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.
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?