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There's a planet in my world's solar system that used to be habitable but was knocked off course. its been frozen for about 4 billion years in the present time.

I'm wondering if the atmosphere would be left after all this time being frozen

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    $\begingroup$ This is why I'm not a fan of things being as realistic as possible. A rogue habitable ice planet is a great idea. Screw the science. Even scientists will enjoy the story. $\endgroup$
    – JBH
    Apr 9 at 3:08
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    $\begingroup$ Can you be more specific, please? Was the atmosphere frozen, or merely the sub-atmospheric part of the planet? Either way, what forces - such as sun-shine or other radiations - might diminish the atmosphere? $\endgroup$ Apr 9 at 22:45
  • $\begingroup$ @JBH "Even scientists will enjoy the story." [citation needed] $\endgroup$
    – Trang Oul
    Apr 10 at 7:15
  • $\begingroup$ I assume you mean habitable to humans, so we mean some sort of very Earth-like as a starting assumption. How "off-course" are we talking? That's, I think, the linchpin here. $\endgroup$
    – Amocito
    Apr 11 at 18:10
  • $\begingroup$ @JBH Um, some scientist think that rouge ice planets could be habitable underneath the surface, due to geothermal vents, the same mechanisms that could have been the initial cradles for life on earth. What you described is actually quite plausible. $\endgroup$
    – Pointy Orb
    Apr 12 at 2:09

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The frozen atmosphere would be present as a layer of ice, which would slowly, very slowly, sublimate away.

It would take more than 4 billion of year to deprive the frozen planet from all of its atmosphere. For a scale, Pluto, with a much slower gravity, still has it, 4.5 billion years after its formation.

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    $\begingroup$ Much, much more slowly than the atmosphere itself goes away, though. $\endgroup$
    – g s
    Apr 9 at 2:44
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    $\begingroup$ Reminds me of "A Pail of Air" by Fritz Leiber. $\endgroup$ Apr 9 at 21:41
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As a gas? No, obviously. Oxygen and nitrogen freeze too, just like water, only at a much lower temperature (54.36 K = −218.79 °C = −361.82 °F for oxygen).

For example, Earth's atmosphere would be converted into a 70 cm (26 in) layer of a mixture of oxygen and nitrogen ice.

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  • $\begingroup$ It would be actually be around 10 meters thick on average. And the pressure at the bottom from the weight of all that ice would be one atmosphere on average. But of course it would accumulate more at low elevations away from geothermal heat sources. $\endgroup$ Apr 9 at 23:10
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I haven't seen the obvious question asked, so I'm going to roll with it here: just how cold are we talking?

Mars receives a little less than 1/2 the energy from the sun per square km and has an average surface temperature well below the freezing point of water and some gasses (like CO2). But most atmospheric gasses are pretty hard to freeze until you get a lot further out.

Let's say a rogue dark star drops through the solar system and bobbles things about a bit, getting rid of Mars and pushing Earth out a little farther than Mars' old orbit. (We'll be nice and let the Moon come along for the ride.)

Pretty quickly the surface temperature will drop and ice will cover the globe. It'll take a while to get all the way there, what with all that geothermal activity, but it'll get there in the end. There will likely be CO2 ice drifts at the poles, maybe some methane lakes and a few other interesting things, but the oxygen stays in the atmosphere until we get down to -183C, nitrogen stays around until -195.8C and so on. The poles may get down that low, but with a good blanketing layer of atmosphere we can hope that it won't end up holding all the world's oxygen in liquid form.

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