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All sorts of different species inhabit the Star Wars universe. How is it that their biological function is compatible with the same atmospheric gases, and that the gravity is the same on all the planets, and the sun is just the right difference from the planet to sustain life?

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This is Plo Koon. Plo is a Jedi master seen in AOTC. Plo wears a breather mask because an oxygen atmosphere is poisonous to him. He wears special goggles because the light on coruscant hurts his eyes and to stop his eyeballs from instantly drying out, blinding him. – Valorum Jan 5 at 19:42
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Note that the planet Yavin (one of only three planets in the original Star Wars) is a gas giant and therefore uninhabitable by humans. The rebel base from which the rebel ships were launched during the Battle of Yavin was located on a moon of Yavin. So right away we see that our brains are biased in remembering only the inhabitable worlds we are shown in the movies and ignoring the uninhabitable. Also recall the cave in the asteroid in Empire required breathing assistance and it seems that the surface of Bespin was not inhabitable - but humans adapted. – Todd Wilcox Jan 5 at 19:54
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There is very little reason to discuss uninhabitable planets in a space opera like Star Wars. What exactly would be the point? "Stuff" pretty much can only happen on planets that are habitable to some degree. – Escoce Jan 5 at 20:03
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In addition to the other comments, in Legends continuity there was also mention of an ancient race that terraformed many worlds to be habitable. I forget the name of the race, but they were the humanoid race that the The Father, The Son, and The Daughter were originally from. As well as Albeloth. All of this occurred over 30,000 years before the movies - long before even the Jedi or The Old Republic was founded. They also created The Maw and the Corellian system. – Omegacron Jan 5 at 20:45
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This is observer bias; we only observe the planets that have life because no one goes to the lifeless planets. A more interesting question is why so many planets in Star Wars are the same terrain over the entire planet. A planet of all deserts, a planet of all swamps, a planet of all glaciers, a planet of all forests... It is an interesting story telling conceit, but not really plausible. – Eric Lippert Jan 5 at 21:27

There's never been any indications that ALL the planets in the Star Wars universe have breathable environments, survivable levels of gravity, etc. We see a lot of those kinds of planets because, well, otherwise our characters wouldn't be on those planets. But we do see gas giants like Yavin and Endor from space.

Yavin in *A New Hope*

There's also plenty of indication that there are planets with radically different environments that still support life. The Jedi Plo Koon, for example, wears a rebreather mask since he doesn't breathe oxygen. While I'm not sure it's ever stated in a canon source, his home planet of Dorin is rich in helium.

enter image description here

As for why there are so many habitable planets, it's not that unrealistic. Star Wars takes place in an entire galaxy. The Milky Way has billions of stars and even more planets. Chances are, there are as many potentially habitable planets in our galaxy.

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@Joshua: eh? Where are you getting such dubious "facts" from? – Martha Jan 5 at 20:20
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@Joshua I don't know how the expected number of planets with oxygen in the atmosphere in the universe could possibly be less than one when one already exists. Even if you say Earth's atmosphere is a 1 in a billion chance, there are 100s of billions of planets just in our galaxy. – TenthJustice Jan 5 at 20:34
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@Joshua "Using measurements from Nasa's Kepler space observatory, scientists led by Erik Petigura at the University of California, Berkeley, estimated that 22% of our galaxy's sun-like stars have rocky planets circling them in the zone where they get roughly the same amount of light energy as Earth receives from the sun. There are around 100bn stars in our galaxy, of which 10% are like the sun." This brings us to probably that at least 2 billion planets are in habitable zone around their sun. And if just 0.1% of those planets have oxygen it is still 2 million planets just in our galaxy. – Vanja Vasiljevic Jan 5 at 20:47
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@Joshua yeah and there are billions or planets that have potential of having oxygen on them in just our galaxy. And there are about 200 billion galaxies in just our our observable universe. Saying that there is just one planet with oxygen atmosphere in whole universe is next to imposible – Vanja Vasiljevic Jan 5 at 21:02
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@Joshua Where did you found that number? If I remember correctly only in our observable galaxies stellar population is 7 x 10^22. – Vanja Vasiljevic Jan 5 at 21:28

