Qui-Gon: We're going down to the planet. What's it like?That science fiction trope where planets fall into two categories: Earth-like (solid, with a human-breathable atmosphere and the same gravity, even if completely barren), and gas giants. And often, the gas giants have Earth-like moons. Even the most inhospitable of Single Biome Planets are not immune. There is, of course, a certain subjectivity regarding the definition of "Earth-like". In some works, "Earth-like" means that a human being can walk around without a pressure suit and not die, but everything else (the presence of animals and plants, the weather, the gravity, and so on) might be vastly different. In other works, "Earth-like" means not only are pressure suits unnecessary, but a person can eat the plants and animals, drink the water, walk around in the gravity without a lot of effort, survive the weather, and so on. Often justified by mentioning terraforming; series without terraforming invariably have many life-bearing planets that are perhaps more similar to Earth than most would consider plausible.note Hell, even with terraforming, there still would not be too many planets in one solar system that might be human-inhabitable without some sort of huge sunlight-focusing and gathering apparatus in orbit, since the planets' distance to the sun seems to be the main factor. There is also some theories that an Earth-like planet would be the ideal type for (carbon-based) life (but again, this is just a theory). Additionally, even if a planet is terraformed, it still would look mostly the same as it did before, except a different color (if Mars were terraformed, it would still have its volcanoes and craters). In live-action TV, this is pretty much inevitable, since creating a convincing alien planetary setting tends to require (a) a great deal of budget and F/X spent on a set that must be built very quickly and most likely will not be used again, and (b) any actors appearing in that setting to don costumes (also expensive, but more potentially reusable) which, if they are convincing full-protection envirosuits, will usually obscure the face and make acting more difficult. (See Rubber-Forehead Aliens for a reverse example of that problem.) Movies, animation and novels, less bound by time and budget concerns, have a somewhat better record, but even there the ability for human characters to see, be seen, and interact with each other without significant inconvenience tends to trump realistic assessment of xenoplanetary environmental conditions. Of course, this makes it problematic if some species wants the resources of a certain planet and are not concerned about the fate of its inhabitants. You'd think that a universe full of uninhabited planets would be a wonderful resource for metals and other minerals, but no. See also Single-Biome Planet. Contrast Death World. Since this trope is so ubiquitous, only Lampshade Hangings, subversions and aversions ought to be listed.
GM: Err... Um... Earth-like?
Obi-Wan: Hmmm. Convenient.
GM: Err... Um... Earth-like?
Obi-Wan: Hmmm. Convenient.
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Anime & Manga
- In Dragon Ball Z, someone steps blithely out onto the surface of an alien world without even bothering to test the atmosphere, and is chastised for it. In fairness though, those someones tend to be Saiyans, half-Saiyans, a Namek, androids and so on — Krillin is the only human. Pretty much all the main characters possess an array of superpowers too, and they only spend a prolonged time on two alien worlds in canon - Namek (which, since there are at least two Namekians living on Earth, presumably has a similar atmosphere) and King Kai's planet, which has some connection to the Afterlife, so atmosphere shouldn't really be a problem.
- Macross Frontier
- Played with: the planet Gallian 4 that Alto and Sheryl go to is described as a planet tidally locked towards the sun by a gas giant neighbor, making the extremities inhospitable.
- The Vajra homeworld is a Class-A planet, virtually identical to Earth except for the three moons, and the Vajra nest/planet-sized rock formation that stretches up from gigantic pillars to form a huge ring around the equator. At the end of the series, Island 1 successfully lands on it, and the Frontier population start a new life there.
- The Macross franchise as a whole actually plays this a bit more realistically than others: while humanity and its allies have spread through nearly half the galaxy, they have only found a relatively small number of already inhabitable worlds on which to form permanent colonies. So while Earth-like planets do exist, they are very rare, meaning that some colonization fleets have had to resort to terraforming less hospitable worlds instead. To make things more complicated, many of the most desirable Earth-like planets are already inhabited by native intelligent life.
- Lampshaded in the first Sound Stage of Magical Girl Lyrical Nanoha Striker S. When the cast visits Nanoha's homeworld, her students immediately express surprise on how similar it was to Mid-Childa.
- Cowboy Bebop dodges the bullet when it comes to climate because every single solid object in the Solar System has been terraformed to one degree or another. What doesn't make sense is the gravity. There is some effort to handwave the gravity problem on ships by giving them a rotating element, but while Faye and Ed should have been Herculean superwomen wherever they went-except Earth and Venus, Spike and Jet should have been puny weaklings. Especially Jet, because he grew up on Ganymede, which is only a bit larger than Mercury.
- Gargantia on the Verdurous Planet outright inverts this. No one had found any habitable planets after Earth, and it is believed to have frozen over.
- Legend of Galactic Heroes
- Played straight. All planets shown in the series could be mistaken for locations in Earth, with those of one faction more thematically attuned to 19th Century Germany.
- Watchmen has a single superhuman in the world, Dr. Manhattan. He teleported to Mars just to have some solitude (with his godlike powers, he has absolutely no problem to survive there), and later returned and teleported Laurie to Mars as well. But, being a normal human, Laurie almost died of suffocation in Mars, before Dr. Manhattan remembered that other people need to breathe and created an air bubble for her.
- Invoked right at the start of With Strings Attached. After Varx offers to whisk Paul to another planet for adventure, Paul (who thinks he's dreaming) jokingly worries that he might be dropped on the Moon or some other inhospitable place. Varx assures him that they have an oxygen planet all picked out. Later, when the four go on the Vasyn quest, the three planets they visit are very Earthlike (one of them being an actual parallel Earth) but again, these were all picked out for them.
- The impact that even minor differences from Earth that apparently habitable alien planets have becomes a major plot point in the Warhammer 40,000 fanfic The Misfits. The planets of the Storm Ravens chapter's home system, Octalia, contain extremely low levels of potassium, so the government needs to import supplements from other star systems. This allows the Big Bad to incite a revolt by exposing the fact that the government has been skimming from the supplement fund and allowing the lower classes to suffer potassium deficiency.
- Subverted in "Frostbite". Orvis II is technically Class M (Earthlike), but it's in an ice age right now. Biri points out that Bajor looked about the same five million years ago.
- Played with in The Next Frontier. The Kerbals have two habitable bodies in their home solar system; the planet Kerbin and Laythe, moon of the gas giant Jool. That Laythe has liquid water at all much less an oxygen atmosphere is the product of a series of very improbable coincidences that surprised even the Kerbals themselves once they figured it out, so imagine their surprise when they send their first Faster Than Light starship to a nearby solar system, take some observations and find eleven planets and/or moons which have oxygen atmospheres. They draw the obvious conclusion: Terraforming. They're dead right, and boy, were they low-balling the number of inhabited planets...
