impossible planets

[Locutus of Bored]

Naboo from 'The Phantom Menace' -

Unless "the planet core" means something different to Gungans than it does to everyone else, the implication was that the entire planet is a fluid filled piece of Swiss Cheese from one side to the other. Skin cancer rates must be through the roof without a magnetic field. I'll grant that maybe when they say "The planet core" they're just generalizing and mean only diving somewhat deeper into the crust of the planet and then coming back up again instead of going through it, because it also doesn't make a whole hell of a lot of sense for the droid army to land on the other side of the planet either when they went totally unopposed. Indeed, the droid's travel time to Theed didn't seem that long, so it couldn't have been that far. Still, you'd think the surface would be pretty damn unstable and constantly sinking anyway with so much porous rock comprising the bulk of the planet's crust.

Pretty much any of the planets from the opening montage of 'Superman Returns' -

Cool as the sequence was with the music and all, it was clear that they just said "screw it, let's toss in some neat looking planets that don't make an ounce of sense." There was the criss-crossed rings planet and the helical orbiting asteroid ringed planet.

The gas giant the long day/night planet was orbiting from 'Pitch Black' -

It had three offset rings, one large one at the equator as you would expect, and then one in the northern hemisphere and one in the southern hemisphere.

 

[Meredith]

Fire and Water from Lexx

 

[The Super Brando]

Pretty much every planet in the Star Wars Universe. All of them seem to be "one climate planets." There's the "ice planet" the "desert planet" the "volcano planet" the "city planet."

And while it could be argued that the volcano planet was a proto planet, planets like Hoth and Tatooine make little sense in the grand scale of things as they're both "single enviroment" planets. Consider how Earth has a wide range of enviroments across its surface, having a planet that's just a desert planet-wide is a little silly.

I don't think this is silly at all. Earth would be a desert planet if not for the tilt on it's axis, allowing seasons.

And what if a planet was farther away from it's sun. It would be an ice ball, and the only areas habitable would be slightly less frozen, but still frozen.

Or what if a planet didn't have several evenly spaced continents. What if it only had a single continent, what if it was a Europe-like continent. Wouldn't that mean it would be mostly forested? Or if it was like Australia wouldn't it be more arid?

In fact, I think one of the coolest things about Earth is that it is so amazingly diverse, and that makes it different from all of the other planets we know about in the universe. That kind of diversity just isn't common. You'd be much more likely to come across a "one climate" planet.

 

[JonathonWally]

Wait, I can do better, here's a paste job from Wookiepedia

Star Wars: Knights of the Old Republic puts parallels between Earth and Tatooine, describing Tatooine as the ancestral home world of Humanity instead of the commonly accepted original home world being Corellia or Coruscant, and describes the climate dying because of unrestrained development and warfare with the Rakata. Lastly, Tatooine is on the galaxy's edge, similar to how Earth is on the edge of the Milky Way galaxy, as shown by the shot of the constellation Orion in the night sky in The Phantom Menace.

What the hell? Earth isn't on the edge of the Milky Way, it's smack in the middle of the stellar disk, about equidistant from the central bulge and the rim, and pretty close to the midpoint of the disk's thickness. If we were way out on the edge, Sol System probably wouldn't have enough heavy elements for large planets to form. Instead, we're right in the galactic "habitable zone" (as it tends to be called, though I find that an overstatement and think "temperate zone" would be a better name), the region of the stellar disk that's believed to be most conducive to the formation of life -- close enough in for heavy elements and large planets, but not so close that those planets are constantly bombarded by comets and radiation as they would be in the denser, more turbulent inner regions of the galaxy.

And Orion is seen in TPM? Whaa? I thought it was a galaxy far, far away. Why would it have our local constellations?

You're free to go edit the wookiepedia page.

 

[Nowhere Man]

Tattooine
Hoth
Coruscant
Mustafar
Kamino

All of those Planets are possible and could have breathable atmosheres. Tattooine: It's a desert planet like Mars and Mars once had an atmosphere that was breatheable unforntunately it was millions of years ago. Tattooine may be a desert, but i;m sure it has some vegitation.

