Do planetary systems in general differentiate terrestrial and gaseous planets by an asteroid belt?
Egads, that is a great question. Maybe the asteroid belt, indicates the beginning of the terrestrial field, yet the end of gravity sufficient to maintain terrestrial planets in stable orbit. Unless they are so distant like Pluto/Charon, that their membership in the solar system is just a tenuous thing, dependent on no greater force peeling them away from the system.
Other than our own, we have very little knowledge of the structure of planetary systems. Planets around other stars have been identified only in recent years, and they are all big (like Jupiter), so making any such generalization about asteroid belts is premature. One thing that has already been observed is that some of these big planets are very close to the star - around Mercury distant.
Mathman got it. IIRC, the smallest extrasolar planet found is still on the order of something the size of Neptune. So, we are far from understanding extrasolar terrestrials & asteroid belts. Stay tuned...the research on extrasolar planets is moving fast!
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