Films — Animation
- The world of Royal Space Force: The Wings of Honnêamise is presumably in another galaxy or system and its "human" population evolved just like Earth's.
Films — Live-Action
- Star Wars. Most notably, pretty much every world visited in the films has Earth-like gravity, regardless of size or composition.
- It's not most definitely not a normal planet, but while the belly of the gigantic space slug in The Empire Strikes Back lacks a breathable atmosphere, it apparently has normal gravity and pressure. Odd, as it's inside an asteroid in deep space. Maybe the Millennium Falcon was able to extend its artificial gravity field or the asteroid field as a whole had some kind of atmosphere.
- Also in The Empire Strikes Back, Bespin apparently has a breathable oxygen-nitrogen atmosphere despite being a gas giant. This is probably actually more realistic than the space slug incident; it's theoretically possible that a thin layer of breathable atmosphere could exist in the upper atmosphere of a gas planet and if it's Saturn-sized or smaller then the effective gravity in its upper atmosphere could be Earth-like. But that'd be an incredibly convenient coincidence.
- At the very least the planet Felucia possessed a very unique looking, completely alien ecosystem. Even though it seems to suffer from similar atmosphere and gravity....
- Likewise Mustafar, though distinctly non-terrestrial in its appearance, has a perfectly breathable atmosphere despite the fact that it is completely covered with active volcanoes and lava rivers. Seriously, not one person coughs in the Mustafar scenes.
- The building had a forcefield which kept in a breathable atmosphere, and likely Anakin and Obi-Wan could use the Force to filter out any toxic gases in the Mustafarian atmosphere after the forcefield collapsed.
- Coruscant, despite being far closer to the center of its galaxy than Earth is to the center of the Milky Way and having three moons, is nonetheless identical to Earth in atmosphere, mass, diameter, rotational period and orbital period. Maybe it even has the same continents as Earth, if you can actually find them underneath the enormous cityscape that literally covers the entire planet.
- The prequels and EU offer some subversions, species that evolved to breathe ammonia or methane instead of oxygen.
- It's an aversion if the human characters have to wear space suits (or at least respirators) when on the surface, as with LV-426 in Alien.
- And in Avatar. The atmosphere has plenty of oxygen, it's just that we can't breathe the atmosphere because the composition of the other elements in the atmosphere more resembles the Earth during the Permian extinction. The Na'vi, along with every other land animal, are able to breathe in this environment despite it being hostile to the vast majority of earth-like life because the Pandoran life actually evolved there. Additionally, just about every plant or animal is inedible/toxic to humans. The low gravity is also why everything is so much bigger on Pandora; it can afford to be, on account of the lower gravity.
- Although there aren't any scenes that actually go out of their way to demonstrate the low gravity, mind.
- Averted in Rocketship X-M, ironically enough given the movies' massive Hollywood Science. Scenes on Mars were filmed in California's Death Valley, which NASA latter used to test equipment due to be used on Mars because of its similarity.
- Lampshaded in Galaxy Quest. When a character opens a shuttle door, another character points out that they don't know anything about the planet. Fortunately, there was indeed breathable air....
- Pitch Black: It's a desert planet, obviously hotter than hot. How it supports such an oxygen rich, earth-like atmosphere complete with rain is never explained.
- The sequel features improbabilities such as Crematoria; indeed, the fact that the "most hostile" planet by Riddick's own account still sports a breathable atmosphere and earth gravity suggests that there may be no celestial bodies devoid of atmosphere.
- Averted in The Day the Earth Stood Still remake. Life-bearing worlds are so rare that the galactic races have agreed that no race must be allowed to destroy their world through pollution or war. Thus, if a race nears this stage, it must be "cleansed" from the planet, allowing this world to try again. Hence Klaatu's appalled reaction when the US Secretary of Defense calls Earth "our world," since the galactic races consider even the concept of a race owning their planet to be blasphemous.
- Older Than Feudalism: This showed up in the world's first space-travel story, A True History by Lucian, an ancient Roman writing in Greek. As it was written before modern astronomy, the Moon was earth-like, albeit filled with all sorts of wacky monsters, but then it gets really weird when it turns out there is also civilization (and people, and trees) on the Sun.
- Animorphs: Mostly averted, with similarities generally being more by analogy, such as Andalite vs. Earth grass despite theirs coming in shades such as red. The Yeerk planet is completely different and unpleasant. Fittingly.
- The Robots-Empire-Foundation universe in the novels of Isaac Asimov has at least 20 million Earthlike planets inhabited by human beings, due to colonization and terraforming of planets by first Earth and then Earth's colonies. After thousands of years, no one is even certain of the location of Earth.
- Played more or less straight, but there are subversions and at least one deconstruction. The Manticore system is extremely unusual for having three Earthlike planets (partly a result of the very large habitable zone created by its Binary Suns), and part of the Star Kingdom's backstory is that Manticore was so Earthlike that a native disease crossed the species barrier and killed the majority of the first wave of colonists. The creation of a constitutional monarchy and noble class was a solution for the survivors wanting to maintain political power over the replacements the colony recruited from offworld. Meanwhile Grayson looked Earthlike from a distance but turned out to have horrifically high concentrations of heavy metals. And San Martin's gravity is so high that people can't live at sea level.
- Done to extremes with the very Texas-like planet of Montana that just happens to have a lot of animals that are almost exactly like real Texas animals. And everyone is a stereotypical ornery cowboy. It was handwaved by describing it as a planet colonised by very Texan-like people determined to preserve stereotypical ornery cowboy lifestyle.
- Subverted in the CoDominium universe. Life-bearing worlds are common, but they aren't always fully compatible with Earth life. Nonetheless, most have at least a few areas where very hardy humans can survive, albeit with high mortality rates (the poles of Fyrstaat and Tanith, the equator of Haven). A rare few, like Sparta, are nearly ideal for Earthlife, if slightly off in gravity and length of day. And some worlds, like New Caledonia, require extensive terraforming. The alien-inhabited Mote Prime requires respirators. The double-planet system of Franklin/New Washington required Earth plants to be genetically altered for the red sunlight.
- "Haven", the titular War World, is pretty much a case of Pournelle getting together with Niven and asking, "Exactly how horrible can we make a planet while still letting humans breathe the air?"
- In the Darkover series, the titular planet itself and the hundreds of planets making up the Terran Empire.