Hoth: It's been pointed out that there are moons and Planets(Pluto is a planet in my mind) that are covered in ice. There could be life in the oceans underneath the ice. Hoth could revolve around the sun at a point where the ice melts for a long enough period of time, so gasses under the surface would seep into the atmosphere. Another explanation could be that Hoth is not always covered in ice and in spring there is vegitation.

Couricant: That's a planet that could have been like Earth and was build up and up, i'd imagine that they have machines that produce oxygen.

Mustafar: Earth was once covered in volcanoes in the early day's and may not have a breathable atmosphere,but that's what the force is for. The sepratists were inside, so that explains that and the droids don't need oxygen.

Komino:That's easy, the whole planet is covered in water. Life alway's evolves from water, it seems to be the magic ingrediant. The life evolved until you get the Kominians,who built platforms to put buildings and houses on. The oxygen from the water and under water plants could create a breathable atmosphere.

As far as other movies or tv. I can't really think of anything.

 

[Trekker4747]

Wait, I can do better, here's a paste job from Wookiepedia

Star Wars: Knights of the Old Republic puts parallels between Earth and Tatooine, describing Tatooine as the ancestral home world of Humanity instead of the commonly accepted original home world being Corellia or Coruscant, and describes the climate dying because of unrestrained development and warfare with the Rakata. Lastly, Tatooine is on the galaxy's edge, similar to how Earth is on the edge of the Milky Way galaxy, as shown by the shot of the constellation Orion in the night sky in The Phantom Menace.

What the hell? Earth isn't on the edge of the Milky Way, it's smack in the middle of the stellar disk, about equidistant from the central bulge and the rim, and pretty close to the midpoint of the disk's thickness. If we were way out on the edge, Sol System probably wouldn't have enough heavy elements for large planets to form. Instead, we're right in the galactic "habitable zone" (as it tends to be called, though I find that an overstatement and think "temperate zone" would be a better name), the region of the stellar disk that's believed to be most conducive to the formation of life -- close enough in for heavy elements and large planets, but not so close that those planets are constantly bombarded by comets and radiation as they would be in the denser, more turbulent inner regions of the galaxy.

And Orion is seen in TPM? Whaa? I thought it was a galaxy far, far away. Why would it have our local constellations?

George Lucas

 

[Christopher]

Pretty much any of the planets from the opening montage of 'Superman Returns' -

Cool as the sequence was with the music and all, it was clear that they just said "screw it, let's toss in some neat looking planets that don't make an ounce of sense." There was the criss-crossed rings planet and the helical orbiting asteroid ringed planet.

Oh, yeah, I hated that. Ridiculously garish and cluttered. The end titles to Star Trek ('09) were almost as bad.

The gas giant the long day/night planet was orbiting from 'Pitch Black' -

It had three offset rings, one large one at the equator as you would expect, and then one in the northern hemisphere and one in the southern hemisphere.

Also the moon's orbital parameters were all wrong. If it were far enough out to have an orbital period as long as that indicated, then the planet wouldn't have been nearly as big in its sky and it wouldn't have been in darkness for nearly as long.

I don't think this is silly at all. Earth would be a desert planet if not for the tilt on it's axis, allowing seasons.

Huh? How do you figure that? We're not a desert planet because we have plenty of water. That's not a function of axial tilt.

And what if a planet was farther away from it's sun. It would be an ice ball, and the only areas habitable would be slightly less frozen, but still frozen.

If you assume it has the same amount of water as Earth, which is a very unwise assumption. There can be planets with hardly any water at all (like Mercury and Venus) or planets that are made mostly of water (like many of Saturn's moons or the exoplanet whose discovery was announced a couple of weeks ago).

