How could you keep a rogue planet warm(ish)?

Question

I'm working on a story right now where darkness plays an important role, and I had the idea to set my story on a rogue planet, a planet in perpetual darkness because it has no sun. However, from what I understand, heat and energy in this kind of system is hard to come by, which could make things a little boring if the entire planet is so cold nothing interesting can happen.

So these are the elements I'm interested in providing to my planet:

• Should be not frozen solid: It doesn't have to as warm as Earth, but warm enough that humans could live with life-support. Antarctic-level cold is about the minimum here, or about -100 °C.
• Should be geologically interesting: I'd like it to be warm enough underground that humans may prefer living there instead of on the surface; I'd also like to see magma, but this is optional.
• Should support life, though it need not be life-as-we-know-it.
• Should be able hold an atmosphere. (EDIT: Originally there was a requirement for a semi-Earth-like atmosphere, but I've relaxed this.)

Here's some more background on the planet, but feel free to adapt this as needed:

• The planet is a (alien) life-supporting planet that was flung from it's solar system long ago.
• The planet does not have to be Earth-sized, and will likely be much smaller.
• Life on the planet somehow survived, though it has heavily adapted to the darkness. It is not necessarily intelligent, though it could be.
• Due to the scarcity of energy, the natives tend to cluster around sources of energy, much like we have deep sea life that clusters around volcanic vents.

To me, adding a moon (or moons, or even sister planet) seems to be the most likely way to make the situation more feasible and more interesting, so i'm especially interested in answers involving moons. (Bonus points(?) if the moon provides low light or is visible somehow.) But any solutions are acceptable. I'm willing to accept some handwaving.

Answer 1

Rogue planets can be kept warm. The key points can be found in the Wikipedia entry on rogue planets.

Interstellar planets generate little heat nor are they heated by a star. In 1998, David J. Stevenson theorized that some planet-sized objects adrift in the vast expanses of cold interstellar space could possibly sustain a thick atmosphere that would not freeze out. He proposes that atmospheres are preserved by the pressure-induced far-infrared radiation opacity of a thick hydrogen-containing atmosphere.

The OP's rogue planet is geologically interesting, any geological activity can produce heat and with a dense hydrogen atmosphere this heat will be retained. If the rogue planet had a large moon, relative to its own size, or was part of a double planet, the tidal forces would drive increased volcanic and geological activity. Large areas of magma could exist akin to Deccan Traps. Good for additional planetary heating.

If similar volcanic activity occurred on the large moon / double planet companion, this could be visible from the planet's surface. This wouldn't provide much illumination, but it could make its presence observable. This won't constitute a glow-in-the-dark moon, but unless you want to go full pseudo-scientific hand-waving, there aren't too many options. Radium moons or a moonscape covered with bioluminescent flora, for example, but neither of which will throw much light. Very much very low light levels.

There is a good chance that a biosphere could develop. Most so in the light of what is known about the extreme conditions where life can survive. But this assumes the evolution of life on rogue planet as such, however, if the planet had its own indigenous alien biosphere it doesn't seem likely it would survive the transition from a planet in a solar system to a rogue planet.

The principal exception would be if the planet possessed sapient lifeforms with sufficiently advanced technology to enable to survive the transition to a rogue planet. Especially if they were capable to preserving and saving enough of their planet's biosphere. Since their planet has been a rogue for a long time this should be enough to allow its lifeforms to adapt the conditions, with a little help from genetic engineering.

Since Stevenson's proposal for habitable rogue planets required a hydrogen atmosphere, any oxygen present would either have to be extremely low or it would constitute a hazard to life and limb. In this case, there shouldn't be very much oxygen in the planet's atmosphere. The rogue planet's inhabitants may use oxygen and so it may be present inside their habitats, but generally in the atmosphere, no.

If these conditions are meet it is plausible that a rogue planet could have temperatures comparable to Antarctica and below, but still at levels to make life viable.

