Baby Planet

An inhabitable planet that is far smaller than astronomically possible, often less than a few functional miles in diameter. As a general rule, a Baby Planet is small enough that you can see its curvature even on the surface. In Real Life, a body this small would be called an asteroid, and would be incapable of supporting an atmosphere (of useful density at a life-supporting temperature), and would probably not even be spherical, but in fiction, these often sport an entire ecosystem awkwardly compressed into the minute available space.

Although Sci-Fi Writers Have No Sense of Scale, they usually aren't this far off the mark on accident; this trope usually comes about either because of limitations in technology's ability to represent planets in a realistic scale, or just for the sake of aesthetic.

Often, this used for purely aesthetic reasons, particularly on cover art for games and CDs. Just as often it's Art Major Physics. In photography, this effect is often created with very-wide angle (or fisheye) lenses.

Bonus points if it's unusually shaped too.

Related to Floating Continent.

Not to be confused with Little Big Planet, It's a Small World After All or Planetville.

Advertising

 * In the UK, British Gas have been running a series of rather fun adverts based on the idea that "your home is your world"; the person whose boiler is up the plonk lives on a tiny planet that is their house, drive and garden!! And their cars and vans take them to other planets and some planets have theme parks...sounds like a cross between The Sims and Super Mario Galaxy in advert form. Some of them are gathered here.

Anime and Manga

 * King Kai's planet in Dragonball Z. Despite being maybe fifty feet in diameter, it strangely has ten times earth's gravity. Based on King Kai's vague explanation, it apparently has the same mass as Earth, heavily compressed. This should technically make it a neutron star, but then, nobody ever accused Dragonball Z of realism. Even if they did, it's still the self-built home of a Physical God, and still set up in a corner of the afterlife, so it might as well be decorated with "A Wizard Did It" in 50-foot-high neon any way the audience looks at it.
 * An episode of Keroro Gunsou had one of Keroro's last-ditch invasion schemes involve stealing garbage and water from Earth to create a mini-planet. We learn it was a "last-ditch" scheme because unauthorized planet creation is against the law, and when the planet creation goes out of control it nearly becomes big enough to qualify as a full planet, leading to the Keronians nearly getting arrested by Space Police officer Poyon.

Comic Books

 * A Silver Age Superboy story had Clark Kent travel to a small asteroid/planet, where he found someone with powers similar to his own. At the time, the preferred explanation for Superman's powers was that Krypton was larger than Earth, so the native race evolved to counteract the more powerful gravity. The boy on the asteroid, it turned out, was from Earth, so on his little world he got to have some of the same benefits Superboy did. The native races of all 3 ecosystems evolved to be identical in size and shape. ..

Film

 * The CGI shots of the Earth in Zombieland (which are apparently the mental images of the narrator).

Literature

 * "Asteroid B-612" from Antoine de Saint-Exupery's The Little Prince may be the Trope Codifier, if not the Trope Maker. It's house-sized, which makes it bigger than most neighboring planets.
 * And he keeps encountering new stuff on it every day, including baobab trees.
 * Justified in Larry Niven's Protector due to use of gravity generators, etc.
 * Played straight and justified in the novel The Collapsium by Wil Mc Carthy: one of the main characters lives on an artificially constructed planet which is only a few hundred kilometers wide. It has a core made out of degenerate matter in the form of neutronium which gives it Earth-normal gravity, holds it in a spherical shape and allows it to retain an atmosphere.
 * The Mushroom Planet in the eponymous novels.
 * The Frederic Brown story Placet is a Crazy Place features a tiny planet with a breathable atmosphere with a core made of degenerate matter to give it its shape and gravity. It even has life forms made of degenerate matter that "fly" through the crust (which is like air to them since they are so dense), causing earthquakes. It obviously suffers from the "what keeps the degenerate matter from expanding" problem, but might be excused since it is a story from the 40s and Science Marches On.
 * Also, the degenerate matter "birds" that cause earthquakes are completely 100% excused by the Rule of Funny.
 * Gary Gibson's Stealing Light has at least one asteroid fitted with a Shoal 'world engine', and their Coreships.

Live Action TV

 * Several episodes of The Twilight Zone TOS had asteroids with normal Earth gravity and a breathable atmosphere.
 * Played straight in "The Lonely"
 * Lampshaded in "Elegy". The astronauts specifically mention that an asteroid shouldn't have Earth gravity and atmosphere.
 * Subverted in "I Shot An Arrow Into The Air". Some shipwrecked astronauts think they're on such an asteroid: they're actually on Earth!
 * Season 2 of Lexx had something like this, a small planetoid with an artificial atmosphere that was a TV studio center. Our 'heroes' wind up there and find that if their ratings slide they'll be in trouble.
 * There was also an episode where a planetoid was so small you could see grazing sheep on its surface from orbit. For unexplained reasons, it had Earthlike gravity.
 * Aversion: In the original Star Trek the Original Series episode "That Which Survives" the fact that a Luna-sized world has Earthlike atmosphere is one of the clues that something's amiss.
 * The official atlas for the Firefly verse says even the small moons, barely large enough to be balls (think Mimas or Enceladus), are terraformed using gravitic technology.

Music

 * Album cover art: The miniature planet on the cover of Fragile by Yes. (On the back cover, the planet breaks up and the population escape in a wooden space glider. This later inspired Jon Anderson's solo album Olias of Sunhillow.)