- In Larry Niven's Known Space universe, there are many planets that are liveable, but not Earthlike. Most of these were seeded with microbes as food sources by the Slaver empire, which died out billions of years ago, explaining why so many of them are biochemically cross-compatible; humans and Kzin, for example, can eat each other. Non-Earthlike worlds, such as high-gravity Jinx with its vacuum-exposed tidal "poles", and Plateau with its single livable mountaintop sticking up out of a high-pressure toxic atmosphere, were settled by humans whose early interstellar probes were rather poorly programmed regarding what kinds of places to green-light for colonization.
- Played with in the Antares series: most star systems resemble Sol, with a single habitable planet. There are no named uninhabitable solid planets. However, it is implied that just like Sol, there are a number of uninhabitable solid planets in most systems - New Providence, for instance, is identified as the seventh planet in the Napier system. Presumably, the other planets are simply unimportant.
- Initially avoided in Madeleine L'Engle's A Wrinkle in Time, where the characters visit in quick succession a two-dimensional planet and a planet where the air has a slightly different oxygenation level; later planets are sufficiently Earthlike for no problems to occur (and Camazotz even has Earthlike trees).
- Played with in the book "Anywhere But Here". The heroes, a married couple, take off for space, going to planets that are already colonized and have people and aliens living on them. However, once the navigation system on their camper truck (yes, a converted camper truck) goes haywire, they quickly end up on a series of planets that aren't on the map. There's one very short stop at a planet filled with some kind of unbreathable (for humans) gas. They get stranded for a few days on a planet with breathable air, but everything melts like plastic when exposed to fire, including some of the rocks, and is inedible. The local water isn't good for them either.
- The Ellimist Chronicles features the interesting case of the Ketrans. Their home planet features giant floating crystals (upon which the Ketrans live) above acid oceans and an atmosphere that seems to consist mainly of hydrogen. When their home planet is invaded they flee in a newly built spacecraft to search for a new home, assuming that All Planets Are Ket Like. But, as anyone familiar with the Animorphs verse knows, the truth is that All Planets Are Earth-Like, and thus inhospitable to Ketrans.
- Played straight in H. Beam Piper's Little Fuzzy series, with the extremely Earth-like planet Zarathustra. Subverted in his work Uller Uprising, with the planets Uller (Breathable air and tolerable gravity, but silicon-based native life) and Nifflheim (hideously poisonous mining planet).
- Subverted by many works by Hal Clement, who would go to great lengths to invent non-terrestrial planets and populate them with believable life forms.
- Also subverted by Robert Forward in his novel Dragon's Egg. The planet in question is a ball of neutronium, the aliens are amoebas, and you STILL empathize with them.
- Averted in Peter F. Hamilton's Commonwealth Saga, where wormhole technology allows the ever present CST to use its exploration wormholes to scout new solar systems for "H-congruous" planets capable of supporting human life. The Commonwealth spans a vast amount of space, and most intervening systems between H-congruous planets are untouched completely by humans.
- This is demonstrated in the first book, Pandora's Star, where an exploration team finds a planet that appears to be H-congruous, but the planet is rejected when the discovery is made that every single plant on the planet shoots or secretes acid. Suspicions are raised when the survey team notices none of the animal life will stand still for any significant length of time on the local grass equivalent.
- Hamilton's other big book series, the Night's Dawn Trilogy, averts this trope as well, with habitable planets being classed as "Terracompatible".
- Fallen Dragon, by Peter F. Hamilton, averts this trope as well. Pretty much no planet is completely suited for human life. For all of them, extensive terraforming is required before sending in the colonists. Amethi was a frozen world with very little atmosphere when the first settlers came, and a scene depicts the startled reactions of a bunch of children who see a cloud for the first time. On Thallspring the soil bacteria and other biota make it necessary for the Mega Corp. running the colonization efforts to clear out all life in large swaths of land via periodic orbital gamma laser "soaks", in preparation for colony expansion. The process is stated to kill all bacteria down to a few meters underground. Santa Chico has a very high oxygen content, and its biosphere is a biochemical and medical goldmine, which is why the colonists modified themselves to adapt and eventually began using Organic Technology, only to revert to a agrarian society of bizarre xenophobic Furries once they grew bored with the Mega Corps periodically plundering their planet.
- Averted in Alastair Reynolds's Revelation Space universe. Most planets are barely habitable and human inhabitants require bases with constant life support to survive. The only planets with breathable atmospheres tend to be the Juggler waterworlds, which have oxygen atmospheres. By far the only really Earth-like planet is Sky's Edge, which has a breathable atmosphere and didn't even require much terraforming, but it's native life is inedible to humans, there are no analogues of vertebrate animals, a lot of the fauna is pretty nasty and dangerous and even the history of the planet's colonization is far from idyllic. Also, Resurgam was once a very earthlike planet - until the local civilization of avian humanoids got wiped out during a mysterious cataclysm.
- Present, after a fashion, in much of Murray Leinster's work. In one of the Med Ship stories, the author notes that though non-Terrestrial ecologies are rarely strongly similar to Earth, they tend to be broadly similar, with grass-like plants, tree-like plants, pollinating flying creatures, prey species, predator species, scavengers of various sorts, etc. Most non-Terrestial ecosystems are somewhat compatible with Earth life, which can keep the Med Service very busy.
- Robert A. Heinlein's Tunnel in the Sky plays with this:
- Averted, in that not all planets are Earth-like. Rod watches a diplomatic envoy from a race of chlorine breathers arrive on Earth before taking the test.
- Discussed between Matson and Rod. Any planet hosting the survival test would be Earth-like to ensure a challenge that can beaten. Students would be formally warned of a hazardous environment beforehand.
- Taken to an absurd extreme by a delirious Rod. The planet is so Earth-like — not just its physical environment, but its animals, plants, etc — he becomes convinced he's actually on Earth, learning otherwise only when the clouds clear up and Jackie points out the stars are different.
- Cleanly avoided in the Star Carrier series. There are Earthlike planets out there, as indicated by the Sh'daar numerical designation for humanity including our preferred atmosphere and the fact that the Agletsch don't seem to have a problem with it, but they are few and far between. Only two human-settled alien worlds allow the colonists to survive unprotected: Osiris and Vulcan (which happens to be located exactly where Star Trek's Vulcan is supposed to be).
- Averted in James White's Sector General novels, about a deep-space interspecies hospital. There are codes for different kinds of species, and some parts of the hospital have atmospheres that aren't breathable by humans.
- Averted in novelisation of I Am Number Four of all places despite most science tropes straight. The Lorien Four tells us that there are eighteen life-sustaining planets in the entire universe. Hence why the Mogadorian want Earth after destroying both his home world and their own.
- Played with in Mark Delewen And The Space Pirates; although the planets visited are Earthlike, the main character can't believe they are and worries about whether he can breathe the air.