EDIT:

There was an earlier question about perception on a Titan-like planet. One possible solution was infrared perception. Not eyes as we know them, but complex arrays of heat sensing pits.

See this question Evolution of eyesight on a planet with a methane biochemistry

If the alien lifeforms on a rogue planet either evolved or modified themselves for infrared perception their world wouldn't necessarily be one of absolute darkness. They might even lack any sense of difference between night and day (without a primary star that is meaningless anyway). The emitted infrared radiation from their moon might provide some additional illumination.

In a double planet system, if you want to tidal forces to generate geological and volcanic activity, the two bodies will need to be close together. This probably means they will be tidally locked, with the same sides facing each other. At least, an apparently stationary moon will be a geographical reference point for the inhabitants.

Answer 2

@a4android's tidal heating answer was my first thought, too. Along those lines, the planet might have multiple moons, or artificial moons. With enough handwaving, a pattern of multiple moon orbits with electromagnetic interaction with each other/the atmosphere could result in some lighting and light effects.

Related, but not identical ideas:

• The "Spiders" from Vernor Vinge's A Deepness in the Sky:

  The plot begins with the discovery of an intelligent alien species on a planet orbiting an anomalous star, dubbed OnOff because for 215 of every 250 years it is dormant, releasing almost no energy. During this period, the planet freezes and its fauna go into hibernation. - https://en.wikipedia.org/wiki/A_Deepness_in_the_Sky

  You could have your planet orbit a pulsing star which wasn't cold enough to let it freeze, but does let it get dark and cold. Or have its wandering route technologically directed to plot a course by stars which could periodically warm it up a bit.

• The Pierson's Puppeteers from Larry Niven's Ringworld books - they put their entire civilization into debt to buy (handwaving technology) from outside the galaxy, so they could sent their planet and its star off into a wandering state - http://www.technovelgy.com/ct/content.asp?Bnum=605

  They have rogueness, and heat, but not darkness. But they could get darkness by being underground, as your other idea mentions. e.g. the humans in The Matrix lived underground for warmth because the sky is reflecting heat, the Eloi and Morlocks in H.G. Wells' The Time Machine have a surface/underground split for societal reasons and potentially for safety (you could have predators on the surface). E.M. Forster's The Machine Stops have people who live underground by ... habit? and are able to go to the surface, but don't because they think it's not for civilized people.

• Jules Verne's Journey to the Center of the Earth has an idea when explaining that the center of the Earth is not too hot and molten, the Earth's heat is surface-only:

  "The earth has been heated by combustion on its surface, that is all. Its surface was composed of a great number of metals, such as potassium and sodium, which have the peculiar property of igniting at the mere contact with air and water; these metals kindled when the atmospheric vapours fell in rain upon the soil; and by and by, when the waters penetrated into the fissures of the crust of the earth, they broke out into fresh combustion with explosions and eruptions. Such was the cause of the numerous volcanoes at the origin of the earth."

  "Upon my word, this is a very clever hypothesis," I exclaimed, in spite rather of myself.

  "And which Humphry Davy demonstrated to me by a simple experiment. He formed a small ball of the metals which I have named, and which was a very fair representation of our globe; whenever he caused a fine dew of rain to fall upon its surface, it heaved up into little monticules, it became oxydized and formed miniature mountains; a crater broke open at one of its summits; the eruption took place, and communicated to the whole of the ball such a heat that it could not be held in the hand."

  If this was at all workable, you could have a cold, dark and frozen planet, but with hot regions generated by ongoing chemical reactions between the surface and atmosphere.

• Greg Egan's short story "Hot Rock" has a rogue planet called Tallulah:

  Tallulah had no sun; it had been an orphan for at least a billion years, drifting untethered through the galaxy. Yet distant astronomers had surmised - and the instruments here and now confirmed - that its surface was awash with running water. [..] when a planet was stripped of its sun, the decay of long-lived radioisotopes could eke out enough warmth over billions of years to keep its core molten. [..]

