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This rogue planet is a cold rocky world where liquid methane replaces water, hydrogen is used in place of oxygen, and a large moon.

I was thinking the large moon would have a large tidal pull that keeps the mantle active and generates tons of geothermal energy for the plants to use in place of sunlight. Or maybe this world could could be a moon that orbits a rogue gas giant. But I'm not exactly sure how all that works.

Either tell me how exactly that works and if it's sustainable, or give me another way the plants can get energy.

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    $\begingroup$ hydrogen cannot be used in place of oxygen, because it is not an oxydizer. $\endgroup$
    – L.Dutch
    May 7 '20 at 17:56
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    $\begingroup$ Camelot 30K by Robert L. Forward posits an organism that lives in extreme cold, and is powered by cosmic rays through successive nuclear transmutation processes. $\endgroup$
    – LSerni
    May 7 '20 at 18:30
  • $\begingroup$ It's not exactly an answer to your question, but I recommend this video as an entertaining approach to the topic: https://www.youtube.com/watch?v=gOu3zGfP-TQ $\endgroup$
    – Luke
    May 7 '20 at 22:09
  • $\begingroup$ How could your details make any difference to sources of energy for plants? (Ignoring that you didn't mean "in place of oxygen and a large moon…") On Earth, all energy comes from the Sun, or you'll be able to explain otherwise. Until then a large moon or geothermal energy might be fun, but how could either help plant life? I suggest they don't work; they're not sustainable and the whole point is that finding another way plants might get energy is your job alone. By the by, if you get round to publishing this how will you reward those who did the work for you? $\endgroup$ May 8 '20 at 21:22
  • $\begingroup$ @RobbieGoodwin, the Sun is just the largest of four heat sources on Earth. In second place is geothermal heating from decay of long-lived radioactive isotopes (eg. U-235), third is tidal heating from the Moon, and in a distant fourth is a tiny bit of residual heat from the planet's formation. $\endgroup$
    – Mark
    May 9 '20 at 1:24
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There's only one real world situation where we think tidal heating is a significant source of energy. That's Jupiter's moon Io. Io doesn't get most of it's energy from its interaction with Jupiter, rather from its interaction with the other moons in the system.

If you really want tidal heating to be the source geothermic activity, then you're going to need a moon of a very large rogue planet in a moon system with several large moons. It's unlikely that any other situation will generate enough energy to drive a volcanic system to feed your hydrothermal vents. (Halfthawed is right that hydrothermal vents are the best model for turning geologic activity into an ecosystem)

Honestly, I don't think tidal heating is the way to go. Earth has remained geologically active for all these billions of years with this one secret: being really big. Planets generate heat when they form, that heat is trapped under many fathoms of insulating rock. It's kept Earth's core hot for, like, 4 billions of years.

Well, to be honest, Earth has a second source of heat: radioactive decay. Radioactive elements in the mantle slowly decay over over billions of years, releasing a little heat as they do so. I've seen models that say that without all the natural uranium, Earth's core would have cooled twice as fast. Between the two effects, it's feasible for a planet a little bigger than Earth to maintain active geothermal vents for tens of billions of years. Longer than the lifetimes of many stars.

The surface of a rogue planet will be cold. Extremely cold. Within a couple of degrees of absolute zero. There's no way around this. The only possible atmosphere will be hydrogen and helium, all other gasses will fall to the ground like snow. There will be no oceans on the surface. Rogue planets take in thousands of times less energy than Pluto. The surface of Pluto, with its seasonal methane snow, would be tropical in comparison. Way too cold for liquid methane.

I disagree with Halfthawed about the feasibility of an insulating atmosphere, if the atmosphere were thick enough to contribute significant insulation, your planet would be a gas giant. Atmospheres make poor insulation anyway, they convect with delivers heat to the outer layers (where it radiates away into space) relatively quickly. What you need for insulation is several kilometers of rock and water ice. All life will have to exist in the sub-surface ocean. We think there are sub-surface oceans on most of the large, rocky bodies in the outer solar system. They seem pretty common.

