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How probable is it that a gas giant planet made of "air" (roughly the same composition as Earth's atmosphere) could exist?

What would it be like? How would it differ from Earth?

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    $\begingroup$ Just a small precision, it would still need to have a solid core. Aside form that, the ''air'' would be located far from the core because it's light compared to other gases. The density decrease as the altitude increase. I'm not sure it's even possible to have a layer made of air thought. $\endgroup$ – Vincent Mar 11 '15 at 23:25
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There are two parts to this question:

  1. Could a massive planet amass large quantities of nitrogen and oxygen?
  2. Could these gases be the main ones the planet is composed of?

Giant planets relatively close to the central star (but beyond the frost line) will accrete large quantities of hydrogen and helium, which were the major components of the initial gas and dust that surrounded the Sun when it first formed. These planets are massive enough that it is simple for them to retain their atmospheres of these light gases.

However, less massive planets (such as the terrestrial planets) will lose any initial hydrogen and helium envelopes via atmospheric escape. The only planets that can possibly retain these gases are the most massive giant planets, which then amass enormous atmospheres. On these planets, there is no possibility of having an Earth-like atmosphere.

Planets further out can have interesting compositions.1 If you move far enough away from the central star, ice giants can forms. Uranus and Neptune are examples (they are not, technically, gas giants). These giant planets are composed of heavier elements than hydrogen and helium - ices, which may include oxygen, methane, sulfur and nitrogen. They also, however, have hydrogen and helium, which, though composing much of their volume, only contribute about 1/5 of their total mass.

Ice giants are better targets for the kind of atmosphere you want. The issue, though, is that the oxygen and nitrogen you want may be in solid form, and even if some of it is gaseous, there's still a large atmosphere of hydrogen and helium to deal with.

The good thing is that exoplanets are not just either terrestrial planets or gas/ice giants. There are several intermediate classes that could help you:

  • Super-Earth: A massive terrestrial planet that could have an atmosphere of hydrogen and helium or other key volatiles (including ices).
  • Gas dwarf: A low-mass gas giant with a less-massive core and atmosphere.
  • Mini-Neptune: A type of gas dwarf, possibly composed of ices.
  • Massive solid planets: Planets that have a rocky core that constitutes a significant fraction of their mass (which can range from Earth's mass to a gas giant's mass) while retaining an atmosphere of hydrogen and helium.

All of those planets sound similar, if not the same. That's because not much is known about them, and their characteristics are largely hypothetical, though some candidates have been observed. Tweak some things and maybe, just maybe, you can figure out a way for one of them to have "air."


1 Although they may have formed further in and migrated outwards.

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