Asteroid fields can be found occupying planetary orbits. They sometimes serve as a base for pirates. Asteroid laboratory can be built on them regardless of system, while Asteroid mine can be built if their system is colonized. With planetology (TL 5), a Space factory can coalesce them into a colonizable large or huge barren planet. If a planet has exploded due to a supernova or Stellar converter then the size of the planet built from the Asteroid field is the size of the original planet.