Karl Schroeder’s imaginary “Lockstep” Civilization or Empire is a group of thousands of human-inhabited planets which periodically go into synchronous suspended animation for 30 years at a time, then revive for a month – a ratio of 360:1. This conserves resources and allows them to travel ‘quickly’ (from a subjective point-of-view) between worlds. Intriguing the planets are those possibly thousands which are sprinkled through the dark void between every star. The suggested number is ~hundreds of thousands of Pluto-to-Moon sized objects and thousands of Earth-sized objects for every star in the sky. A vast territory, all within a few light-years at most.
The Lockstep Empire is a reasonable solution to the problem of interstellar travel & communications – if we can’t develop warp-drive. It’s also a good exposition of the inter-stellar (“between the stars”) territory that civilizations could access and populate. Schroeder has rightly noted that many thousands of large objects probably exist between the stars, though maybe not so many Neptune to Jupiter sized ones, however, as WISE has demonstrated there seem to be fewer brown-dwarfs and super-Jovians Out There.
Here’s a short-story set in the Lockstep universe, from Tor: Jubilee, by Karl Schroeder
The kind of materials captured by the small denizens of inter-stellar space depends on how they formed. Two main options: via direct collapse out of protostellar nebulas or via scattering from forming planetary systems. In either scenario they might capture large amounts of the primordial nebula’s hydrogen/helium gas, along with ices, silicates and metals. One could have a large population of “micro-Neptunes” – Mars-to-Earth mass objects with gas-planet atmospheres. Internal geothermal heat will make it very hard for such objects to condense their helium atmospheres, but hydrogen might cool sufficiently to form oceans and ice-caps. Imagine such dark, eerie oceans of ultra-cold liquid, lit by stars twinkling through helium atmospheres…
Marshall Eubanks notes in the the comments:
In a short submission to Nature in 1999 (http://www.nature.com/nature/journal/v400/n6739/full/400032a0.html), David Stevenson pointed out that an Earth-like nomadic planet in deep space, with an Earth-like amount of radioactive heating, could have a thick hydrogen-helium atmosphere and a surface temperature high enough to support liquid water on the surface, even though there was no Sun and the planet would appear to have a temperature (as seen from the outside) of only 35 K. So, there might be “dark, eerie oceans” of regular old water on some dark Earths, oceans we might conceivably be able to sail without much protection, although we would need some sort of breathing apparatus.
Stevenson’s paper is available here, slightly longer than the “Nature” edit, on the author’s web-site: Interstellar Planets