  Tallulah was created by long-lost 'Builders', who created a 'deep rock' femto-scale technology, which turns iron and nickel into a hundred billion varieties of weird particles at 90% mass to energy conversion. The story has future-humans and aliens landing on it, trying to work out how it works.

• The Matrix again, it has 'humans combined with a form of fusion' used as batteries to power the machines. That kind of idea, perhaps the life forms on the planet huddle together for some kind of unusual heat creation process. They can go exploring over the surface, but not spread out and live or stay there for long in isolation.

• Not exactly related to temperature, but Terry Pratchett's The Dark side of the Sun has a sentient planet called The First Syrian Bank. It's a digital computer intelligence formed by plate tectonic shifts creating circuitry in a silicon based crust (IIRC). If you went with an approach like that, the planet could have its own unknown technology for keeping warm, and a motivation of keeping warm surface regions to encourage aliens to land, live, trade, communicate. At least fulfills the The planet supports life, though it is not life-as-we-know-it. part, and has lots of scope for why the temperature isn't very human friendly.

• Peter F. Hamilton's Void Trilogy has a couple of variants of an ancient alien Arkship/warship crash landed on a planet and being embedded in the surface and lived in by other people. If it was a powerful enough ship, handwavy technology, it could have been enough to drive the planet out of orbit, energy levels drop a bit and cool down, now it's the main source of light and heat - heating the surrounding earth and melting rock underground around it - but not the entire planet. Life forms life within a region around the ship, without necessarily knowing what it is, or knowing about it at all (if it's underground).

And of course, there's blunt technology - the planet is frozen and inhospitable, but the aliens have enormous nuclear reactors in craters. Each crater is a region of habitable land/underground, and a gaseous atmosphere kept from leaking out by gravity/frozen atmosphere dome overhead. Some nuclear reactors over time have gone wrong and melted regions near them, some attempts at drilling underground, trying to find resources, nuclear testing, have left relatively short lived molten regions deep underground.

Answer 3

I don't have time to research and write a full answer, so this will have to do, sorry.

A low mass Brown Dwarf Star with a habitable moon might allow what you need, due to orbital shenanigans the brown dwarf was ejected from it's solar system with it's moon.

Now heat on the moon is generated internally by tidal forces, and some heat from the brown dwarf (depends on how near it is). The brown dwarf can have as much luminescence as you like, from as bright as sol to no visible light, just some infra-red (which sounds like it suits), and still be a brown dwarf.

You've a rogue pair, and one is technically a star, but it might give you a habitable rogue world.

Answer 4

Suppose that the planet had almost completely frozen over with the remaining life clustered around geothermal vents. Then if one ore more moons, either through collision or by entering the Roche limit, were to break up and impact the planet, there would be significant heating. Keep your impacts small enough to not be crust-melting/atmosphere stripping but large and frequent enough to melt the ice and trigger a bit more geological activity. You can then have a (relative) bloom in native life for a geologically short period and it's clustered around vents, as you wanted.

Light is a major problem. The best I can come up with is a glowing molten crust, which would of course have to be another body. So I would definitely go for a collision between moons as the source of the meteors and the molten moon, which is also what you wanted.

The numbers are unlikely to pan out if you calculate this exactly but you did say you're game for hand-waving.

Answer 5

I had an idea that might be slightly out of the box. How about an Oort-cloud style set of asteroids and meteors in a fairly unstable orbit, such that the planet is bombarded relatively often.

Whilst this presents some difficulties for the continued existence of life, the effects may not be that great, as the primary cause of extinction in events like this is a dust cloud blocking out light.

Answer 6

Try setting it in the distant past, before space was the frigid wasteland it is today: https://arxiv.org/abs/1312.0613 interview: https://www.nytimes.com/2014/12/02/science/avi-loeb-ponders-the-early-universe-nature-and-life.html?_r=0