- Played straight in the Russian children's story "Journey To The Morning Star". Given that the story was written in 1960 and meant for kids, this is expected. The Lado system in the Coma Berenices constellation has three planets, all of which are (or were) Earth-like. The first planet, Aeo Tau (the titular "Morning Star"), is a virgin world full of prehistoric creatures such as dinosaurs. The second planet, Sino Tau ("Thunder Star") is a lush paradise with an advanced race of Human Aliens who are constantly at war with one another. The third planet, Etheri Tau ("Cold Star") is the oldest of the three and is no longer capable of supporting life, although it was once much like Aeo Tau. The Etherians are Human Aliens who have lived underground for thousands of years but must now relocate to Aeo Tau to survive. The warlike Sinots invoke The Right of a Superior Species and threaten any Etherian who sets foot on Aeo Tau with death.
- In Jack Chalker's "Four Lords of the Diamond" novels humanity discovers a solar system with four Earthlike planets - completely unheard of. It's only at the end of the series that they discover that the four planets were artificially constructed by an alien race as nurseries for their young.
Live Action TV
- Doctor Who
- Nearly every alien planet ever featured on the show has Earth-normal gravity and atmosphere.
- The First Doctor serial "The Web Planet" has the distinction of being set fully in an alien galaxy, with (aside from the Doctor and his companions) a full insectoid supporting cast and environment. The Earthlings (and Gallifreyans) did not wear space suits, but at times did suffer from weakness owing to the difference in atmospheric composition.
- It was parodied in the First Doctor serial "The Daleks' Master Plan", when the Doctor was nervous about the effect of poisonous gases in the atmosphere of a city on his companions. It turned out to be an industrial city Oop North, and the gases were just then-normal levels of air pollution.
- In the Second Doctor serial "The Moonbase", the characters wore special atmosphere suits to explore the Earth's moon. They were never seen again. The show's writers most likely did it because it was set somewhere that the viewing audience already knew didn't have a breathable atmosphere.
- Space suits showed up again much, much later, in the Tenth Doctor serial "The Impossible Planet" / "The Satan Pit. He didn't make them on his own though, but borrowed them
- In the Tenth Doctor episode "Midnight," the title planet is totally unlivable ( except it turns out it isn't, but that's beside the point); it's apparently one big lump of crystal with no atmosphere and a star that emits very deadly rays. But Professor Hobbes goes on about this like it's remarkable for a star system NOT to support life. The Doctor's spacesuit makes a return in the Mars-set episode "The Waters of Mars".
- The Eleventh Doctor is seen using the same suit again in "Hide", though equipped with a brand new helmet design this time around. Some preview photos from an episode of the upcoming eight series show the Twelth Doctor, Clara and other allies wearing several spacesuits of the same design (presumably kept in storage until then, along with the Doctor's own).
- Battlestar Galactica (2003) has had a relatively small number of Earth-like worlds actually shown for a show of its type (Caprica, Kobol, New Caprica, Tauron from "Razor", and the Algae Planet). The other 10 colonies may or may not all be Earth-like (it's possible a military base shown in a flashback was on Picon). We also have references to moons and planets from the First Cylon War that apparently have snakes and other creepy-crawlies, so we can assume they're Earth-like. However, BSG has also shown a windswept, dusty red Mars-like planet, an ice moon, and a few gas giants. Also, the Algae Planet was originally conceived as far more primordial than the budget would have allowed for. We can speculate that whatever world the colonists originated from might have been the model by which the 13th Tribe or the other original 12 Tribes terraformed their newly settled planets, especially since only Earth-identical wildlife is ever mentioned (whether or not the humans originated on Earth or on Kobol or somewhere else altogether).
- Star Trek:
- Star Trek: The Next Generation may have had the ultimate subversion: Earth, 3.5 billion years ago, just prior to the first protein being formed by two amino acids. Presumably, if not for Q's abilities, Jean Luc Picard couldn't have breathed on that surface.
- Of course, the First Humanoids spread DNA to most life-bearing worlds to ensure parallel Hollywood Evolution. Many alien planets have been described as having the equivalent of orchids, vultures, cats, and other terrestrial-specific animals as well as Human Aliens and Rubber-Forehead Aliens. The Klingons even have coffee.
- Averted in Star Trek: The Original Series. The planet Elba II had a poisonous atmosphere that would kill humans breathing it before very long, "The Way to Eden" had a planet that was technically habitable (right sunlight and air quality), but all the flora excreted a deadly acid, and the fruit was lethal.
- Zigzagged in The Wrath of Khan. The planet Khan was found on had been Earth-like but had since become fairly toxic.
- Averted in Star Trek: Voyager episode "Demon". A class-Y planet is colloquially known as a 'Demon' class planet because it's as harsh as the class-N Venus.
- The fact that the recurring Benzite species in The Next Generation and Deep Space 9 are one of the few species shown that can't breathe normal air without a special device implies that their homeworld, Benzar, is not Earthlike.
- But there are also episodes where the reason for the crew to look closer is that the planet, or an area on it, is Earth-like when it shouldn't be (Sufficiently Advanced Aliens did it).
- And frequently enough in all Star Treks, "only" one or two planets in a system will be "Class M" (that is, Earthlike) and no one ever beams down or lands on a planet without checking first. Sometimes the checking is offscreen, but if the away team know how to dress, they probably also know the air won't kill them them the second they arrive.
- Then, there are the in-betweeners (planets that are nearly Earthlike and can support the away team with the busted shuttlecraft but only for a short time is a common setting). So all planets in Trek aren't Earthlike — but there are certainly a lot of them, and close to Earth, and it is a striking coincidence that the planet a shuttlecraft crashes on almost always happens to be something that can support the away team long enough for them to be overdue getting back and the others to go find them.
- In episode of Enterprise, Tucker crash lands on a small planet that is habitable while it is night time. Once the planet rotates to face the sun, it is uninhabitable for human life, as it is too hot.
- The Tholians (seen in TOS and Enterprise, and referenced in the other series) were hermaphroditic, insect-like humanoids who lived in an environment of around 480 Kelvin. Presumably their homeworld was very much unlike Earth.
- Zigzagged with the Breen. The thermal suits they wear constantly seemed to suggest their homeworld was a frozen wasteland, but according to Weyoun the world of Breen is actually quite temperate (and presumably Earthlike).
- Stargate SG-1:
- Most of the planets in the series are fairly Earth-like, though there are some exceptions where the team has to wear special space suits to explore them. This is explained as the Goa'uld (and before them, the Ancients) having terraformed the planets millennia in the past. And once again, Stargates are placed by once-humanlike Precursors who would have no reason to put one on a world that would kill you the second you exited.
