Complex sentence: Kepler-9b and 9c are thought to have formed at a greater distance from their star, beyond the "frost line", and then migrated inward due to interaction with the remains of the protoplanetary disk.
Split into simpler sentences: Kepler-9b and 9c are thought to have formed beyond the "frost line". They are then thought to have migrated inward due to interactions with the remains of the protoplanetary disk.