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    $\begingroup$ Radioactive decay is by far the larger of the two heat sources. Kelvin calculated that the Earth could be no more than 40 million years old if the only source of heat was that of formation. $\endgroup$
    – Mark
    May 9 '20 at 1:20
  • $\begingroup$ Could ammonia exist in liquid form at those temperatures? If not is there any liquid suitable for life at those temperatures? $\endgroup$ May 10 '20 at 2:08
  • $\begingroup$ If you think tidal forces isn't a good way, then what about plants powered by wind? If this world was volcanically active enough to make hot areas to mix with the cold, there should be plenty, right? $\endgroup$ May 10 '20 at 3:45
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    $\begingroup$ if it is volcanically active that is enough by itself. see "black smokers" $\endgroup$
    – Jasen
    May 10 '20 at 6:00
  • $\begingroup$ @ChickenpeepChickenpeep If by "Those Temperatures", you mean about 2K, the natural temperature of something that's been in interstellar space for millions of years, no, you cannot have liquid ammonia. You cannot have any known liquid, or even any imaginable liquid. There is no chemistry, no movement, no change, no wind, and certainly no life. You have to raise the temperature for something to happen. If you want to raise the temperature on the surface, volcanoes are indeed your best bet, but probably only in small patches. $\endgroup$
    – Luke
    May 10 '20 at 8:53
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First of all, if you have liquid methane, you are in range of temperatures where life as we know it cannot exist.

Then hydrogen cannot replace oxygen, because it doesn't oxidize other elements, apart maybe some alkaline metal.

Then, last but not least, plants use as source of energy for their chemical processes. A rogue planet has no star, and thus no source of light.

Even tidal force, though they can produce some heat, would produce at most some far infrared radiation. Being that typical of thermal process and not of electronic transitions, it won't be useful for sustaining any equivalent of photosynthesis.

The only way for some sort of life to be present is that, provided that the other issues are somehow hand-waved, the tidal heating would produce some sort of geological activity and that would release some chemical which could be oxidized as source of energy. But those organisms would not be plants.

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  • $\begingroup$ Oxidization does not require oxygen, but only a more electronegative element or molecule. While hydrogen does oxidize in some instances particularly heavy metals, there are many instances where talking about losing hydrogen is equivalent to oxidation particularly for hydrocarbons. The issue with hydrogen is that it does not have enough binding points (only one) to replace oxygen (three) for maintaining strong multi-atom bonds. $\endgroup$
    – user110866
    May 7 '20 at 21:45
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Hydrothermal Vents

But first, let's clarify something. In your question you state:

Either tell me how exactly that works and if it's sustainable, or give me another way the plants can get energy.

It doesn't work, it's not sustainable (see L.Dutch's answer) so I'm now going to give you another way for plants to get energy.

For starters, we're using liquid water, not liquid methane. In order to keep this temperature, we're going to be giving the planet an insulative atmosphere, so hydrogen is actually a good choice in that regard. We'll still have a suitable amount of oxygen though, but this is actually kind of moot because life isn't going to exist on the physical surface or in atmosphere, it's going to exist in the ocean. Given the insulation in the atmosphere, it's possible for this planet to mantain a hot core, and said hot core can leak into the ocean via hydrothermal vents. Hydrothermal vents are perfect for life, seeing as they provide a decent range of temperatures around them not to mention unique chemicals. The way your plants are going to get their energy is from these chemicals - not from light, that's not really possible on a rogue planet. Basically, these things will be more similar to plankton.

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How about the planet being half-rogue? Not attached to some particular star, but gravitationaly bound to a large cluster containing a great deal of giant stars? You'll get a lot of frequent and near supernovas, but surviving that, it may be liveable.

Other idea: the planet travelling before or behind the edge of a star-forming shock wave. There will be always a lot of young, bright stars around.

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I had a couple thoughts... lets see here...

