If the planets were evenly distributed over a cubic light year then there would be one Pluto or larger size planet every 2200 astronomical units or one for every cubic two light week volume.
This situation would not be like islands in the Pacific Ocean for Polynesian colonization. The reason is that there is almost no friction in space. If we have a colonization ship that gets up to 5 to 20% of the speed of light, then it would make no sense to spend fuel to slow down at a nomad planet and then use the same amount of fuel to speed up again. Other consumable supplies should be carried onboard and recycled as efficiently as possible.
For long term colonization the nomad planets would require a civilization not wanting to be near the solar energy of a star.
If they have a power source that was efficient and independent of a star, then this could be conceivable. Still for major colonization of particular nomad planets there would have to be something useful about the particular nomad planet. It is not efficient to develop a colony in space and then move resources over many light weeks or light months.
The resources on Earth or at any particular location need to be worth using at that location. There are no resource limitations that need resources to be moved from a moon to a planet or from Pluto to Earth.
There could be a need for secret base and the hidden construction of a space fleet but those kinds of future scenarios would be dependent on some hypothetical political situation.
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