Not to mention that suns differ too. An Earthlike distance from a cooler star would be freezing; an Earthlike distance from a hotter star would be roasting. And there are all sorts of other variants possible too. A more distant planet with a denser atmosphere, a higher greenhouse effect, could still be habitable, for instance.

Or what if a planet didn't have several evenly spaced continents. What if it only had a single continent, what if it was a Europe-like continent. Wouldn't that mean it would be mostly forested? Or if it was like Australia wouldn't it be more arid?

Well, Europe isn't technically a continent at all except by the geopolitical conceits of European mapmakers. Geologically speaking, it's more of a large peninsula of Asia. (Really. The Siberian peninsula at the other end of Eurasia is roughly the same size as the whole of Europe, but nobody calls it a continent.)

Aside from that, though, when all of Earth's continents were combined into one mass called Pangaea, the interior was one huge desert, because the moisture from the oceans simply couldn't reach it. Climate is influenced by many complex factors of geography, like where the mountains are, what the airflow and ocean current patterns are like, etc. Every continent has a range of different climates in different regions.

If there were only one small continent the size of Australia or Europe, it would be hard to judge its climate without more information, but even it probably wouldn't have a single invariant climate across the whole thing. Australia and Europe certainly don't.

In fact, I think one of the coolest things about Earth is that it is so amazingly diverse, and that makes it different from all of the other planets we know about in the universe. That kind of diversity just isn't common. You'd be much more likely to come across a "one climate" planet.

But as a previous poster cogently pointed out, those planets aren't habitable. A planet with a biosphere is bound to be more complex than that, both because it would need a more complex environment to produce a biosphere and because a biosphere would in turn make it more complex. A "one climate" planet with people living on it would have to be either dying or artificially settled.

And as I pointed out, the exoplanet discoveries we've made over the past two decades have shown us that we can't make any assumptions about what might or might not exist out there. There's so much we haven't found yet, but the planets we have found have been constant sources of surprise.

All of those Planets are possible and could have breathable atmosheres. Tattooine: It's a desert planet like Mars and Mars once had an atmosphere that was breatheable unforntunately it was millions of years ago.

No, we have no reason to believe Mars ever had a breatheable atmosphere. Yes, it had a denser and wetter atmosphere once -- and that was billions of years ago, not millions -- but breatheable means containing oxygen, and oxygen is only produced in such large quantities by life.

Also, when it did have a denser atmosphere, it wasn't a desert yet.

Komino:That's easy, the whole planet is covered in water. Life alway's evolves from water, it seems to be the magic ingrediant.

Actually it's not as simple as that. Oceans can be deserts too. ("Desert" doesn't mean hot or dry, it fundamentally means "deserted," lacking life.) On land, you get a desert when there isn't enough water. In the sea, you get a desert -- a zone where almost nothing lives -- when there isn't enough sediment runoff from the land. (Lovely symmetry there, eh?) Oceanic life needs minerals to sustain itself, to build bodies from. A water world without land masses might be just as barren as a rocky world with no water. Or at least, any life that existed on such a world might be limited to the depths, around thermal vents spewing minerals into the water from the molten mantle, like the places where life first arose on Earth.

But the Kaminoans aren't sea creatures. They're extremely thin, lanky bipeds that walk on land. Something that skinny couldn't swim, and life on a world without land would have no reason to evolve into a bipedal form. Clearly Kamino can't be the world they evolved on. They'd have to be settlers from somewhere else.

 

[Gotham Central]

Pretty much any of the planets from the opening montage of 'Superman Returns' -

Cool as the sequence was with the music and all, it was clear that they just said "screw it, let's toss in some neat looking planets that don't make an ounce of sense." There was the criss-crossed rings planet and the helical orbiting asteroid ringed planet.

Oh, yeah, I hated that. Ridiculously garish and cluttered. The end titles to Star Trek ('09) were almost as bad.

The gas giant the long day/night planet was orbiting from 'Pitch Black' -

It had three offset rings, one large one at the equator as you would expect, and then one in the northern hemisphere and one in the southern hemisphere.

