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I am making a rogue planet. Right now it is a super-earth with a lot of radioactive materials in its core. It rotates very quickly as well. However, it needs to be habitable. And not just the deep ocean, the surface of the planet, exposed to the atmosphere, needs to be habitable enough for intelligent life to develop, discover fire, and advance.

I was wondering if this was possible with a thick enough atmosphere with enough greenhouse gasses to trap geothermal heat and thus keep the planet habitable. Obviously regular geothermal heat won't do in such a scenario; its simply not hot enough. However I do have an idea as to how to make it hot enough. Give it a stagnant lid (much like Venus) for its crust. Global resurfacing events fail to occur because the planet is spinning too quickly, thus making its crust much thinner at the poles than the equator. A would-be resurfacing event would instead melt the poles first and this would ease the pressure from the rest of the crust (Coriolis Currents induced in the lava by the same fast rotation allow it to circulate the upper mantle such that the entire upper mantle is cooled by this, preventing it from simply melting equatorial crust later) The atmosphere traps the heat emitted, creating a hot period which slowly cools over time, until the next polar resurfacing event. (Effectively "seasons")

Is this plausible by the mechanism described, even allowing for extreme rotations and radioactivity? If not, is there another way that doesn't involve aliens or magic or advanced technology, and retains the solid surface with liquid water, with enough energy availability for intelligent life to evolve?

I don't mind changing huge factors about the planet if that's necessary as I haven't established much else about it yet, other than that its a rogue planet, has a space-faring civilization, and is heated from below. I also don't mind unlikely but possible configurations so long as its explainable without advanced tech/aliens/magic, though importantly, it needs to be self consistent and allow intelligent life to not only evolve, but advance in their technology as well, to at least the ability to send craft into orbit around the planet.

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  • $\begingroup$ Mostly no.. It would require a very hot planet(thus surface very volcanic and not stable), OR an enormously thick insulating atmosphere (thus stable and comfortable temp, but not inhabitable, at least by humans) $\endgroup$ – PcMan Jan 3 at 6:49
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Yes, they can.

This paper notes that a thick hydrogen atmosphere, protected from being stripped by solar radiation due to lacking a star, could provide enough insulation to allow surface temperatures above the freezing point of water.

A satellite can provide extra warming. as if they have a large moon then it can do a lot of tidal warming. Europa, a Jupiter moon, famously has oceans because of it's tidal warming.

So, there is less need of the fast spinning planet with lots of radiation. It doesn't make complete sense anyway. The magma under the surface is gonna heat up away from the poles and erupt there anyway.

Lava

Convection currents in the mantle don't tend to be that large. Just a thousand kilometers across or so. The cycles can take millions of years- a very distant release valve under the poles would do little to relieve that, as it would take tens of millions of years for it to reach there.

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  • $\begingroup$ Upvote! Tidal heating from the moon is a slick idea. $\endgroup$ – Willk Jan 1 at 15:45
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Nepene Nep's answer mentions a paper https://ui.adsabs.harvard.edu/abs/1999Natur.400...32S/abstract[1] which says that a rogue planet could be warm enough for liquid water using life. But that requires a thick hydrogen atmosphere. A thick hydrogen atmosphere is inconsistent with an oxygen rich atmosphere because the hydrogen and oxygen would burn to make water.

Here is a link to a more popularized dicussion:

https://planetplanet.net/2015/06/04/real-life-sci-fi-world-8-the-free-floating-earth/[2]

Based on this longer article:

https://aeon.co/essays/could-we-make-our-home-on-a-rogue-planet-without-a-sun[3]

So a planet with a dense hydrogen atmosphere would be uninhabitable for humans for lack of enough oxygen to breathe in the atmosphere. And fires wouldn't burn without a lot of oxygen in the atmospehre. And any Earthly or extraterrestrial life forms with enviromental needs similar to those of humans would also require an oxygen rich atmosphere.

So how are you going to have large multicelled animals on your world, including some which have intelligence and culture, if there isn't oxygen in the atmosphere for them to breath, or oxygen for fires?

Either you resarach alternate biochemistries to find a way to make large multicelled anaerobic organisms function, or you are going to have to have an oxygen rich atmosphere on your rogue planet. Which means it can't have a hydrogen rich atmosphere, which means it can't use a hydrogen rich atmosphere to conserve heat.

