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So, after multiple users informed me that my previous question was a little to broad, I decided to narrow my focus a little bit.

Using only hard science, could there be a planet composed, with the exception of a small, rocky core, entirely out of gas, that could support human life? I don't just mean one in which spacecraft could venture. No, I mean a fully breathable, habitable, earth-like world; One which you could live in without any sort of advanced equipment (assuming you had food to eat, water to drink, and solid ground to stand on). This doesn't mean the entire planet has to be livable; Just a small, verticle sliver or localised area would be fine.

The main things to take into consideration are...

  • Radiation: The solar wind streaming through space would normally turn the planet into an inhospitable wasteland. This radiation would, somehow, have to be low enough that a stable population could exist, even if it suffered from higher rates of birth defects and a shorter average lifespan.

  • Gravity: The reason I said "Planet made of gas" instead of "Gas giant" is because any human on the latter would crumple under their own weight. To avoid this, the new planet would have to be comparitively small, making the gravity roughly that of Earth's, so that the denizens wouldn't be to stout/elongated, and of more normal preportions.

  • Pressure: The pressure has to be within reasonable means for a human to survive, although, in the case of scuba divers, the addition of certain gases would increase this.

  • Air: Gases, especially reactive ones like oxygen, don't like to stay in neat, precise bands but instead mingle. It would have to be a high enough percentage for a working, active person to safely breath for years on end, and of course free of any dangerous or toxic substances like Carbon Dioxide.

  • Cold: The level of heat, due the specific story I'm working with, is more variable, with anything that a society living today endures acceptable.

Please comment if you need any clarification.

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  • $\begingroup$ It's worth pointing out that gravity on Saturn is only slightly higher than Earth. I don't remember the exact number, but it was something like if you weighed 85 pounds on Earth, you'd weigh 96 pounds on Saturn. So you could potentially have a gas giant that doesn't crush you. $\endgroup$ – AndyD273 Jan 1 '16 at 9:12
  • $\begingroup$ Do you think the hard-science tag would be appropriate for your question? $\endgroup$ – HDE 226868 Jan 1 '16 at 15:07
  • $\begingroup$ "free of any dangerous or toxic substances like Carbon Dioxide" That would be a hard requirement as oxygen itself is rather toxic given sufficient dosage. Carbon Dioxide is also an important part of what we breath out. Our planet's air consists of 78% nitrogen, 21% oxygen and a lot of trace elements, one of which is carbon dioxide at 0.04%. What you want to do is mimic the concentrations on earth. Onto another point, reactivity of an element and how it "likes to mingle" are not related, the "mingle" factor is dependent mostly on temperature. $\endgroup$ – Selenog Jan 4 '16 at 13:24
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From a hard science perspective, a planet made of gas that is small enough to avoid the problems associated with a gas giant would be too small to hold a gaseous envelope unless it was extremely cold (in which case gasses like oxygen and nitrogen would have frozen out, leaving you with an atmosphere of hydrogen and helium).

The alternative would be to look at engineered worlds instead. The closest thing to what you are looking for that I can find is the "gravitational balloon", which is pretty much what it sounds like: a balloon filled with breathable gasses which is inflated inside an asteroid and uses the gravitational "pressure" of the asteroid to maintain its integrity (not to mention the mass of the asteroid provides shielding, protection from micrometers and a degree of temperature control). There is an entire site devoted to the mechanics and mathematical exploration of the idea here: http://gravitationalballoon.blogspot.ca

Note that you can build a gravitational space balloon as large or small as you like, the author envisions large asteroids riddled with voids filled with these balloons and housing billions of people, but there will be a range of balloons of all sizes depending on the materials available, population to be housed and the political, social and economic conditions of the setting.

