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So I was watching Issac Arthur videos recently, about colonizing the other planets of our solar system. And it was pointed out that Neptune and Venus have atmospheres and gravity such that an airship could be built that would float in the atmosphere, and allow people inside to live in earth-like gravity.

Which got me to thinking- those airship cities would be so much cooler if people could go outside, stand on the deck, and breath the atmosphere.

But I'm not much good with science. So is it possible, scientifically speaking, for a gas giant / ice giant to have an atmosphere people could breath, at a gravity people could be comfortable in? Is there a possible gas giant / ice giant configuration that would allow that at some point?

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    $\begingroup$ We know that Earth only has oxygen in its atmosphere because it was put there by life. The scientific majority believe that an oxygen atmosphere cannot form without life (although there are some scientists who disagree). It's hard to imagine (but of course not impossible) that life would form on a gas giant. $\endgroup$ – cowlinator Nov 6 '20 at 0:23
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    $\begingroup$ Does this answer your question? Can a gas giant have its own habitable zone? $\endgroup$ – sphennings Nov 6 '20 at 1:20
  • $\begingroup$ There have been some carefully worked out speculations. Sagan's "Cosmos" included some stuff on this. I don't have the book at hand so can't give chapter citations. $\endgroup$ – puppetsock Nov 6 '20 at 1:22
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    $\begingroup$ @sphennings question does not appear to be a duplicate, the linked question is about whether a gas giant can act as a mini-star to warm its satellites, not whether the gas giant's own atmosphere is habitable. $\endgroup$ – KerrAvon2055 Nov 6 '20 at 1:28
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    $\begingroup$ Not a duplicate IMO. This is about a gas giant being habitable in itself, the other question is about gas giants/brown dwarfs having a habitable zone around them where an orbiting satellite would be habitable. $\endgroup$ – Monty Wild♦ Nov 6 '20 at 1:46
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Oxygen is one of the most common elements in the universe and in planets. The Earth is nearly 30% oxygen in mass - and I mean the whole planet, not just the atmosphere.

However, there are a few problems when it comes to having available oxygen in a gas giant:

  1. Oxygen is one of the most reactive elements in the table. Not being a picker, it will bind to almost anything. And it will stay bound. Most of the oxygen in Jupiter for example is in water molecules.

  2. Oxygen may be one of the most common elements in the universe as I said above, but it is still far behind hydrogen. Gas giants are mostly hydrogen, so practically all the oxygen will bind to that to form water.

  3. Then there are ice giants, like Neptune and Uranus. According to Wikipedia, an ice giant is "a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur." But then the article for Uranus states that its atmosphere is still mostly hydrogen and helium, with some methane in it. Oxygen is about as heavy as methane, but since it binds to other atoms it becomes heavy and precipitates, so you would only find oxygen at depths were you'd die from other causes. Even then it would not be O2.

The reason we have so much breathable oxygen in our own atmosphere is basically a fluke. If you have that much oxygen in a lifeless planet it is unlikely to develop life because oxygen is toxic, speciallly to microbes. We had life kickstarted in oceans, where oxygen concentrations are low, and that life oxygenated the world. Thay also led to many lifeforms disappearing due to the toxicity involved but hey, you can't have an omelet without causing a major extinction event.

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Welcome to Worldbuilding SE, my friend! Although I am a bit of a newbie to the site myself, I think you will fit in here fine!

This is a very basic answer but here goes nothing lol...

TL;DR

No, we don’t have enough oxygen.

Long version:

The only reason we have oxygen here on earth is because life brought it, as pointed out by @cowlinator.

Thus, we would have to artificially introduce enough oxygen to support life. The problem is we don’t really have enough oxygen to spare.

Neptune is ~17 times larger than Earth, so we would need possibly 20-30x the amount of oxygen, which we don’t have...

There are many ways to make oxygen, but all of the, require pre-existing oxygen:

  • Photosynthesis requires carbon dioxide.

  • That cool manganese/hydrogen peroxide trick, once again, requires hydrogen peroxide.

  • etc. etc.

So the only conceivable way to do this would be through subatomic rearrangements, where you rearrange the electrons, protons and neutrons in an atom to create a new one. Since this is completely impossible by today’s technology, I’m going to have to say...

No, it not possible...

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  • $\begingroup$ Gas giants atmosphere is mostly hydrogen. So even if you do get the oxygen, all that will happen is the gas giant will become a somewhat-less-giant water world. $\endgroup$ – user79911 Nov 6 '20 at 4:50
  • $\begingroup$ 'all of the, require pre-existing oxygen' - I suspect the distribution of basic elements is more or less flat across all the planets. The Earth isn't the oxygen magnet, is it? It merely has some of its oxygen moved from compounds to the atmosphere. $\endgroup$ – Igor G Nov 6 '20 at 8:33
  • $\begingroup$ The author had asked if such a planet can exist, not if we humans can make such a planet with technology. $\endgroup$ – Darth Biomech Nov 7 '20 at 11:04
  • $\begingroup$ @DarthBiomech, I think you are right, but when I read the question it sounded like he was specifying Neptune... $\endgroup$ – fartgeek Nov 7 '20 at 22:53

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