Video Games

 * Spore, although the scale is more realistic in earlier stages; it gets increasingly tiny relative to the player as you advance through the stages of the game. Actually, the planet is tiny to begin with. They only look regular-sized from close up, as evidenced in Galactic Adventures.
 * To give you an idea of scale: You can find Earth. The UK is about the size of an average spaceship.
 * Super Mario Galaxy not only has baby planets; it has baby galaxies. As in, galaxies that're not much bigger than a large paddock.
 * Judging by the background of the levels you go into, those baby planets are only a small part of a much larger galaxy. For example, in Gusty Garden Galaxy, there is clearly a normal sized planet beneath the Baby Planets.
 * Ratchet and Clank
 * The Special Stages of Sonic 3 and Knuckles.
 * The cover of Sim City Societies.
 * Populous the Beginning
 * The Prince Planet in Katamari Damacy (very obviously based on The Little Prince's,) as well as Earth itself if you get big enough to notice the curvature.
 * The cover of Animal Crossing: Wild World, and in addition, in-game the ground curves downward into the distance.
 * The freeware game Frozzd features dozens of such planets.
 * The American cover art for Bullfrog's Theme Park game.
 * Cosmic Osmo.
 * In Yoshi's Island, you fight Raphael the Raven on one of these.
 * Spacebuild maps for Garry's Mod have very small planets generally no bigger than about the size of a football field. This is because the maximum map sizes available are not particularly big.
 * The Tiny Bang Story.
 * Albion. The titular planed is considerably smaller than Earth, but is mostly made up of exceptionally heavy metal, making the planet's mass just slightly smaller than Earth's.
 * Angry Birds in Outer Space

Web Animation

 * Anything Can Happen On the Moon.

Web Comics

 * In The Inexplicable Adventures of Bob, the planetoid of Fleen follows this trope, even to the point of depicting The Little Prince's asteroid floating nearby.
 * Prospit and Derse from Homestuck: If the size of the towers on their respective moons is any indication, they are smaller than a Death Star, though still quite big.
 * This ultimately applies to almost every planet in the Incipisphere; the Lands are likely less than a hundred kilometers in diameter, small enough that at a fairly distant view the players' homes are still visible, giant tower status notwithstanding. However, the Battlefield is a full-sized planet.

Western Animation
"Duck Dodgers: As I was saying, this planet ain't big enough for the both of us! *pushes Marvin off* I claim this planet in the name of the Earth! Cadet: A-a-a-a-Big Deal"
 * Briefly alluded to in an episode of Futurama where a planet implodes, leaving behind something the size of a large boulder with a few of its native animals pathetically hanging on. As far as the cast is concerned, this is an acceptable level of survival for them.
 * And appears again in "The Mutants Are Revolting" with Mrs. Astor's asteroid. Has both an atmosphere and normal gravity.
 * And inverted when the Sun seems to have Earth gravity. And is a lava planet.
 * The Clanger Planet.
 * Isn't that supposed to be the moon?
 * It has vaguely moonlike craters, and in one episode a Lunar Module lands and an astronaut plants a flag (which the Clangers adopt as a tablecloth,) but it's referred to by the narrator as a "star" and exists among other similar worldlets. But since the Clangers occasionally leave it for the space above without any breathing problems, it runs strictly on Cartoon Physics.
 * The Transformers Generation 1 portrayal of Cybertron. Curvature could often be seen, and buildings could be seen from space. When Cybertron was moved into Earth's orbit, it was shown to be smaller than the moon - close enough for a plane-bot to fly to in under a minute, and yet the entire planet could be easily seen whenever it was in frame. Asteroid-sized is generous, and yet it's shown to have gravity comparable to Earth.
 * And then in the comics it's the size of Saturn. Way too far the other way, guys.
 * Arguably, that issue is justified by the fact that Cybertron itself is the body of Primus. Surely as a Physical God, he could whatever size he pleases, regardless of what the laws of physics say.
 * In Duck Dodgers in the 24½th Century, Planet X gets reduced down to a rock that's barely large enough for Dodgers and Marvin Martian to both stand on. Nonetheless, it still has air, and some weird kind of gravity that allows someone to fall off.


 * All the planets in Tiny Planets.

Real Life

 * Theoretically, you could have things like this. They just would have to be on the larger end of this scale (or be very deep), and have constant support (artifical atmosphere, seeding life, etc). We have no idea how this would work in practice, but we have a decent idea that it could work.
 * One way that probably wouldn't work would be compacting a large planet to a smaller size. The escape velocity of an object depends on how compact it is; that is, the ratio of its mass to radius. So the more you compact a planet, the higher the surface gravity would be - the Earth itself compacted to 3 miles across wouldn't be a black hole, but we certainly wouldn't suggest walking around on it.
 * Baby Planets might be able to be constructed once we attain Magrathean construction technology by creating a world around a superdense core. Considerably less mass than the Earth would be required given the distance from the surface to the gravitational center would be significantly less, so long as the gravitational pull of the core was not enough to decompose the shell of nuclear matter around it, and as long as the diameter is large enough that tidal forces (difference in gravity between feet and head) don't cause serious trouble.
 * Icy moons, such as Europa and Enceladus, are theorized to be capable of housing aquatic life and ecosystems. Europa is a bit smaller than earths moon, and Enceladus is about the size of the british isles. However, as small on the scale of celestial bodies they are, they are still much larger than a true Baby Planet.