- And then, there are those examples where Planets are absurdly present-day Earth-like to the point where Ikea furniture would fit right in. Particularly common in later seasons and "Atlantis".
- Lampshaded in "Prodigy", when Carter's arrogant young cadet emerges from the Stargate and says, "So this is another planet? ... Doesn't look that different from home."
- They actually played with/subverted this trope in the teaser for a Season 9 episode. Most of the alien worlds, as stated, look almost exactly like the woods around Vancouver so the audience wouldn't be too surprised to have an episode open with a Jaffa running through one of these alien forests... until he gets hit by a truck and you find out that he was on Earth All Along.
- O'Neill takes a swipe at this (and at California Doubling) at one point. "Why do they always look like Canada?"
- In "Heroes, Part 1", SG-13 is doing a standard recon mission, and the team leader Col. Dixon takes bets on what they're going to find on the planet. One of his mauve shirts pipes up with "trees". Dixon says he's "disqualified for being a smart-ass."
- Double Subverted in season one's "Solitude" when Sam and Jack are trapped in an ice cave. Sam gets a look at the surface outside and declares it's an ice planet and there's no way out. In actual fact, they're on Earth, in Antarctica, which was a temperate zone when the Beta Gate was built there millions of years ago.
- Stargate Universe averts this and Human Aliens by having most worlds visited barren and lifeless and mostly only good for picking up the one or two useful natural resources before moving on. And SGU is a good example of why this trope falls under Acceptable Breaks from Reality: barren rocks really aren't that interesting of a setting. However, sometimes the ship stops at a planet automatically, and the team will explore others that happen to be in range. The ship rarely stops at nasty ones, but with these others, you're rolling the dice. And as always, the then-humanlike builders of the gates and the ship have no reason to put gates on useless or deadly worlds.
- Likewise, all the inhabited worlds in Firefly were deliberately terraformed, though they each have their little quirks.
- Sliders, which dealt with Earths in parallel universes, mentioned once or twice that the nature of the wormhole would keep it from dropping the heroes in universes that were patently incompatible with human life. Or inside rocks. Although it did once briefly drop them on an Earth that was covered completely with fire. Of course, they were standing in the one safe spot, and the fire was alive and followed them.
- Aversion: In Dans une galaxie près de chez vous, where a dysfunctional crew of French-Canadians tries to find one of these, it turns out there's everything besides Earth-like planets!
- Averted by Babylon 5: the titular station has segments that rotate at different rates to simulate the gravities of various resident alien races, and entire sections dedicated to non-oxygen-breathers. There are even special toilet facilities for methane breathers! B5 was one of the first science fiction series on television to not immediately assume that aliens were capable of breathing oxygen and speaking English. On the other hand, there are some species, such as the Soul Hunters, who can process both nitrogen-oxygen and alien atmospheres.
- Space 1889 Mercury, Venus, Mars and the moon can all support human life quite easily (if not necessarily comfortably), although only Mars has any sort of civilization on it — Mercury has only very basic life, such as trilobites, Venus has sentient lizardmen, although they are only at stone age level. Mars has an ancient civilization based around their canals, although they have lost the technology necessary to build new canals or even to maintain their cities. The Moon can also support life, although there is no atmosphere on the surface — it only exists nearer to the Moon's core.
- GURPS Space arguably goes too far in the other direction. Enough that the fourth edition had you deciding whether you wanted an Earthlike planet to start with and then designing the physical characteristics with that in mind. The main alternatives being gas giant, rocky moon/asteroid, ice moon, or toxic terrestrial.
- Justified in the Traveller RPG, where Earth life (including Transplanted Humans and Uplifted canines) were spread by the Ancients.
- In BattleTech, 'Earthlike' is variable. No planet is exactly like Earth in terms of comfort for humans, which is commented on by those who are lucky enough actually travel there. Most of the settled worlds are fairly close, though there are wild exceptions, like the domed cities of Sirius V (the atmosphere is poisonous. Justified in that Humans would naturally pick the Earth-like planets to settle first.
- The Star League in its heyday did in fact do rather a lot of terraforming and other mega-engineering projects. Then the Succession Wars started and military budgets took priority over such expensive flights of fancy and massive terraforming projects stopped.
- Warhammer 40,000 averts this trope pretty impressively. The Imperium of Man classifies planets into several different categories, some of which are Earthlike. Even within a category there is enough variation that most planets aren't a Single-Biome Planet. And being the kind of universe it is, most of the settled planets are not, in fact, habitable. The Imperium seems to absolutely love settling worlds that humans can't actually expect to survive on.
- Spherus Magna in BIONICLE. When The Shattering happened, it was divided into three SingleBiomePlanets — one jungle, one mostly desert, one ocean. Although it's fair to assume not all inhabitants survived the event, most species retained a breeding population, and were able to function on the new planets with no apparent alterations to the atmosphere or gravity. Furthermore, two of these "planets" are actually moons of the bigger planet, yet the inhabitants didn't seem to suffer from a 28-day long day-night cycle. Then when Spherus Magna was restored to its Earthlike state, there was no mention of increased gravity whatsoever.
- Averted and played straight in SimEarth. Depending on the player actions, the planet could be just like Earth, or something else altogether. Life that evolves on the planet will be Earth-like however.
- In the old Star Trek game for the NES, players could technically only land on planets with breathable atmospheres, but all that it actually meant is that the designers only made planets with oxygenated atmospheres. The rest are just there as a backdrop.
- Freelancer has roughly a 50/50 ratio of Earth-like planets and somehow uninhabitable worlds. On one hand, you have planets such as Cambridge or Stuttgart, full of fertile farmlands, or Cura, a planet full of heavenly beaches; but on the other hand, you also have planets like Pittsburgh, a barren, deserted wasteland punctuated with mines, or California Minor, a little frozen ball under terraforming.
- Most of these planets are covered in rocky pine forests.
- In the Homeworld series, while Hiigara falls under this trope, Kharak is only partly habitable, being a harsh desert planet that only the poles are comfortably habitable, while maintaining .98g gravity and breathable atmosphere. Justified in the fact that since the original Hiigarans were exiled there, the planet must've been chosen specifically as being harsh, but not too inhospitable.
- Meteos averts this trope. There is only one Earth-like planet in the game; every other planet is pretty much unique and un-Earth-like in its own ways. Some of these are not even planets at all. Intelligent life, in this series, have sprung up on dwarf planets, asteroid clusters, dimensional rifts, interstellar gas clouds, neutron stars, and even mythological realms.
- Mass Effect notably averts this; the vast majority of the planets encountered are actually very hostile to human (or most) lifeforms. This makes those inhabitable worlds that can be found all the more valuable.
