From: gdpusch@NO.xnet.SPAM.com   
      
   Hop David  writes:   
      
   > Gordon D. Pusch wrote:   
   >>  wallacethinmintr@eircom.net (Russell Wallace) writes:   
   >>   
   >>> 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?   
   >>   
   >>  Gas-giant planets and "terrestrial" planest are believed to form by   
   >>  different physical mechanisms. "Terrestrial" planets are believed   
   >>  to form by collisional accretion of "planetesimals," whereas "gas giant"   
   >>  planets are believed to form by accretion from a sub-disk embedded within   
   >>  the main protoplanetary disk that itself resembles a "mini solar system."   
   >   
   > Do Uranus' moons lie in her equatorial plane?   
      
   Yes. Tidal friction tends to damp out an inner moon's orbital inclination   
   relative to the planetary equator on timescales short compared to the   
   lifetime of the solar system.  High orbital inclination satellites   
   usually have fairly large orbital radii, and are generally thought   
   to be relatively recent "captures." (One exception to this is if   
   their inclinations are being "forced" by some  orbital resonance.)   
      
      
   -- Gordon D. Pusch   
      
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