  1. Natural fission reactors. They've happened on Earth, and given that the closer you get to the center of our solar system the denser the planets, I'd expect the insides of Mercury to be radioactive as hell. As if having the Sun for your next door neighbor didn't make things hot enough. Perhaps a liquid core that was relatively active such that fissionables were brought into close proximity, generated some heat, and then separated fairly regularly. Regular enough to keep the core molten. Not terribly sustainable. The core would eventually get low enough on fissionable material that the core would solidify and then everything grinds to a halt.
  2. An ecosystem that sleeps when it's too far from the sun. There are bacteria that can be completely dormant in extreme environments and then "wake up" when things settle into something more survivable. Make your entire planet's ecosystem like that. It only "wakes up" when relatively close to a star.
  3. A nigh fanatical dedication to the Pope!
  4. Apparently Jupiter was big enough to burn like a sun for quite some time before it finally "went out" and settled down to being a gas giant. If your rogue planet was actually a bare-minimum sized rogue star... it would admittedly be hard to justify having flora and fauna. It has been done however, see also: "Dragon's Egg" and "Starquake" by Robert L. Forward. I believe the books revolve around a rogue white dwarf with an ecosystem and intelligent life. Their neutron-based biology worked so much faster than our chemical biology that the human explorers in orbit around the white dwarf watch them go from stone age to flying up to visit them (from within a white dwarf's gravity well(!!!)) over the course of a month.
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I don't think this scenario is feasible for several reasons:

  1. It is highly unlikely that a reasonably terrestrial world would have a hydrogen-rich atmosphere, it just would not have enough gravity for sustaining it for a long time.

  2. Even if you get your chemistry right, the chemical reactions at lower temperatures are usually a lot slower. It took about 3.5b years to develop something resembling plants as we know it today, so for your planet it will take an order of magnitude longer unless you have some magic catalyst. In that timeframe any power source your rogue planet might have would certainly run out.

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Hard Radiation!

Ra-di-a-tion. Yes, indeed. You hear the most outrageous lies about it. Half-baked goggle-box do-gooders telling everybody it's bad for you. Pernicious nonsense!

Your lonely rogue has a heavy heart; chock full of uranium and other radioactive elements. It is hot down there! Also there are alpha particles and their friends flying around. This is the source of your hydrogen - it is stripped away from primordial methane, ammonia and water that was trapped in your world at its formation.

You could have some of the hydrogen and "oxidized" carbon (here called oxidized because of the double C=C bone, not because of oxygen) form naturally. Higher up you could have your plant equivalents catalyze that with ionizing radiation capture photochemistry. Like our green weeds capture visible light and plow that energy into stripping hydrogen off of oxygen, your plants capture ionizing radiation released by radioactive decay.

https://chemistry.stackexchange.com/questions/107363/energy-capture-photochemistry-with-ionizing-radiation

Yes, yes, they laughed on the chem stack when they told me that beta particles are too energetic to do chemistry with. Mad, they called me, mad! But I'll show them. I'll show them all!

mmm, yes. Back to the chalkboard. Your plants capture energetic emanations - maybe from radon working its way up from below - and strip hydrogen from its home, releasing it into the atmosphere and creating carbon and nitrogen molecules. The stuff of life.

The nice thing about this is then you have hydrogen for your critters to breathe, and they use it to reduce the big molecules made by plants back to methane and ammonia. The circle of life!

You have now a reducing world. There is much written about such and much on the stack exchange. Here is one. Ammonia planets.

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  • $\begingroup$ I think downvote must be because C=C is not really oxidized. $\endgroup$
    – Willk
    May 9 '20 at 2:20
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Thermoelectric Plants

The temperature difference could be exploited to generate electricity via the Seebeck effect. Thermoelectric generators are about as efficient as chlorophyll so, with some handwaving, you can have plants that face the ground rather than the sky.

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  • $\begingroup$ Where does the heat come from? $\endgroup$
    – Mark
    May 9 '20 at 1:21
  • $\begingroup$ @Mark: The tidal forces from the moon mentioned in the question. Alternatively, the planet could have a core with a higher percentage of radioactive elements than the Earth's. $\endgroup$ May 10 '20 at 2:13

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