Created By: StarSword on January 20, 2014 Last Edited By: Iapetus22 on January 25, 2014
Troped

Tidally Locked Planet

One side of a planet always faces its star, the other always faces away.

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Trope

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Tidal locking is the result of a body (a planet around a sun 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 sunward 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 sun-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.

Examples:

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    Film 
  • Supplementary material for Star Trek: Nemesis indicates that the Remans evolved on the dark side of tidally locked Remus, explaining their photosensitivity.

    Literature 
  • In the Star Wars Expanded Universe 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 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.
  • 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's 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.
  • 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.
  • In the Star Trek Expanded Universe novel Twilight's 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 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.

    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.

    Tabletop Games 
  • 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.
  • In Mage: The Ascension the Euthanatos had a base on tidally locked Mercury. This was destroyed In-Universe when, out-of-universe, Mercury was revealed to not actually be tidally locked.

    Video Games 
  • In Kerbal Space Program, Mun is tidally locked to Kerbin, Duna and Ike are mutually tidally locked, and Laythe, Vall, Tylo, Bop, and Pol are tidally locked to Jool.

    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 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.)
  • 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.
  • 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.)

Indexes: Adverbly Adjective Noun, Otherworld Tropes, Settings, Speculative Fiction Tropes, Tropes in Space

Community Feedback Replies: 37
  • January 20, 2014
    StarSword
    Picked Tidally Locked for the title to make it easy to use in a sentence (x planet in y work is Tidally Locked). Another possibility would be to go for Adverbly Adjective Noun and make it Tidally Locked World.
  • January 20, 2014
    zarpaulus
    • 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.
  • January 20, 2014
    kjnoren
    The name should be Tidally Locked Planet or equivalent. Sentences are still easy to write "X is a Tidally Locked Planet orbiting the sun Y in work Z".

  • January 20, 2014
    StarSword
    ^Okey-dokey.
  • January 20, 2014
    Bisected8
    • Inverted in RWBY, where the moon's phases (and shape) change because it rotates (instead of being tidally locked, as it is on Earth).
  • January 20, 2014
    freesefan
    Literature

  • January 20, 2014
    DAN004
    Non Rotating Planet for more clarity?
  • January 20, 2014
    dalek955
    • 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.

    For the Ciaphas Cain example, see also its entry in Language Equals Thought.

    See also Hailfire Peaks, which tide-locked planets tend to be on the macro scale.
  • January 20, 2014
    kjnoren
    ^^ Tidally locked is a preexisting term, and the planet does rotate. However, the rotation around its own axis takes the same time as its orbit around the planet/star it is orbiting.
  • January 21, 2014
    Arivne
    Tabletop Games
    • 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 Celcius) 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.
  • January 21, 2014
    crazysamaritan
    Also known as a Twilight Planet, in reference to the perpetual twilight experienced by the narrow band between the sun-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.

    ^ should be in the description.

    From How To Write An Example, Keep it an Example, the 1965 entry isn't an example. The part about Issac Asimov's short story can stay, but here's my stab at rewriting for Real Life:
  • January 21, 2014
    freesefan
    ^Yes it is an example. Clarke, Asimov, Vonnegut, and others wrote fictional stories featuring a tidally-locked Mercury. The linked page has a list. Your recommendation to move the example to Real Life is inappropriate because Mercury isn't really tidally locked.
  • January 21, 2014
    StarSword
    As luck would have it Star Trek has a couple examples. Adding them.
  • January 21, 2014
    BOFH
    Video Games
    • In Kerbal Space Program, Mun is tidally locked to Kerbin, Duna and Ike are mutually tidally locked, and Laythe, Vall, Tylo, Bop, and Pol are tidally locked to Jool.
  • January 21, 2014
    BOFH
    Literature
    • In Proxima, Per Ardua is tidally locked to Proxima Centauri.
  • January 21, 2014
    crazysamaritan
    ^^^^ What you didn't read, I am now posting here:
    • Keep It An Example: Making a blanket statement on the behavior of '70s Live Action TV may be interesting information, but it technically doesn't add anything new as an example. If you feel the information is important, then add it to the description instead of the examples. Examples are about specific works and instances in them; the description is the general behavior of the trope. See Needs A Better Description.
  • January 21, 2014
    freesefan
    ^ You remain incorect. I did not provide a blanket statement, I provided a specific example—examples in literaturre of Mercury being written as a tidally locked planet—and a link to a page that has a list of specific stories. What you cut and paste does not have any relevance to what I wrote. I'm not sure how a list of fictional stories that employ a certain trope can not be viewed as examples of that trope. And, once again, you are incorrect in saying the example should be moved to Real Life, because the example is not about Real Life. Mercury is not tidally locked. The example cites instances in fictional stories.
  • January 21, 2014
    SeptimusHeap
    OK; coming here from Ask The Tropers:

    In my mind, that example is not Real Life. Works are not Real Life. Never ever. That said, it's a blanket example indeed - "Examples are about specific works and instances in them", that is a blanket statement, plus I don't think these authors (or literature) are the only thing making this mistake. Put that in the description.
  • January 21, 2014
    StarSword
    @free and @crazy: First, knock it off.

    Second, I sent a query to Ask The Tropers for a moderator ruling.
  • January 21, 2014
    freesefan
    I provided the names of specific authors and linked to a page that lists specific stories. Still am not sure why that is not specific enough. But since it apparently isn't—do I have to write out individual story titles?
  • January 21, 2014
    Tallens
    The entry on Hagalaz from Mass Effect 2 is incorrect. That planet has a 92 hour rotation in a roughly standard year orbit. Here's the info on it.
  • January 21, 2014
    randomsurfer
    ^^Yes. Weblinks Are Not Examples. If they are all examples, they should all be individually bulleted and described. Lumping them all together like that is the epitome of Blanket Statement.
  • January 21, 2014
    NagatoUzumaki
    Western Animation
    • In ''Ben 10 Ultimate Alien, Mykdl'dy is a tidally locked planet with one side burning, and the other side frozen.
  • January 21, 2014
    freesefan
    ^^ Well, that's pointless, as all I am going to do is copy the information on the Wikipedia page, but okey doke. Guess I'll do that tomorrow.
  • January 21, 2014
    StarSword
    ^Candi put one together in Ask The Tropers for what I think is the big one, that Asimov story. I'll use that and use the real life example crazysamaritan came up with.
  • January 21, 2014
    StarSword
    @Tallens: My bad. I remembered the game wrong.
  • January 22, 2014
    Rotpar
    Supporting Septimus Heap: add the blanket statement about Mercury to the end of the description, then just do proper examples by author on the actual list.

    Also, please don't just copy-pasta off of anybody, Wikipedia included.
  • January 22, 2014
    Antigone3
    In Mage: The Ascension, the Euthanatos had a base on tidally-locked Mercury. (Which got burned away when Mercury's real-world rotation overrode the assumption that it was locked — I don't recall offhand just what caused this to happen.)
  • January 23, 2014
    StarSword
    Hats? Examples? Description thoughts?
  • January 24, 2014
    JonnyB
    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.
  • January 24, 2014
    Tallens
    • In the Star Trek Expanded Universe novel Twilight's 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.
  • January 24, 2014
    Iapetus22
    I fixed the real life example about Mercury (it rotates 3 times every 2 orbits, not once). I also added the note about large moons being tidally locked to their planet, which unlike a tidally locked planet, gives them a normal day-night cycle.
  • January 24, 2014
    Tuckerscreator
    • In the science-fiction literature 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.
  • January 25, 2014
    StarSword
    Hats? Description thoughts? Examples?
  • January 25, 2014
    Iapetus22
    • Added an example from the Night's Dawn series. I think the key element of this trope is not the tidal locking (which is essentially universal for large moons), but the resulting duality between the nearside and farside climate. (Perhaps this should be explicitly stated?) The moon I added demonstrates this, but most tidally locked moons would not.
    • I clarified the real life examples.
    • I tried to add a hat, but I'm new here, so I can't do that quite yet.
  • January 25, 2014
    Iapetus22

  • January 25, 2014
    StarSword
    Gonna launch at 9PM EST-ish. Got dinner and a movie before that.
http://tvtropes.org/pmwiki/discussion.php?id=swur0lebs7hbpjr0rzeamec2&trope=TidallyLockedPlanet