Artemis - The Fifth World?
Centauri Dreams has a piece on a hypothetical world called Artemis that
may have existed between the solar orbits of Mars and Jupiter until the
movements of the giant planets in the early days of our Sol system
destroyed this world.
The full item here:
http://www.centauri-dreams.org/?p=648
The Abscicon 2006 abstract here:
http://abscicon2006.arc.nasa.gov/abstract/id/152.doc