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For your consideration, a star system inside a dark nebula with a potentially habitable planet. Being a dark nebula, we Earthers can't see light shining behind the nebula. Further and therefore:

  • The nebula is large enough that the potential inhabitants won't see any stars in any direction. This previous question appears to discuss "normal" nebula and I'm not enough of an astronomer to know whether this is an absolute for dark nebula. But I'm assuming if we can't see through it, it's potentially large enough that no one at the center could, either.

  • The gravity of the sun would, I believe, draw the surrounding nebulous cloud into itself... which would suggest that it would draw the material inside the orbital planes of the planets.

  • It is therefore my assumption that as the planets orbit, surrounding material is constantly pulled into the atmospheres where I believe it would incinerate. The leading point of atmosphere would be aglow with burning debris that may even pass around the atmosphere and leave a bit of a thermoluminescent trail behind the planet. Dust consisting of mostly Carbon would be constantly falling.

But...

Could such a planet have a thick enough atmosphere and a strong enough magnetosphere to block enough of the nebular material from falling and allow for an inhabitable 70% water planet?

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    $\begingroup$ I'm pretty sure the fact that it's in the center of a nebula won't affect the habitability of a planet that much. Nebulae are extremely spread out and are only opaque because they are also obscenely large. $\endgroup$ – Gryphon Jan 9 '18 at 0:15
  • $\begingroup$ Oh, @Gryphon, but that would take the fun out of my world! Say it ain't so! $\endgroup$ – JBH Jan 9 '18 at 0:32
  • $\begingroup$ The habitability of the planet will depend more on its relationship with its primary. Essentially the same as any other planet. Expect spectacular skies due to a more or less continuous rain of debris turning into shooting stars. Like the leonids or other meteor showers the year round. If sapients evolve, the rest of the universe will be occluded. They will live in a space surrounding by darkness. $\endgroup$ – a4android Jan 9 '18 at 4:51
  • $\begingroup$ @a4android, is the density of a dark nebula so great that my world won't have trouble filling up with sediment, so to speak? (Now that I think about it, with enough Clarkian Magic, does it make sense to dredge a world?) $\endgroup$ – JBH Jan 9 '18 at 5:04
  • $\begingroup$ Planets are usually massive enough to vastly outmass the amount of matter falling on them. Meteoric dust could be a fair part of the sediment. This depends on the rate of mass infall & the timescale over which it happens. Trivial bit: "Clarkean" is the preferred adjective. There is an 'e" in Clarke's name, after all. $\endgroup$ – a4android Jan 9 '18 at 5:38
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Definitely. First, the nebular material must get into the solar system. (It certainly couldn't be there to begin with - when a star forms, it eats a hole in the nebula. So, this system had to have moved into a nebula later.) A system like our own, outside a nebula, has something called the heliosphere, which is where the star's solar wind dominates.

The effect of the solar wind and radiation pressure means that this dust will not be able to simply fall down into the planet's atmosphere. And it's usually the charged, high-energy particles from the solar wind that are the problem with regards to holding in atmosphere.

The dust itself will have some effect on the planet, not sure what, because of the way dust interacts with the weather (in complex ways I'm not going to even pretend to understand.) (and this leads further into effects on life.)

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Newly formed stars tend to expel the nebula around them, if the nebula is dense enough for there to be an accretion disk in spite of the solar wind the planets in the system will have decaying orbits due to "friction" with the accretion disk (the dense planets are orbiting much faster than the disperse gas/dust so ploughing through this medium will slow the planet down) and their increasing mass (more mass + same inertia = less velocity).

I don't think they'll remain in orbit long enough for life to evolve however they may be habitable for a cosmologically short period of time, although my timeframe estimates are pure speculation since I don't have the means to computationally model any of this.

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