Also the moon's orbital parameters were all wrong. If it were far enough out to have an orbital period as long as that indicated, then the planet wouldn't have been nearly as big in its sky and it wouldn't have been in darkness for nearly as long.




Huh? How do you figure that? We're not a desert planet because we have plenty of water. That's not a function of axial tilt.



If you assume it has the same amount of water as Earth, which is a very unwise assumption. There can be planets with hardly any water at all (like Mercury and Venus) or planets that are made mostly of water (like many of Saturn's moons or the exoplanet whose discovery was announced a couple of weeks ago).

Not to mention that suns differ too. An Earthlike distance from a cooler star would be freezing; an Earthlike distance from a hotter star would be roasting. And there are all sorts of other variants possible too. A more distant planet with a denser atmosphere, a higher greenhouse effect, could still be habitable, for instance.




Well, Europe isn't technically a continent at all except by the geopolitical conceits of European mapmakers. Geologically speaking, it's more of a large peninsula of Asia. (Really. The Siberian peninsula at the other end of Eurasia is roughly the same size as the whole of Europe, but nobody calls it a continent.)

Aside from that, though, when all of Earth's continents were combined into one mass called Pangaea, the interior was one huge desert, because the moisture from the oceans simply couldn't reach it. Climate is influenced by many complex factors of geography, like where the mountains are, what the airflow and ocean current patterns are like, etc. Every continent has a range of different climates in different regions.

If there were only one small continent the size of Australia or Europe, it would be hard to judge its climate without more information, but even it probably wouldn't have a single invariant climate across the whole thing. Australia and Europe certainly don't.



But as a previous poster cogently pointed out, those planets aren't habitable. A planet with a biosphere is bound to be more complex than that, both because it would need a more complex environment to produce a biosphere and because a biosphere would in turn make it more complex. A "one climate" planet with people living on it would have to be either dying or artificially settled.

And as I pointed out, the exoplanet discoveries we've made over the past two decades have shown us that we can't make any assumptions about what might or might not exist out there. There's so much we haven't found yet, but the planets we have found have been constant sources of surprise.

All of those Planets are possible and could have breathable atmosheres. Tattooine: It's a desert planet like Mars and Mars once had an atmosphere that was breatheable unforntunately it was millions of years ago.

No, we have no reason to believe Mars ever had a breatheable atmosphere. Yes, it had a denser and wetter atmosphere once -- and that was billions of years ago, not millions -- but breatheable means containing oxygen, and oxygen is only produced in such large quantities by life.

Also, when it did have a denser atmosphere, it wasn't a desert yet.

Komino:That's easy, the whole planet is covered in water. Life alway's evolves from water, it seems to be the magic ingrediant.

Actually it's not as simple as that. Oceans can be deserts too. ("Desert" doesn't mean hot or dry, it fundamentally means "deserted," lacking life.) On land, you get a desert when there isn't enough water. In the sea, you get a desert -- a zone where almost nothing lives -- when there isn't enough sediment runoff from the land. (Lovely symmetry there, eh?) Oceanic life needs minerals to sustain itself, to build bodies from. A water world without land masses might be just as barren as a rocky world with no water. Or at least, any life that existed on such a world might be limited to the depths, around thermal vents spewing minerals into the water from the molten mantle, like the places where life first arose on Earth.

But the Kaminoans aren't sea creatures. They're extremely thin, lanky bipeds that walk on land. Something that skinny couldn't swim, and life on a world without land would have no reason to evolve into a bipedal form. Clearly Kamino can't be the world they evolved on. They'd have to be settlers from somewhere else.

Actually desert pretty exclusively has to do with dryness. A place is called a desert because of the absense of water not life. After all, most deserts on Earth are not lifeless.

 

[Christopher]

Actually desert pretty exclusively has to do with dryness. A place is called a desert because of the absense of water not life. After all, most deserts on Earth are not lifeless.