- "Vast majority" here meaning that only three planets shown in the first game are safe for humans, in a game shows around a hundred. Most of the planets you visit require spacesuits and sealed environments for survival, and sometimes not even your Powered Armor/space suit will keep you alive for very long. The 80%+ of planets you don't visit are often even more inhospitable.
- It's also noted on the info screens for several planets note that what makes them unsuitable for humans make them perfect for volus/hanar/elcor etc.
- A few planets actually are more like subversions, too. Like Nodacrux, the planet with a much higher oxygen concentration than Earth's. It's enough for humans to feel plenty comfortable, like being in a hyperbaric chamber...but it makes thunderstorms absolutely devastating to anything they catch in their path, and vegetation goes up like tinder in amounts that make the California wildfires look like campfires. Not to mention huge insects and plants that give off pollen that causes death by allergy in seconds.
- Even planets that humans actively colonize aren't all Earthlike. Eden Prime, which you visit at the beginning of the game, is pretty close to Earth, except for its sixty-four hour days.
- In Mass Effect 2, most of the planets you visit are at least partially Earthlike. Key word being visit: you only ever touch down on planets with some kind of habitation, even if it's just a tiny mercenary resupply point. Also, the Blood Pack have a base set up on a world that's filled with gas that's toxic to all your crew members but the Vorcha can live there fine.
- Space Quest III: The Pirates of Pestulon sort of averts this with the Planet Ortega, which while the atmosphere is breathable, the surface is too hot for humans who don't have special clothes.
- Averted in the Master of Orion series. Terran class planets exist, but so do Toxic, Desert, Ocean, Tundra etc. etc. Terraforming can help make them more habitable.
- Subverted in Earth 2160. The game takes place on a number of celestial bodies in the Solar System and around the nearby stars (one mission takes place on a very large comet), but of these, only 2 planets are actually suited for human habitation. One of the is a desert planet reminiscent of the one in Stargate (complete with ancient alien pyramids), while the other is a very Earthlike planet called Eden. The main characters are visibly surprised when they see a video from the surface, and some even suspect that it's actually old Earth footage.
- All of the above locations are made a bit less pleasant by the Morphidian presence.
- For most part, StarCraft averts this, as the vast majority of planets in the Koprulu Sector are said to be largely inhospitable to human life. In the original game, only Aiur appears to be anything close to an Earth-like planet, with most other planets supporting permanent human habitation depicted as semi-arid wastelands. According to the backstory presented in the manual, the colony ships were originally sent to colonize a habitable planet that was one year away from Earth, but ended up traveling blindly through space for twenty eight years when their navigation systems shut down. The supercomputer controlling the ships forced them to land on the nearest habitable planets before life support systems failed.
- Expanded universe materials and the sequel game expand the number of Earth-like planets, with Umoja, Agria, and Meinhoff being notable examples. Korhal IV also used to be a nice place to live, but most of the planet was flattened by nuclear missiles before the start of the first game.
- WarCraft only has two explicitly known planets, Azeroth and Draenor, both of which are very earth-like. However, Draenor was ripped apart by magical experiments and drained of its life by demonic influence, which makes it a very alien place... with the exception of Nagrand, which somehow remained untouched. What else is left of the planet still qualifies as earth-like as far as this trope is concerned. Life is still possible (even though half of the zones have no water or any plantlife other than herbs, but that's more an issue of Gameplay and Story Segregation).
- The Lunarian Moon (no, that's not redundant) in Final Fantasy IV has a perfectly breathable atmosphere and its gravity is identical to the heroes' homeworld. Just ignore the man-sized viruses and the killer plates of flan.
- Invoked in Ultima Underworld II, when Iolo expresses concern that one of the facets of the gem might transport you to a planet of poisonous gas or an ocean floor.
- Averted in Sid Meier’s Alpha Centauri—Planet is called "earthlike" at the very beginning, yet some of the many problems you face in the course of colonizing Planet are the poisonous atmosphere (90% nitrogen rather than 78%, combining with higher gravity and a thicker atmosphere to give you nitrogen narcosis if you try to breathe it), inedible plantlife, and hostile (very, very hostile) native fauna. And the game doesn't hide the fact that even such "earthlike" planets are very rare.
- The planet in the Spiritual Successor Civilization: Beyond Earth is also poisonous to humans. Instead of xenofungus, colonists have to deal with poison gases that can easily kill unprotected humans. There are three main approaches to dealing with the planet: Harmony (biologically modifying humans to survive there), Supremacy (turn humans into cyborgs who aren't as susceptible to the hostile environment), and Purity (aggressive Terraforming in order to make the new planet into a second Earth).
- The Escape Velocity games feature all kinds of planets, including gas giants. The most you'll ever see of one, though, is a little pre-rendered image. Many of the ones that you can actually interact with are habitable for one or another reason.
- Star Control 2 averts this trope: it has at least 50 types of planets. The kinds that support (humanoid) life are rare; many—not all—races come from home planets of these types (and among them (water worlds as are known there) are included those with temperatures so high that lack oceans as well as those so cold their water is frozen). Each type has common characteristics—likely minerals, ranges of size and strength of gravity, tectonics, and so on. All but gas giants can be landed on and explored via Lander vehicles.
- Galactic Civilizations II, with all of its expansions in, finally adds toxic, ocean, high grav, barren and other world types. However, you can research technologies to colonise all of them.
- Except possibly on the highest "usable worlds" setting, nearly all of the worlds you encounter are not only not earthlike but completely uninhabitable quality 0 gas giants or tiny balls of scorched or frozen rock. Although there are weird mega-events that occasionally roll through and turn an entire system into medium to high quality potential colonies, regardless of the size or nature of the planets.
- Star Ocean: Till the End of Time is guilty. Although there are a variety of planets not suitable for humans to live on (or those where it'd be very difficult for them to thrive), the main party always conveniently lands on one where they can breathe the air and move around comfortably despite any effects of the gravitational fields.
- Subverted in the Halo series. It's revealed in the EU that it takes decades of terraforming to make a colony world suitable for human use.
- Also the Grunts evolved on a planet with a methane atmosphere.
- Space Colony averts this with no planets capable of supporting life without assistance, the planet range from barren, to volcanic and a few that have a bit ''too' much life on them.
- Averted in Star Ruler where some planets have a charred appearance and are clearly not Earthlike.
- Comprehensively averted in Space Empires IV, in which you set your race's preferred atmosphere and planet type (ice, rock or gas giant) and start out only able to colonise those.