What about having the rouge planet heated by a large moon? Would that provide enough e heat? If a moon of size A wn't provide enough heat, you could double the size of the moon to size B. If a size B moon isn't enough, you could double the size of the moon to size C, and so on.

And eventually the size of the moon might have to be increased so much that the moon becomes the planet, and the habitable planet becomes a moon of the moon-turned-planet.

Thus you could have a roughly Earth or super Earth sized object orbiting a much larger super Earth, or mini Neptune, or ice giant, or gas giant, planet which is also a rogue in interstellar space.

There has been considerable discussion of the potential habitablity of hypothetical exomoons orbiting hypothetical giant planets in other solar systems, although as far as I known no exomoon discoveries have been confirmed yet.

Discussions of the habitability of other worlds seem to be about habitablity for carbon based liquid water using organisms in general, and not for large multicelled land dwelling plants and animals in particular. So most scientific discussions of habitability discuss the broad conditions for Earth like life in general instead of humans.

Humans and similar intelligent beings in particular, should should be able to evolve and flourishon only a minority of the worlds which have some forms of life.

And most of the discussion of the potential habitablity of planetary mass exomoons in other solar systems discusses situations where those exomoons and their exoplanets orbit within the circumstellar habitable zones of their stars.

As far as I know, the main discussion of the habitablity of other worlds which focuses on habitability for humans (and thus for most of the aliens in science ficiton) is Habitable Planets for Man, Stephen H. Dole, 1964, 2007.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf[4]

As I remember, the discussion of possible habitable double planets or planet sized moons of giant planets is very, very brief.

Comparatively little attention is given to hypothetical exomoons of exoplanets orbiting farther from their stars than the circumstellar habitable zones of those stars, where the majority of the heat would be supplied by tidal heating and internal radioactive decay.

And of course a rouge exomoon orbiting arogue exoplanet in interstellar space would be the extreme example of a exommon outside the circumstellar habitable zone of any star, and would need to get almost all its heat from tidal heating and internal radioactive decay.

Anyway, scientific studies of the potential habitability of exomoons should be consulted.

A good example is "Exomoon Habitability Constrained by Illumination and Tidal Heating", Rene Heller and Roy Barnes, Astrobiology, Volume 13, number 1, 2013.

http://faculty.washington.edu/rkb9/publications/hb13.pdf[5]

Here is a link to a non scientific discusson about life on an exomoon orbiting an exoplanet, though in this case they are supposed to be in the circumstellar habitable zone of their star.

https://planetplanet.net/2014/11/18/real-life-sci-fi-world-6-pandora-from-the-movie-avatar-the-habitable-moon-of-a-gas-giant-planet/[6]

And here is a link to an article discussing life on the moon or planet of a brown dwarf.

https://planetplanet.net/2014/10/09/real-life-sci-fi-world-4-earth-around-a-brown-dwarf/[7]

A brown dwarf is an object intermediate in mass between a giant planet and a very low mass star. Brown dwarf's are just barely massive enough to fuse deuterium in their cores for part of their existence, but are not massive enough to fuse ordinary hydrogen. The minimum mass for a brown dwarf is approximately 13 times the mass of Jupiter and the maximum mass for a brown dwarf is approximately 75 to 80 times the mass of jupter.

The article discusses the habitability of a planetary mass object orbiting its brown dwarf star or planet, depending on your point of view, close enough to be within the circum brown dwarf habitable zone. But what if a planetaru mass object, either a moon or a planet depending on definitions, was outside the circum brown dwarf habitable zone and thus would be a bit too cold for life except for its internal heat and especially tidal heating. If there are other planetary mass objects orbiting the brown dwarf the tidal interations might induce enough tidal heating to keep one or more ofthe planetary mass objects warm enough for life.

Of course if a rogue planetary mass object orbits around a rogue giant planet or rogue brown dwarf in interstellar space far from any luminous star, the light it receives from outside should be from starlight from distant stars, which shouldn't be enough for much photosynthesis and for the production of an necessary oxygen rich atmosphere. Figuring out a way to solve that problem may be hcallenge for a science fiction writer.

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