There are a small number of potential capital worlds that are positioned in nebulas near rift zones in such a way that no other planets are visible from the starting position.

Players unlucky enough to start in such a position will be unable to send fleets anywhere and can't explore or expand unless another player messages them (this gives them a potentially valid fleet destination - the other player's capital.)

I would suggest that all potential capitals be placed a healthy distance away from rift zones.