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I am imagining a special ecosystem on a rogue planet (a planet that wanders throughout space without orbiting a star or other heavenly object) but I would need your help to fill in the gaps and realize if a world like this would be possible.

Imagine a jupiter sized gas giant: lets call it Erebus.
Erebus would have few moons orbiting around it.
One of it would be the size of Mars and would have a dense atmosphere, which would keep the heat inside. The moon would be habitable, thanks to the heat trapped, water resources and other elements that allow the construction of organic things. The moons name would be lyra.

Life could be possible and could last even longer then if Lyra orbited a basic star, because the star would "die" one day (leaving the planet to freeze). But life would last longer on a moon that orbits a rogue planet: Erebus heats Lyra because its tidal forces slowly (and relatively) stretches Lyra, causing it to produce heat. This "heat production" wouldn't wear out, because the force that causes Lyra's stretching is gravity. So thanks to Lyra's composition and Erebus's tidal forces: life is possible, but... we miss one thing. We miss light in all of this.

As Erebus wanders throught the galaxy: Lyra is enlighten by starlight. But it isn't enough to cause photosynthesis or eyesight, and won't allow a complex biodiversity to emerge. This is why I thought Erebus could be a light source. At the beginning, I thought erebus could've been a brown dwarf that slowly produces light. But a brown dwarf isn't eternal.

That is why I need your help:
Could there be a way in which Erebus radiates light just like the sun (without necessarily radiating heat and not thanks to thermonuclear fusion) thanks to a chemical or other reaction? What could it be? Or could something else (that orbits around Erebus) radiate light?

Lyra would have a day/night system like planet Earth as it wouldn't be tidally locked towards erebus because it would have an eccentric orbit so it would be able to rotate on itself while orbiting erebus. This would only happen if it was erebus emitting light but if it isn't erebus that radiates light, how could lyra sustain an day/night system?

My basic questions are: how could Erebus produce light forever (in a self sustaining way) or if not what would produce light in an eternal way? What reactions could able this process to happen?

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    $\begingroup$ /Erebus heats Lyra because its tidal forces slowly (and relatively) stretches Lyra, causing it to produce heat./ excellent! $\endgroup$ – Willk Dec 8 '17 at 22:46
  • $\begingroup$ Let's change his "eternal" to a million years. Is it any of it doable for that amount of time? $\endgroup$ – Len Jan 30 '18 at 20:23
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Your world is the equivalent of Jupiter's moon Io, which has tidally induced heating and volcanism. Then when the volcanoes go off, the charged particles they blast out provoke huge auroras on Jupiter.

https://www.space.com/29248-jupiter-auroras-volcanic-moon-io.html Jupiter's auroras

Jupiter's auroras, which are sparked by particles from the planet's moons as well as the sun, are thousands of times more energetic than Earth's and many times bigger than Earth itself. They're also constant, but every once in a while they grow to an incredible intensity — the result not of a solar flare but of volcanic activity on Io, according to the new study.

Superbright, constant auroras around your rogue planet can serve as your light. It would be quite a light - even on Earth auroras can be all different colors, moving and shifting. Auroras thousands of times brighter (like Jupiter's) could be a plausible sun substitute. Or the aurora can be around your inhabited moon. Or both, and sometimes they collide! Whoa!

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This "heat production" wouldn't wear out, because the force that causes lyra's stretching is gravity.

Yes it would. The energy for tidal heating comes from a combination of Erebus's spin, and Lyra's orbital energy. Over time, depending on the initial conditions, Lyra will either crash into Erebus, move far enough away that the tidal heating becomes insignificant, or have its orbit circularize, again such that tidal heating becomes insignificant.

That is why i need your help: Could there be a way in which erebus radiates light just like the sun (without necessarily radiating heat and not thanks to thermonuclear fusion) thanks to a chemical or other reaction? what could it be? Or could something else (that orbits around erebus) radiate light?

Any chemical source of light will last far less time than the heat & light from gravitational contraction & deuterium fusion in a brown dwarf. Auroras are also out of the question, as they are primarily powered by solar wind interacting with a magnetic field; i.e., you need a star to create auroras on a planet. If Erebus is a rogue, there will be no regular auroras. And if Lyra has a thick enough atmosphere to retain enough heat to make it habitable on the surface, you wont get the kinds of world-escaping volcanic plumes providing an alternative source of charged particles to create auroras such as Io provides for Jupiter.

My basic questions are how could erebus produce light forever (in a self sustaining way) or if not what would produce light in an eternal way? What reactions could able this process to happen?

There is no way and nothing. Every source of energy will run out eventually.

Your best option for an exceptionally long-lived light source is a red dwarf. Red dwarfs have sufficiently long lifetimes that none of them have ever yet had time to die in the history of the universe. Small ones could conceivably last for several trillion years. That ought to be close enough to eternity for just about anybody.

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So, you use the word "eternal," which makes me think that you're missing some details. As Logan stated in their reply, there is no such thing as an eternal energy source. This is because of the principle we call "the first law of thermodynamics," or "conservation of energy."

You seem to suspect that energy gained from tidal heating won't run out, but it will. Heat from tidal heating is heat gained as the orbiting body moves closer and further away from the body it is orbiting. As the distance changes, the force of gravity acting on the orbiting body increases and decreases, stretching the body's interior and causing friction which then translates to energy in the form of heat. As you might guess, a perfectly circular orbit will not experience tidal heating, because the distance between the two bodies doesn't change.

A similar thing is happening between the Earth and the Moon. You know that the Moon controls the tides, as the water on Earth is pulled up by the Moon's gravity, forming bulges of water (one on the side of Earth facing the Moon and a smaller one on the opposite side, as the Earth is pulled away from the water on that side). The Earth is rotating through these bulges, which act similar to brakes and are slowing the rotation of the Earth down. However, the rotational energy that is lost in this process has to go somewhere, and is translated into the Moon's potential energy relative to the Earth. In other words, the Moon is moving further away from the Earth as a result of this slowing. We've actually measured how fast the Moon is spiraling away from us: 3.8 cm a year.

So, the energy gained from tidal heating would have to come from somewhere. Where it's coming from depends on the particulars of the system. It might cause Lyra to decrease the height of it's orbit, for example.

The same applies to your question about Erebus producing light. Light is a form of energy and that energy has to come from somewhere. You wont be able to produce light without some process that releases electromagnetic radiation, and that process will eventually run out, no matter what it is we're talking about.

If you are trying to set up some sort of place with life on it which will literally last forever, you either need to rely on fictional and unrealistic sources of energy, or redefine what the word "life" means. If you want to rely on unrealistic technology, then an inter-reality portal could constantly be pulling energy from another reality where conservation of energy doesn't exist. If you want to redefine "life," then that's an entirely different can of worms.

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