The ultimate size of a gravitational balloon was calculated by Dani Eder, and is pretty fantastic. Technically, this is a construct entirely filled with gas (a central core of hydrogen in this case), using some membranes to prevent the gasses from mixing and an outer shell of steel to provide counter pressure for the balloon, the maximum size is 240,000km in radius, providing something like 1400 X the usable surface area of Earth, plenty of room for your setting.... http://yarchive.net/space/exotic/bubbleworld.html

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  • $\begingroup$ My understanding is that Saturn's atmosphere isn't breathable but otherwise habitable in a balloon. $\endgroup$ – Loren Pechtel Jan 1 '16 at 22:19
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Do you actually require a planet? If not, read The Integral Trees and The Smoke Ring by Larry Niven. Both are set in a gas torus being pulled off an uninhabitable planet, the whole thing kept in check by being pretty close to a neutron star. It's not permanently stable and probably couldn't evolve an intelligent species before it dissipates but if it were seeded from elsewhere things could evolve.

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  • $\begingroup$ This might be better as a comment. $\endgroup$ – HDE 226868 Jan 3 '16 at 16:53
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Radiation

Need not be a problem, if the planet orbits quiet, settled down star. Note that even in our Solar system, Sun radiation is pretty survivable unless there is a solar flare. So you are looking at K or M stars (you might wait a trillion years or so for the flares to settle down — but on the upside, you'd have very stable star with many trillions of years ahead).

Alternatively, change your "small rocky core" to a "small metallic core" and you can have a full featured magnetic field, making it a non-issue even for Sun-like stars.

Gravity

Let's take Saturn as a model — its "surface" gravity is $10.44 m s^{-2}$, which is practically the same as Earth. Note that gravity does not depend on the size of the planet alone, but on its mass (linear) and radius (inverse square). Assuming compatible density distribution (which is not true, especially when under pressure gasses undergo phase change into metallic form, but anyway), mass depends on the volume, which is radius cubed. $r^3$ for mass is cancelled by $r^{-2}$ for the radius and the dependency is nicely linear — for planets with the same density, gravity increases linearly with the radius.

Since the mass is density times volume, gravity depends linearly on density (at the same radius). Therefore, if you increase the (average) density of the planet, you need to scale down the radius by the same amount to keep the gravity the same.

Pressure

No problem, just pick the height you need. Remember the barometric formula. Changing temperature will make it more complicated, but you can still pick up comfortable pressure.

Air

This is where things become interesting. Let's assume (for simplicity) a planet made of oxygen. Taking Saturn as an example, it consists mostly of hydrogen, which has a density $0.09 kg\cdot m^{-3}$ (at 1 bar and "normal" temperature). Oxygen's density is $1.4 g\cdot m^{-3}$ (its atomic weight (16) times the hydrogen density). Thus in order to keep the gravity at $1g$, recall that the planet's radius should be 16 times smaller than Saturn's — that is $60000 km/16 \approx 3750 km$, i.e. about 60% of Earth.

Now, you do not want to live at 1 bar of pure oxygen atmosphere — it is unhealthy and dangerous — anything combustible will combust rather violenly. You'd need 0.2 bar partial oxygen pressure, which means moving upwards just by 12km or so.

Or you can mix in 75% nitrogen, and it will not change the numbers much and make the atmosphere very Earthlike.

Cold

Again, let's take Saturn — at 1 bar pressure, the temperature is 134 K, way too low for unaided human life. But since the planet's gravity and dimension is quite comparable to the Earth, so just increase the insolation to Earth level (or slightly more, due to the absence of greenhouse gasses).

To summarize

You have a planet made of oxygen (and some other gasses, like water vapour which you'd rather need for survival), slightly (60%) smaller than Earth, with Earth like insolation. It's perfectly habitable, if you manage to float at the correct level (i.e. using hydrogen or helium balloons). If you drop anything, it will be crushed by the pressure — but long before that, increasing oxygen pressure will make it burn (if combustible) rather explosively. At the pressures at the centre, oxygen will likely turn into metallic form, which is going to create some magnetic field, useful (but not absolutely necessary) to protect from solar flares.

