From: henry@spsystems.net   
      
   In article <3f7cd130.138801132@news.eircom.net>,   
   Russell Wallace  wrote:   
   >Something I'm curious about: All the outer planets seem to have   
   >trillions of objects in orbit around them, in a smooth continuum from   
   >moons comparable in size to our own, down to microscopic dust specks.   
   >Yet all the inner planets seem to be completely devoid of orbiting   
   >material, apart from the three moons Earth and Mars possess between   
   >them. Does anyone know the reason for the discrepancy?   
      
   The two types of planets form differently (as is obvious anyway, given   
   their radically different compositions).  The major moons of the gas-giant   
   planets probably formed in place, as part of the formation of the planet,   
   although there are one or two probable exceptions and the details are not   
   well understood yet.   
      
   A lot of the smaller odds and ends are probably captured asteroids, and   
   there again the outer planets are favored:  their large masses combine   
   with their greater distance from the Sun to give large gravitational   
   fields (more properly, areas of influence) with quite fuzzy edges, thus   
   making it much more likely that a passing object will wander through the   
   fuzzy area and possibly end up captured.   
      
   The formation of the inner planets doesn't seem to leave any room for   
   parts of them to remain in orbit.  And their areas of gravitational   
   influence are small, with relatively sharp edges, so it's rare for them to   
   capture anything.  (Not impossible, but rare.)   
      
   Earth's moon is the result of a huge impact late in Earth's formation, a   
   relatively unlikely accident.   
      
   Mars's moons seem to be captured asteroids, and just how that happened is   
   deeply mysterious.  Their orbits are fairly close and fairly circular,   
   which is not at all what you'd expect for captured objects.  Worse, they   
   seem to be *outer-belt* asteroids, and how they ended up at Mars is   
   another puzzle.  Again, this is probably the result of some unusual   
   accident.   
   --   
   MOST launched 1015 EDT 30 June, separated 1046,         | Henry Spencer   
   first ground-station pass 1651, all nominal!            | henry@spsystems.net   
      
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