- Averted in the X-Universe. There are a fair number of Earthlike planets (most of which are colonized by the Argon), but plenty are decidedly not Earthlike and still inhabited. The Boron homeworld Nishala is an ocean planet with an ammonia atmosphere, and the Terrans' Lost Colony Aldrin is an airless planetoid that presumably uses pressure domes or the equivalent. The Terrans also have settlements on several objects in the Sol System other than Earth. And that's before you get into the many planets that are absolute uninhabitable hellholes covered in lava flows or what-have-you.
- While Endless Space probably has more Earth-like planets than would be possible in Real Life, there are also a lot more planets that are inhospitable, although colonization is possible after extensive research. Even terraforming may be possible closer to the end of that research branch, although gas giants and asteroid belts (which can also be colonized) can never be turned into hospitable planets.
- The Beta Caeli system in Alien Legacy has not one but two Earth-like planets, named after the Greek goddesses/Titanesses Rhea (mother of the gods) and Gaea (earth goddess). Gaea is the third planet like Earth but lacks a satellite. Despite this, it has plant and animal life, even though it would be difficult for them to evolve without tides. Rhea is the second planet like Venus but has the same conditions and even life forms as Gaea. The game implies the presence of a Moon-sized satellite prevented a Venus-like greenhouse effect. The rest of the planets are similar to Solar System planets. All planets (including satellites and asteroids) except for gas giants are colonizable. However, the late-game reveal that the two habitable planets were seeded by the H'riak may imply that this is not a natural occurance. The existence of the Centaurians also implies a habitable planet in the Alpha Centauri system.
- Averted in No Man's Sky—many of the planets players will encounter will be lifeless rocks, with only 10% of them being viable, earthlike worlds; and of that only 10% of those worlds will have flourishing "garden"-type ecologies. That doesn't necessarily mean that the non-earthlike worlds (or barely-earthlike worlds) will be useless, though: a barren world may have more valuable resources in it than an earthlike world.
- Played with in The Mandate as the Osmani faction had the unfortunate luck of having their colony ship land are a very hazardous world. This forced them to become cyborgs in order to survive.
- Averted in Outpost. No star has an Earth-like planet in that game, at least until you develop terraforming and the friendliest one are those that are Mars with the Serial Numbers Filed Off, with planets there being identical to Real Life Solar System bodies (including even two moons and two minor planets) in everything but surface features. The game's demo showed different planets, but it was one of the features missing on it (see entry for Outpost2 for more.)
- In Spore, all the planets are rocky and have plant life (albeit some very bizarre ones) on them. Justified, as each planet is where your creature will live and evolve.
- Darths & Droids tries to explain this trope as applied to Star Wars. When unexpectedly asked about what Naboo is like, the Game Master automatically responds: "Um... Earth-like?", and the players notice that it's "convenient". Later, when they approach Tatooine, they ask the GM if it's "conveniently Earth-like again", and he hastily assures them that it's a planet-wide desert. Only, for the purposes of this trope, it's still Earth-like enough.
- "So it's Mad Max World?"
- Later subverted somewhat with Naboo, as well, as they figure out that, while it may have an Earth-like surface, its geology would have to be radically different from ours for some of the things they do in the game to be possible.
- Ronin Galaxy: Just another average day in Japa- Whoa! Those are some huge potholes. That look suspiciously like Moon craters...
- In the "GOFOTRON" arc of Sluggy Freelance, the cast visits the Punyverse, Another Dimension consisting of about one hundred planets, all but a few of them inhabited, and packed together within easy traveling distance. The strip actually addresses the oddness of this, with Riff saying, "I've never seen a universe so ... deliberate." Later justified when it's revealed that the Punyverse did not evolve naturally, but was actually created for an alien science project.
- Earthsong justifies this one fairly well - all the aliens encountered are from Earthlike planets for the simple reason that planets must make themselves Earthlike before they are given the secret of supporting life.
- This Irregular Webcomic! strip demonstrates what will happen if this trope didn't exist.
Man, this is the hundredth tide-locked airless rockball orbiting an M-dwarf we've found in a row!You forgot that system with the uninhabitable gas giant with no interesting features whatsoever.
- Nexus Gate: The worlds of Taivas and Cielo.
- Many of the planets and moons in Orion's Arm are rather earthlike, but it justifies it in that most of them were terraformed in some way. However even if you include the terraformed planets they are still greatly outnumbered by the non earthlike planets in this universe, ultimately making this an aversion. The non earthlike planets in Orion's Arm can be anything from dead rock worlds, to planets covered in atmospheres of strange gasses and pressures, yet thriving with (non earthlike) life.
- In the '90s X-Men cartoon, a passing Shiar ship was bored that they had to map out a section of the universe filled with uninhabited worlds. This was the section that Dark Phoenix fried. Note, this is more of a case of softening her Face–Heel Turn, as in the original comics she did destroy inhabited worlds; and some people consider that unforgivable.
- Phineas and Ferb is especially odd — even Mars is Earth-like enough for the human characters to survive, not to mention an asteroid that had air and normal Earthly gravity. Of course, the latter had a milkshake bar on it, so presumably the aliens terraformed it in some way. (It Makes Sense in Context. Sort of.)
- Averted in Star Trek: The Animated Series. Personal environmental force fields allow the crew to explore planets without atmospheres while wearing nothing but a uniform. (A yellow line around each character was easier to animate than spacesuits, which are what the live-action Treks use.)
- In Futurama, the only non-Earthlike planets shown so far are a few moons and asteroids without atmospheres, and one high-gravity (but otherwise Earthlike) planet. Even the world with three giant suns, apart from being a bit warm at full noon, was perfectly livable to humans.
- In StrokerAndHoop the detectives encounter an elaborate con with a woman pretending to be an alien needing sapphires to return to her home planet "Sapphiria". Turns out she really is an alien and needs sapphires. At the end one of the detectives crash lands on an alien planet, sees a jogging alien and immediately tries the same con, claiming to be from the planet "Goldia" and he needs gold. In a few seconds he says "Wait, this isn't oxygen" and collapses.
- Although not an exact example, the discovery of Zarmina (aka Gliese 581 g), the first extrasolar planet potentially capable of supporting Earth-like life, after just under twenty years of searching has some scientists convinced that Earth-like planets are actually pretty common.
- Subverted in that while it could possibly (we don't know for sure yet) support Earth-like life, it's still very different from Earth. Just for a start, the planet is tidally locked around its red dwarf sun, meaning that one side is always facing the sun while the other is in perpetual darkness. This means that the only habitable zone is the terminator, where it would be an unending dusk/twilight. Furthermore, the surface gravity is significantly higher (1.5 g is the current estimate), although still within a (relatively) comfortable range.