Then why was Gilligan's jungle island referred to as "an uncharted desert isle?" It's basic etymology. The word desert comes from the Latin dēserere, "to forsake or abandon." "Desert" means "deserted." It meant empty and abandoned before it meant dry and arid. It took on the connotation of a dry wasteland by association, but that's not where the word originally came from. We rarely use "desert" to mean "deserted" anymore, but it lives on as a fossil usage in the expression "desert island," which does not mean a dry and arid island but rather an uninhabited island.

And if you'll check the Dictionary.com link I provided above, you'll see that it does list the following definitions for "desert" (emphasis added):

A barren or desolate area, especially:

a. A dry, often sandy region of little rainfall, extreme temperatures, and sparse vegetation.

b. A region of permanent cold that is largely or entirely devoid of life.

c. An apparently lifeless area of water.

Heck, I thought I was using the idea of an oceanic "desert" in a figurative way to illustrate my point, but it turns out it's a valid and recognized usage. Desert means barren, lifeless. On land, barren means lacking water. But in water, barren means lacking minerals and nutrients. So yeah, oceans have deserts too.

 

[Nowhere Man]

Wherevertheres water,there's life. That is the case on Earth and most likely the case in the rest of the universe. The life may be microbeable, but it is life. I;m just sugesting that these planets are possible,not that they are probable. The truth is that we have no idea how the Kominians evolved, we evolved from small mouse like creatures that look nothing like us. The other thing that we don't know is how old these planets are and whether not these planets were colonized. Maybe the Kominians came from somewhere else and just colonized Komino or just used Komino for Cloning.

 

[Christopher]

^Yes, but we evolved to fit our environment. That's how it works. Evolution is a process of adaptation to environmental demands. There is no reason in hell why a planet with no solid surfaces would produce a bipedal humanoid species, let alone one too skinny to float or swim. It's not impossible for some kind of life to exist on a world without land, but it's certainly impossible for the Kaminoans to be native to such a planet.

 

[DWF]

I'm surprised nobody's mentioned the impossible planet from Dr. Who's Impossible Planet/Satan Pit two parter, it was a planet in a fixed position to a black hole.

 

[Timelord79 (he/him)]

Yes an everyone in that epsiode commented on it being impossible.
In truth it was artificial. made by beings to old for a time lord to remember.

 

[Myasishchev]

But the Kaminoans aren't sea creatures. They're extremely thin, lanky bipeds that walk on land. Something that skinny couldn't swim, and life on a world without land would have no reason to evolve into a bipedal form. Clearly Kamino can't be the world they evolved on. They'd have to be settlers from somewhere else.

Alternatively, there are five big continents on the other side of Kamino and the face seen in that wookipedia article is just displaying the Kaminoan Superpacific. Although you'd think there'd be an island or two.

Or maybe it isn't a Superpacific. That lanky body structure kind of might suggest low gravity. What's the lower limit for O2/CO2/N2 atmosphere retention? Mars didn't keep the bulk of its N2, but it's easy to forget, with all the mystique attached to the planet, that Mars is truly tiny. Not a third of our surface area, barely a tenth of our mass. Even if we magically imported a breathable atmosphere and water to it for a few thousand years, we still couldn't fit more than two or three billion people on it--and they'd all get skin cancer.

On the minus side, even a planet as big as Venus (more than 90% Earth mass) doesn't have proper plate tectonics. Some stuff I've read recently seems to indicate that Earth might be at the cutoff for a planet to have regular geological activity induced by its own weight. Although afaik Venus would have a magnetic field if it had a decent rotational period. Fortunately, a lot (all?) of the terrestrial planets we can see so far are significantly bigger than Earth, and should have no problem in any respect.

