I'm working on a story where interplanetary travelers occasionally have to deal with space spore contaminating their ships. It's a very old form of life, and has spread throughout much of the galaxy. The system from which the spore originates, therefore, shouldn't fit the traditional -mold- (Thank you, I'll be here all week!) of an alien planet. More of a habitable nebula.
I require a rich and diverse ecology within an environment with the following constraints:
Near enough to vacuum conditions many forms of life can survive or spend a cycle of life in empty, interstellar space.
A large cohesive volume of without the confines of gravity, yet still have pressure gradient capable of producing liquids, solids, and gases.
My original idea was to have a supermassive blackhole surrounded by a huge nebular cloud. An entire life-filled habitat would exist within the cloud, at some orbital distance and the material. Rather than collapse into a planet or protostar, the material remains spread across the orbital plane. The atmosphere, oceans, and landmasses can be made of anything, (hydrocarbons, tholins, etc) so long as they can exist at all. I would allow 'shepherd planets' to keep everything in line, so long as life originates and flourishes in the nebula. Could material in orbit attain the density required without having to aggregate? Is such an environment even feasible?