Astronomers were surprised by the existence of hot Jupiters because, according to the planetary system formation theory available at the time (based only on the Solar System), these giant, volatile-rich planets should not have been able to form so close to their parent stars. The expectation was that Jupiter-type planets should form in the more distant, cooler regions of the protoplanetary disk, where the volatiles that make up much of their composition are able to survive. So astronomers suggested that perhaps hot Jupiters formed much farther away from their parent star and subsequently migrated inward to a closer orbit. The mechanism by which a planet could migrate over such a distance must involve an interaction with gas or planetesimals in which orbital angular momentum is somehow transferred from the planet to its surroundings, allowing it to spiral inward.