As for impossible planetary ecologies, I'd like to ask what the hell is up with the terrestrial life on Pandora. We have the Na'vi, with a quadrupedal chordate body plan with an airway above the foodway like ours and every other vertebrate--on Earth--and then we have all the other verebrate Pandorans--which have airways beneath the foodway and are sextupedal. It's not impossible, just really strange that two, fully different vertebrate phyla would have grown up without one (my money's on the quadrupeds) outcompeting the other entirely. Additionally, have any of the ancillary materials identified the poisonous and/or asphyxiating gas that apparently permeates Pandora's atmosphere? If the intent was to suggest a non-O2 atmosphere, they failed. Oxygen is very obviously the driver of the running-jumping-fighting metabolism on Pandora.

Edit: and I almost forgot the psychotically implausible network between creatures. It's just remotely possible (not plausible) that one species of pseudo-plant or their pseudo-horses might co-evolve with the Na'vi to provide a repository for their memories in exchange for upkeep--it would require the Na'vi to be stupid enough to maintain a steady state civilization for several million years to permit that to occur--but everything, even wild animals, have these USB connections. That's where Avatar went right from science fiction to out-and-out fantasy. It was a story about elves fighting U.S. foreign policy.

 

[Christopher]

On the minus side, even a planet as big as Venus (more than 90% Earth mass) doesn't have proper plate tectonics. Some stuff I've read recently seems to indicate that Earth might be at the cutoff for a planet to have regular geological activity induced by its own weight. Although afaik Venus would have a magnetic field if it had a decent rotational period.

I've read that a planet as small as Venus could have plate tectonics if it had oceans to provide the pressure and lubrication that drive the process. So yes, Earth is close to the theorized cutoff point, but Earth and Venus are very nearly the same size, and there's sure to be a degree of wiggle room.

Fortunately, a lot (all?) of the terrestrial planets we can see so far are significantly bigger than Earth, and should have no problem in any respect.

That's a misleading statistic, since our detection methods are not yet sensitive enough to find planets as small as Earth or smaller. So the fact that all known exoplanets are at least several times larger than Earth is an artifact of the limitations on our technology, and cannot be taken as evidence of the actual ratio of planet sizes.

However, it is reasonable to conclude from existing evidence that superterrestrial planets are common in the galaxy, regardless of whether they're the majority.

Also, subterrestrial bodies could have plate tectonics if they were tidally kneaded moons of massive Jovians. Since Jovians in stellar habitable zones are now known to be common, we can't rule out the possibility of life evolving on low-gravity worlds.

As for impossible planetary ecologies, I'd like to ask what the hell is up with the terrestrial life on Pandora. We have the Na'vi, with a quadrupedal chordate body plan with an airway above the foodway like ours and every other vertebrate--on Earth--and then we have all the other verebrate Pandorans--which have airways beneath the foodway and are sextupedal. It's not impossible, just really strange that two, fully different vertebrate phyla would have grown up without one (my money's on the quadrupeds) outcompeting the other entirely.

Clearly, Pandora's evolution shaped the Na'vi into the necessary form for human colonists to find them sexually desirable, because... umm... Ah, hell, a wizard did it.

Additionally, have any of the ancillary materials identified the poisonous and/or asphyxiating gas that apparently permeates Pandora's atmosphere? If the intent was to suggest a non-O2 atmosphere, they failed. Oxygen is very obviously the driver of the running-jumping-fighting metabolism on Pandora.

My guess is it's carbon dioxide. Even with adequate levels of oxygen in an atmosphere, an excess of carbon dioxide would still cause asphyxiation in humans. When people suffocate, it's the CO2 poisoning that gets them long before the oxygen runs out. And it's entirely plausible that an otherwise Earthlike planet could have CO2 levels much higher than ours currently are. Indeed, some of the nastier scenarios for the consequences of global warming involve the release of suffocating levels of CO2 from oceanic clathrates or some such things.

 

[JonathonWally]

As for impossible planetary ecologies, I'd like to ask what the hell is up with the terrestrial life on Pandora. We have the Na'vi, with a quadrupedal chordate body plan with an airway above the foodway like ours and every other vertebrate--on Earth--and then we have all the other verebrate Pandorans--which have airways beneath the foodway and are sextupedal. It's not impossible, just really strange that two, fully different vertebrate phyla would have grown up without one (my money's on the quadrupeds) outcompeting the other entirely.

