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Atmospheres like Earth's or Mars would freeze and precipitate towards the surface if the planets were to be moved very far away from the Sun, since the light that reaches that far is incredibly dim and provides close to no heat.

Could a planet, say 44 AU away from the Sun, have a volcanic atmosphere if it was VERY geologically active?

This way all the heat that doesn't reach the planet would come from inside of it, that's the idea behind this at least. Granted this won't last forever, so...

What, if anything, could prevent the planet from cooling down too fast?

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    $\begingroup$ worldbuilding.stackexchange.com/questions/62378/… isn't this question similar to this? Is there something I am missing? $\endgroup$
    – Nepene Nep
    Apr 3 at 11:52
  • $\begingroup$ @NepeneNep The question itself isn't that close, but the accepted answer is packed with useful information, thanks for sharing the post ^^! $\endgroup$ Apr 3 at 12:00
  • $\begingroup$ Helium clathrates could generate atmospheric helium, which the planet can hold if it's massive enough. Then the planet doesn't need to stay warm. $\endgroup$ Apr 3 at 22:56
  • $\begingroup$ We ask users to wait at least 48 hours before awarding best answer, as there's an international community which you otherwise might exclude from answering. $\endgroup$ Apr 4 at 1:51
  • $\begingroup$ Assuming that the furthermost planet of a solar system would be 44 AU from its star is making an assumption which actual solar aystems of other stars do not validate. In this list - en.wikipedia.org/wiki/… - the extreme example is about 5,800 AU, followed by about 660 AU and about 650 AU. Thus there may be rare examples of systems where the outermost planets are hundreds of AU from the star. $\endgroup$ Apr 5 at 21:52
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Boatloads of Uranium!

It works for Earth. It can work for your world too!

https://en.wikipedia.org/wiki/Decay_heat#Natural_occurrence

Naturally occurring decay heat is a significant source of the heat in the interior of the Earth. Radioactive isotopes of uranium, thorium and potassium are the primary contributors to this decay heat, and this radioactive decay is the primary source of heat from which geothermal energy derives.

Earth is heated by the decay of elements inside it; mostly uranium, potassium and thorium. Those have been in there since the get go keeping things hot ever since. That method would work anywhere. You dont have to be close to the sun for radioactive decay to generate heat.

Your world happens to have coalesced around great chunks of uranium and other radioactive elements. I think L.Dutch had a scenario where giant meteorites of radioactive elements came flying away from a supernova - I do not recall how that was made to happen and am worried it was deemed impossible (something about plasma?) but I like it and so assert that it can happen. These got together and made your planet. It is hot and it is staying hot.

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    $\begingroup$ But wouldn't that make the planet radioactive? I'm assuming OP wants human explorers to visit said planet(?) $\endgroup$
    – Len
    Apr 3 at 20:31
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    $\begingroup$ @len earth is not radioactive, rock is good at blocking radiation. $\endgroup$
    – John
    Apr 3 at 20:37
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    $\begingroup$ Note if it has that much uranium it is way to far out for it to be its natural orbit, outer planets are mostly composed of lighter elements. It could be a captured world though. $\endgroup$
    – John
    Apr 3 at 20:40
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    $\begingroup$ @John , so you're not talking unusually high amounts of uranium; higher than on Earth? If not then aren't we back to freezing? $\endgroup$
    – Len
    Apr 3 at 20:44
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    $\begingroup$ @Len heat will travel through stone radiation will not, in fact to have enough radiation to penetrate the crust the crust would be molten, so too hot for what is desired. you literally can't have enough radioactivity to threaten things at the surface and still have the surface cold enough to live on. $\endgroup$
    – John
    Apr 3 at 23:49
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Give it a really big moon.

Or you could have it be a moon (rather than a planet) like Io orbiting a larger planet, though with that option it's liable to be tidally locked which can cause it's own issues.

The tidal forces generated by the body orbiting it will push & pull it helping to keep it warm longer.

Io is a good example of how it works, the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean moons (quoted directly from its Wikipedia page).

You might also consider having any atmosphere in subterranean pockets closer to any warmth left in the core (not really an atmosphere) which gives the opportunity for biomes like the Movile Cave.

And then there's also heat from biological activity, any sufficient quantity of microbial extremophiles (subsurface or other) can plausibly generate enough heat of their own to have a non zero impact.

What keeps earths core hot? Scientific American says.

(1) heat from when the planet formed and accreted, which has not yet been lost.

(2) frictional heating, caused by denser core material sinking to the centre of the planet.

(3) and heat from the decay of radioactive elements.

So there's another option, more radioactive elements than other planets have, I'm not sure how you justify that though, planets are made the way planets are made & that's pretty much the same way for all of them so their make up is probably going to be pretty uniform .. baring the lighter elements stripped of by the solar wind for those that form nearer their star.

But perhaps it could be the product of the collision of two proto planets where the heavier elements had begun to sink to the core & it's a chunk from one of them from where radioactive elements had settled to, we think some asteroids with high metal content were made that way.

If you want life of some sort on your planet then a significantly higher concentrations of radioactive elements than on Earth may be problematic so you may want to pass on that option.

You could mix & match any or all of those.

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Why not? We have several examples. Neptune, at 30.1 AU from the Sun has a considerable atmosphere - indeed, it's mostly atmosphere. Its moon Triton also has a bit of an atmosphere, as does Pluto, at an average 38.5 AU.

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    $\begingroup$ Pluto's an especially good example, with an atmosphere about 4x as thick at perihelion (~30 AU) as at aphelion (~40 AU) due to it seasonally sublimating/freezing. A brighter star or close binary would allow the same atmosphere to be sustained at a further distance. $\endgroup$ Apr 4 at 0:24
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    $\begingroup$ @Christopher James Huff: And if Pluto or Triton were more massive - say roughly Earth mass - they could hold on to more of that nitrogen, and perhaps some hydrogen & helium too. $\endgroup$
    – jamesqf
    Apr 4 at 17:54

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