- Gliese 581 g's existence has been called into question. Other teams examining the system have been unable to confirm its orbit. That said, the presence of planets potentially like Gliese 581 g has some interesting ramifications for the type of habitable worlds possibly present in the universe. If it's common for larger red dwarfs to have planets like that, then the majority of habitable planets may be those orbiting red dwarfs (since they vastly outnumber more luminous stars).
- Due to observing how life exists in the most extreme environments on Earth, more scientists are beginning to ponder whether perhaps a planet doesn't need to be exactly like Earth to support some form of life, albeit a form of life very different from what we're used to. Carbon-based water-dependent life is likely, even if the secondary elements used (S, P, and in lesser quantities Fe, Ca, Na, etc.) in their biochemistry is different. Europa for example might evolve life in its vast underground ocean, though it would almost certainly be blind and use echolocation, perhaps, for there's no sunlight down there and energy is provided by tidal heating. Unfortunately for them, even if they rose to intelligence, they'd be trapped under a thick worldwide roof of ice and never know of the planet they orbit, their home star, and the wider universe. Carl Sagan even posited that life could arise in the upper atmospheres of gas giants and evolve there, as creatures incredibly different from us. However again, they'd be unlikely to form spacefaring civilizations, for the tool-making, agriculture, metallurgy, etc. that we're capable of would be impossible on their world, or at least be a lot less effective.
- That's not necessarily the case. While they'd be handicapped in terms of fire while only being able to work in a water environment, there are other ways to develop advanced technology that don't require open flames. They might develop technology based much more around the use of electricity, which is easier to conduct in salty sea water, and which has been partially utilized by sea life on Earth. Even fire might not be totally off-limits on ice-covered ocean worlds, since they could presumably carve out water-free chambers in their planet's ice layer (and use flammable materials that can be found in water environments, such as methane hydrates).
- NASA's Kepler space telescope is dedicated to searching for planets around other stars. As of February 2011, after surveying 156,000 stars, it has possibly (these discoveries are hard to be 100% certain about) found five planets that are Earth-like in terms of size and distance from their parent star. About 50 other found planets are much larger than Earth, but do orbit at about the right distance to make life possible (and so some could be — purely hypothetically — gas giants with habitable moons and the like). Considering the total number of possible planets found by Kepler is about 1,200, this seems like very much an aversion of All Planets Being Earthlike. On the other hand, with only 156,000 stars surveyed, and between two and four hundred billion stars in our galaxy, this discovery suggests that the total number of Earth-like planets out there may be very large.
- It's important to keep the limitations present on existing telescopes in mind. One of the reasons why Kepler has found so many "Neptunes" and "Super-Earths" is because they're easier to find than Earth-sized planets (never mind those even smaller than Earth in size). Alas, Earth-size or smaller planets are easier to find when they are close to their stars.
- And on December 2011 scientists announced that Kepler found Kepler-22b, a world twice the size of Earth and parked well within the "Goldilocks" orbit that would allow for water to exist in liquid form (meaning it could generate carbon-based life similar to our own). Even better, the Kepler-62 system announced in April 2013 has two planets around 50% larger than Earth within the habitable zone.
- Enter Kepler-186f, Kepler-438b, and Kepler-442b a trio of worlds announced in March 2014 and January 2015 respectively, very similar in size to Earth and that orbit stars smaller than the Sun within their habitable zones.
- Venus. Often called Earth's "sister planet" due to having an extremely similar mass, and favourable position relative to the Sun. The extreme heat and pressure make surface colonies unlikely (although there's always the possibility of terraforming it in the distant future), but various other surprisingly sensible ideas exist. Notably, at an altitude of 50km, the atmosphere is friendly enough to allow humans to go outside without a pressure suit (although masks would still be needed, and given that the clouds are made of sulfuric acid you'd need protective clothing as well). Serious proposals for floating cities at this altitude have been made.
- Mars. Even if it's smaller than Earth, has a very tenuous atmosphere, no running water to speak of -except maybe very shortly and occasionally-, and low surface temperatures compared with (most of) Earth's regions, it's the friendliest of all Solar System bodies, without accounting for the possibility of terraforming it. Also, while it's still controversial, it could have had an ocean in the distant past and it may become somewhat habitable for a time in the far future, when the Sun's luminosity increases.
- Alien life, if it exists, is likely to have some properties in common with Earth's life, if only because some biological life-strategies are too effective to pass up. Photosynthesis, for example, is an efficient way of acquiring energy, and a large surface area is advantageous for that, so it's likely that some form of "vegetation" with broad structures to collect light (i.e. "leaves") would arise on any world with land-based multicellular organisms.
- Additionally, carbon's rather unique bonding abilities and water's solvent abilities, specific heat and other useful properties are fairly rigorous physical chemistry based reasons for them being necessary for any form of life.
- Despite all the science and fact-based theories out there, the fact of the matter is we will never be able to conclusively pin down how much a planet can differ from Earth itself and still stay Earth-like (for instance, ratio of ocean to land - most of our oxygen comes from the ocean), until we actually find Earth-like worlds to compare (or terraform our own). This is not to say that our theories aren't good science, they are. We just don't have any real-life examples to closely analyze to compare results yet, and there are always surprises ahead no matter how well-checked your math is.
- Compared to before the 1990's, when without solid proof of other exoplanets scientists were still bandying around the possibility that our planetary system itself could be a rare fluke among stars, it's pretty much a statistical certainty that there are other earthlike planets out there that should meet all the criteria to be Earthlike. Whether or not any actually do have life, etc? That's to be seen.
- Averted with the very first exosolar (read: not in our Solar System) planets discovered, the class known as pulsar planets. These planets are orbiting pulsars (a type of neutron star that sends out massive amounts of electromagnetic radiation, generally in the X-ray and gamma wave ranges) and are the likely remains of the core of a companion star which had its layers blown off when the fellow star it was orbiting went supernova, or are the fused-over bits that coalesced after a supernova explosion, and even if they weren't blown to pieces or ejected from their star systems, the radiation sent out by pulsars would blow off any atmosphere and sterilize any rocks still in orbit around these star remains.
- Averted for much of its geological history by the Earth itself, which didn't acquire its atmospheric oxygen until about 2.4 billion years ago, and spent 300-400 million years after that event frozen from pole to pole. "Earth-like" is relative.
- Assuming life on alien worlds evolves at roughly the same rate as it does on Earth, we likely won't ever find an Earth-like planet with native complex multicellular life orbiting a star much larger than about 1.4 solar masses because the amount of time a star spends on the main sequence falls rapidly with increasing mass. At 1.5 solar masses, a star will spend less time on the main sequence than it took for complex multicellular life to evolve on Earth.