable features and strange life-forms. A Challenge adventure adds that it is "nodding" a little, thoughnote .
  • Classic Traveller Double Adventure 2 "Across the Bright Face". The planet Dinom is an interesting variation on this. Its north pole points toward its star, so it looks like it's on its side. Its northern continent is always in sunlight (up to 260 degrees Celsius) and its southern continent in darkness (and goes almost as low as absolute zero). Between the north and south continents there's a temperate zone where life can exist.
  • Pathfinder's Golarion setting has the planet Verces, where the habitable central zone is settled by a union of bio-augmented transhumanists, predominantly pastoral Pure Ones, and God-Vessels. The sun-blasted Fullbright and the ice flats of the Dark Side are largely avoided as Death Worlds, since most creatures that can survive the temperature extremes are serious bad news for any traveler.

    Video Games 
  • In Kerbal Space Program, Mun is tide-locked to Kerbin, Duna and Ike are locked to each other, and all five of Jool's moons (Laythe, Vall, Tylo, Bop, and Pol) are locked to their primary.
  • Planet Bryyo in Metroid Prime 3: Corruption has this characteristic. According to the lore data, 48% of the planet's surface is in perpetual daytime (thus very hot), 48% is in perpetual nightime (thus very cold), and the remaining 4% has a temperate climate that allows the existence of cliffs and jungles. Interestingly, the tidal lock was a result of a planet-shattering war that decimated the population and left the survivors as primal savages.
  • Mass Effect
    • The Shepard Trilogy games note in the flavor text for numerous planets that they're tidally locked.
    • Elaadan in Mass Effect: Andromeda is a large moon locked to both the gas giant it orbits and another nearby moon. As a result the sun never sets for the playable area, making it a barely habitable desert (except for the krogans, who can live practically anywhere). One wonders why nobody seems to have settled in the twilight zone where the temperatures would be far more tolerable.
  • In Battleborn, Tempest the throneworld of the Jennerit Imperium is an artificially tidally-locked planet, set in orbit around the star, Solus.
  • In Evolve, the planet Shear was going to be one of these but was changed later in development as the writer felt it added nothing of value to the narrative.
  • The setting of the Borderlands series, Pandora, is apparently tidally locked according to Word of God, but it's an odd case. For one thing, its habitable zone is not the twilight band, but the night side, and this zone does seem to shift very slowly. (This shift caused, for example, the area near T-Bone Junction to change from a vast ocean to a desert.) Second, the planet experiences pseudo-seasons due to its very eccentric orbit. (Never mind that tidal locking and eccentric orbits are not compatible, as it requires too much variation in rotation speed.) And finally, Pandora has a false day-night cycle caused by the rotation of the moon, which radiates intense heat from one side only, and its phases. While Pandora's moon Elpis is not locked and rotates quite rapidly, the Helios space station that hangs above Elpis is locked to a position that always faces Pandora, so it can always fire "moonshot" deliveries to the planet surface.

    Web Original 
  • Inverted in RWBY, where the moon's phases (and shape) change because it rotates (instead of being tidally locked, as it is on Earth).

    Western Animation 
  • In Ben 10: Ultimate Alien, Mykdl'dy is a tidally locked planet with one side burning, and the other side frozen.

    Real Life 
  • All large moons are (or will eventually be, if they are young) tidally locked to the planet they orbit. This does not affect the surface conditions / climate in the same manner as a tidally locked planet, as the moon then experiences one day-night cycle each orbit. (Smaller, irregular moons may or may not be tidally locked, depending on their size and distance.)
  • Tidal friction applies to all bodies orbiting each other (the universe doesn't care about our definition of "planet"). However, with planets usually being both more massive and further away from their primary, this takes a very, very, very long time. Nobody knows yet how long it will take for the Earth, but several times the current age of the universe is a bare minimumnote .
  • This includes the Moon, which is locked to Earth. As a result, until the Space Age, nobody on Earth knew what the far side looked like. (It actually wobbles a tiny little bit on its axis, meaning we see slightly more than 50% of the surface, but not much. Animation and explanation) Incidentally, this is what's usually meant by the phrase "the dark side of the Moon". It means the "invisible" side which doesn't move, not the unlit side which shifts to create the Moon's phases.
  • Pluto's largest moon, Charon, is tidally locked to its planet as would be expected, but the size difference is so small compared to similar systems that Pluto is also tidally locked to Charon. For this reason, some astronomers argue that Pluto and Charon's tidal lock being mutual makes Charon not a moon of Pluto, but a dwarf co-planet with Pluto.
  • As noted in Literature, until 1965 Mercury was believed to be tidally locked to the Sun, and several major science fiction authors wrote stories featuring this. In '65 radar measurements revealed that the planet actually rotated three times every two orbits. (The combination of motions from rotation and revolution means that an observer on Mercury would see one passage of the sun across the sky, one local day, every two local years.)
  • Most, if not all, of the planets orbiting very close to their starsnote  are expected to be tidally locked. While that sounds like very bad news in terms of habitability, at adequate distance of its star if it's dim enough you could have an eyeball Earth, with an ocean in the subsolar pointnote 
This is particularly relevant for red dwarf stars: because they are dim, a planet has to be close to be in the "Goldilocks zone" (allowing liquid water) which in turn is close enough to expect tidal locking.