Clearly, Pandora's evolution shaped the Na'vi into the necessary form for human colonists to find them sexually desirable, because... umm... Ah, hell, a wizard did it.

Additionally, have any of the ancillary materials identified the poisonous and/or asphyxiating gas that apparently permeates Pandora's atmosphere? If the intent was to suggest a non-O2 atmosphere, they failed. Oxygen is very obviously the driver of the running-jumping-fighting metabolism on Pandora.

My guess is it's carbon dioxide. Even with adequate levels of oxygen in an atmosphere, an excess of carbon dioxide would still cause asphyxiation in humans. When people suffocate, it's the CO2 poisoning that gets them long before the oxygen runs out. And it's entirely plausible that an otherwise Earthlike planet could have CO2 levels much higher than ours currently are. Indeed, some of the nastier scenarios for the consequences of global warming involve the release of suffocating levels of CO2 from oceanic clathrates or some such things.

I don't think it's too out of the blue to think that most of the life on Pandora and Pandora itself was "intelligently designed"

And I don't mean by a "God" so much as something probably engineered everything there.

From their Tree God thing showing kind of all-knowing intelligence, to most lifeforms on the planet having built-in USB ports to allow "data transfer"

I mean, even the atmosphere isn't that toxic. It looks to be just outside of the human breathable range. Considering humans don't have to wear anything protective, just an oxygen mask. And they can obviously stand outside and just hold their breath.

My guess, since there's all that lush plant life, it's probably not excess C02 but most likely nitrogen or argon.

 

[Christopher]

I think that excess nitrogen or argon would have a narcotic effect, not a toxic one per se. I haven't seen the film yet; how was the effect of the Pandoran atmosphere described/depicted?

And since plants thrive on CO2, couldn't high levels of that gas cause the lush plant life?

 

[Hyperspace05]

Additionally, have any of the ancillary materials identified the poisonous and/or asphyxiating gas that apparently permeates Pandora's atmosphere? If the intent was to suggest a non-O2 atmosphere, they failed. Oxygen is very obviously the driver of the running-jumping-fighting metabolism on Pandora.

Yes, they have. The 'Avatar: activist survival guide' has that information. Here is a quote from it:

"... The nitrogen-oxygen atmosphere is 20% denser than our own. It [also] contains so much carbon dioxide (more than 18%) that humans who breathe it directly will rapidly lose consciousness and die. ... "

It is a neat little book - here is some other info on Pandora:

World	Diameter (km)	Mass	Surface gravity	Atmospheric density   Surface pressure
Earth	12756.2	        1	1	        1	              1
Pandora	11447	        0.72	0.8	        1.2	              1.1

 

[Trekker4747]

Yeah, it was my interpetation that the air on Pandora wasn't "toxic" so much as it was just "not breathable" to humans, Jake was exposed to it, and breathed, it for a short time before chickie got his mask to him and he weathered the experience nicely as-if he wasn't exposed to anything harmful. So the air on Pandora was just simply not oxygen and nitrogen it was probably oxygen and some other element that can't be "breathed" by humans.

I don't see why the location of their nose to their mouth should have any effect on suggestion on how evolution occured on that planet considering the diverse locations of airways and such that are located on animals just on Earth. Nor do I think the number of limbs they have means anything. Evolution is odd.

 

[Locutus of Bored]

I think that excess nitrogen or argon would have a narcotic effect, not a toxic one per se. I haven't seen the film yet; how was the effect of the Pandoran atmosphere described/depicted?

You can hold your breath for however long you personally are able to do so with no harmful effect, but if you start breathing you begin gasping immediately. According to the orientation briefing when they landed on Pandora after twenty seconds of breathing you lose consciousness, and after four minutes you die.

You only need a rebreather mask to survive and the atmosphere has no noticeable effect on bare skin. Soldiers run around in t-shirts in the jungle with no apparent problems.