In my opinion not all planets in the galaxy far far away are habitable, it's just that we are mostly shown planets that are. There wouldn't be much of a story if everyone on a planet were dead.

an alien wearing a breathing mask and goggles

Kel Dor's species have to wear masks since they breathe something other than oxygen (helium and Dorin gas).

a group of Gungans

Gungans are an amphibious species so they are also quite different from humans

Many planets wouldn't be able to evolve intelligent life. For example, Hoth was an ice planet and home to primitive tauntauns and wampas.

icy Hoth

Another example is Mustafar, which is a volcanic planet.

fiery Mustafar

As for why all species are pretty much the same humanoids with the same biological functions, it wouldn't make a good story if everyone had to wear space suits all the time when they were on other planets. Regarding gravity, it would be weird if Jedi came to some planet with G15 and couldn't move an inch, wouldn't it?

But that's just my take on the thing :D

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1. Hoth clearly did have native species, they just weren't necessarily intelligent native species. 2. "Amphibious" means "able to breath in both water and air", so your statement about Gungans is a tautology at best. Perhaps you meant "aquatic"? – Martha Jan 5 at 20:18
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Yeah my bad. I just coped from wiki without thinking about it. I meant that Gungans are amphibious which makes them different that humans. – Vanja Vasiljevic Jan 5 at 20:23
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Mustafar wouldn't be a nice place to live for humans, but it did have a breathable atmosphere and appeared to have "normal" gravity. – T.J.L. Jan 5 at 20:27
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@Jasen amphibious, amˈfɪbɪəs/, adjective, relating to, living in, or suited for both land and water. – Gusdor Jan 6 at 9:42
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"There wouldn't be much of a story if everyone on a planet were dead." Haha – Lightness Races in Orbit Jan 6 at 12:51

It's now part of the Legends universe and non-canon, but the video game Knights of the Old Republic establishes that

an ancient race called the Rakata used powerful technology to terraform many planets to suit their desires.

This is also the explanation for why many of the planets shown in the films are "single-climate" planets (e.g. desert planet, snow planet, forest planet), something that would be incredibly unlikely in real life.

You can read about the Rakata here: http://starwars.wikia.com/wiki/Rakata, but I highly recommend actually playing the game.

Aside from that, the films and other media cover life-sustaining planets because completely uninhabitable planets are not very interesting settings and Hollywood hates putting masks on actors.

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To be fair, a planet entirely covered in snow is unrealistic whether you've terraformed it to specifications or not. You can't terraform out physics. A global weather control grid, on the other hand, could sufficiently suspend disbelief. – Lightness Races in Orbit Jan 6 at 12:51
    
@LightnessRacesinOrbit: en.wikipedia.org/wiki/Snowball_Earth – Emil Jeřábek Jan 6 at 13:06
    
@LightnessRacesinOrbit it is feasible to have a planet which is completely covered with ice/snow. If the equatorial regions were cold enough, then there would be snow there. Hoth may be so cold at the poles, that no life can survive there. – Jason Hutchinson Jan 6 at 20:08
    
Also remember that much of Earth's seasonality and inconsistent weather patterns is due to its axial tilt. A less tilted (or non-tilted) planet would have much more consistent temperatures, and if a bit further from the sun (or with a less warm sun) could easily hang out in the temperature range that would have consistent snow. (It would probably have to have a variable temperature from -25°C to 5°C, or something like that, so that snow still "snows", but that's entirely feasible.) – Joe Jan 6 at 22:29

They don't all contain human-breathable atmosphere. The Kel Dor breathe helium and Dorin gas, and are poisoned by oxygen-rich environments (plus carbon dioxide and nitrogen in Legends canon). They wear a filter mask to compensate.

In Legends canon, there are a couple of different atmospheric types. Type I is human-breathable. Type II is potentially toxic, but can be managed without aid in the short term. Type III require a breath mask, and Type IV requires a full environment suit.

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Not all of the planets are habitable, as many have said before me, however the story only takes place on these planets with breathable oxygen and gravity similar to (or the same as) Earth's.

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