There has evolved a species of turtles large enough to maintain an atmosphere of their own and travel interstellar space. We have already discovered how they don't collapse under their own gravity, but we don't know how they reproduce.
Somehow, while still being oxygen breathers, these turtles have grown large enough to carry an atmosphere with them, and enough vegetation growing on them to maintain the oxygen levels in that atmosphere that they became known as the world turtles. Each one carrying a self contained ecosystem of which they are a part.
Turtles however are egg laying species, not normally known for the tender care of their young, but the young won't be large enough to maintain their own atmosphere when they hatch.
Normally turtles go back to the beach where they were born, but planetary landing by thousands of world turtles laying eggs very quickly becomes destructive, so while they still go back to the same system that they initially evolved in, the planet itself is long since gone.
How do young world turtles get an atmosphere? Even if the mother raises them, they'll need to top up atmosphere as they grow.
If you're committing a mother to rearing her children, how big does she need to be and how many young could she support?