Now the question is, could such a planet occur naturally? The answer is almost certainly no. You'd need a protoplanetary disc consisting primarily of oxygen (and nitrogen), but no or little hydrogen (which will burn in the oxygen planet, providing much needed water vapour). It is difficult to imagine such circumstances. The planet would have to be deliberately constructed. Also, any life will lack solid materials (such as carbon), since everything will fall down to the core (although, carbon will combust and at least part of it will be recycled in the form of $CO_2$). It would have to be imported and kept floating.

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  • $\begingroup$ Great answer, but are you saying that the planet, which is made out of gas, would only have to have an area 60% of Earth's, a planet made of rock and metal, to share the same gravity? $\endgroup$ – Cthulu472 Jan 3 '16 at 16:47
  • $\begingroup$ @Cthulu472 (radius, not area) I agree it's strange, but taking Saturn's mean density of $0.687 g/cm^3$ and multiplying by 16 you get the density of $11 g/cm^3$ - about twice as much as Earth's. On one hand, gas is compressible really well, liquid metal (making much of Earth interior) not so much. On the other hand, oxygen would turn to metallic state somewhere deep and thus not be a true gas anymore (but still it could well be more compressible than liquid iron). On the gripping hand, I've neglected the existence of solid Saturn core. $\endgroup$ – Radovan Garabík Jan 6 '16 at 18:26
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You can forget about radiation and gravity as problems, if you want to. As others have pointed out already, a magnetic field could deflect the radiation (just like Earth's), and gravity at the surface of a gas giant isn't necessarily that extreme.

As for the other factors: it seems that what you'd need is a situation where pressure is approximately one bar, gravity is approximately on G, and oxygen content is approximately 20%, and for all of these conditions to occur at the same altitude within the planet's atmosphere. Saturn actually comes reasonably close, except for the oxygen content (it has practically none).

Making a semi-educated extrapolation, we could guess that a planetary body could exist that would fulfill all your requirements. It would be smaller than Saturn, closer to its sun, and have an atmosphere more similar to Earth's, with nitrogen, oxygen, water vapor, and carbon dioxide. Humans could survive there by floating around in the layer where pressure ~ 1 bar. I don't know if there is a plausible way for such a planet to form; it certainly hasn't happened in our solar system. If you can't think of anything either, just say aliens did it, or ignore the question altogether.

One would assume that, in order for humans to live in this environment, there would have to be (at least) some kind of plant that has adapted to live here, too. An organism like a living zeppelin, which breaks down water molecules using photosynthesis and collects the hydrogen inside a membrane for buoyancy, could provide both a food source and a stable platform for people to float on; a rich enough ecosystem of these zeppelin-plants would allow humans to survive with any level of technology down to and including zero. If tapped carefully, a large enough zeppelin-tree could be made to "leak" a small amount of hydrogen to be used as a cooking flame or energy source. I say "carefully" because tapping hydrogen faster than it can be replenished would cause the tree to sink, and of course if it overheats, the whole thing could go down like the Hindenburg.

Interesting features of this world:

  • It's much, much bigger than Earth. Habitable surface at least 100x greater.

  • Metals and heavier elements, even trace metals to support body chemistry, may be hard to come by, since they normally don't float around in the air. Perhaps some of the largest, oldest "trees" have sunk roots all the way to the surface, or close enough to gather some of these trace minerals and transport them to their leaves, which humans must then eat in order to get calcium for their bones, iron for their blood, etc. This would also tend to keep technology limited to naturally-occurring materials. Human bone might be the hardest substance available, which would have interesting cultural implications. ^o^

  • The most primitive people would have no control over their movement through the atmosphere; they'd float around wherever the air currents take them, drifting with the trees. They could thus experience more extreme seasonal variations than if they existed at a fixed latitude, because they drift sometimes near the polar regions, sometimes near the equator.

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