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Because stars kill stuff with radiation, flares, etc, then die taking planets with them, might it not be easier for living things to live on a rogue planet's moon? Say you had a rogue Jupiter-like planet with tidally heated moons like Europa orbiting it. Europa stays warm inside just from gravity/going around Jupiter. Wouldn't that end up being a safer and longer-lived environment for life or would that not work due to lack of energy?

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  • $\begingroup$ Bump up the gas giant to a brown dwarf, and we've got something viable. Otherwise, I don't think any life is theorized to be possible to thrive in two-digit kelvin range. Although if the moon had own energy sources (geothermal, nuclear), that wouldn't be so impossible. $\endgroup$ – SF. Jun 4 '16 at 2:30
  • $\begingroup$ @SF. "Although if the moon had own energy sources.." ..you mean as described in the question? $\endgroup$ – Andrew Thompson Jun 4 '16 at 4:24
  • $\begingroup$ I consider the heating source for today heating to be the Jupiter like planet. And thus not the moons own energy source. $\endgroup$ – Hennes Jun 4 '16 at 10:22
  • $\begingroup$ Yeah, from what I've heard, you can get a lot of energy from tidal heating. This would, I believe, last a very long time. However, my guess/understanding is also that over billions of years, objects in this arrangement will eventually part or collide (for eg, won't the moon eventually leave its orbit, Mars will eat one moon and lose the other). $\endgroup$ – eingrossgeek Jun 4 '16 at 16:51
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If the right chemical bath spews from hydrothermal vents on the floor of an ocean on this moon, then perhaps it could host creatures like the ones found around such vents on Earth.

Here is a quote from an article on the subject in NASA Science News:

Instead of photosynthesis, vent ecosystems derive their energy from chemicals in a process called "chemosynthesis." Both methods involve an energy source, carbon dioxide, and water to produce sugars. Photosynthesis gives off oxygen gas as a byproduct, while chemosynthesis produces sulfur... Because they offer an alternative way for life to meet its fundamental need for energy, these vent ecosystems have piqued the interest of astrobiologists -- scientists who study the plausibility of life starting elsewhere in the universe.

The level of tidal heating required for volcanic activity on that level is seen in our solar system on Io, Jupiter's inner moon. In Io's case, it would not remain in an orbit eccentric enough for these forces to act on it in this way if the other large moons of Jupiter weren't preventing its orbit from circularizing. So for such a model there would need to be several moons around this rogue planet, orbiting in a particular way.

There may also be conditions under which rogue planets could support life without any moons involved, as investigated in the article The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space:

We find that a rogue planet of Earth-like composition and age could maintain a subglacial liquid ocean if it were ≈3.5 times more massive than Earth, corresponding to ≈8 km of ice. Suppression of the melting point by contaminants, a layer of frozen gas, or a larger complement of water could significantly reduce the planetary mass that is required to maintain a liquid ocean.

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It wouldn't work. Heat is a form of energy, but thermodynamics means that not all forms of energy are equally valuable. Any form of energy can be converted to heat, but you need temperature differences to turn a fraction of the heat into usable energy.

The hypothetical moon has only a limited amount of heat generation, which also means there are only very small heat differences. Life needs energy in other forms: light for plants, chemical for animals. Converting a very small fraction of a little bit of heat simply can't provide that.

Tidal heat generation occurs most strongly in liquids (oceans), but those are also the environments in which temperature differences are hardest to obtain. The combination of tidal forces and temperature/density differences drive currents which causes mixing, and thus a decrease in temperature differences.

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  • $\begingroup$ It may be that you are correct about the particular case, but your answer doesn't sufficiently demonstrate that. It is not definitively known that life needs light. You do not explain why the system would lack chemicals needed. $\endgroup$ – called2voyage Jun 8 '16 at 15:24
  • $\begingroup$ @called2voyage: light and chemicals are examples of different forms of energy. The examples show that there indeed is no mandatory form of energy, but all life needs energy in some form other than heat. $\endgroup$ – MSalters Jun 8 '16 at 15:33
  • $\begingroup$ But why would chemicals be unavailable? $\endgroup$ – called2voyage Jun 8 '16 at 15:39
  • $\begingroup$ @called2voyage: As my chemistry teacher used to say, everything is a chemical. The problem is the availability of energy in chemical form. There's no mechanism to produce those. On earth, such chemicals are predominantly produced by plants and consumed by animals. There are some hot vents in the ocean that support life, but those are really hot (heated by magma, not by tidal warming) $\endgroup$ – MSalters Jun 8 '16 at 15:47
  • $\begingroup$ There are also psychrophiles that eat metal. It doesn't seem that plants or extreme heat are needed. $\endgroup$ – called2voyage Jun 8 '16 at 15:48

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