Odysseus wrote:

What constitutes an "independent" orbit?

An orbit which is gravitationally dominated by central star of the
planetary system. A body which orbits a larger planet such that the
center of mass of the two-body system resides inside of the larger body
is considered a planetary satellite or a "moon", and not a planet. For
example, Jupiter's moon Ganymede is larger than both Mercury and Pluto
and if it were by itself orbiting the sun, it would probably be referred
to as a planet. However, it orbits Jupiter and therefore is not a
planet.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 13th Annual NEBRASKA STAR PARTY *
* July 23-28, 2006, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************