PlotPlanet:

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   Add planet to current axes.
   The planet will be a solid color sphere. The color may be input as a
   structure with color data and a texture map, see Earth.mat
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   Form:
   hSurf = PlotPlanet( r, radius, color )
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   Inputs
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   r        (3,1)     Location of planet in figure frame
   radius   (1,1)     Planet radius
   color     (:)      Color, i.e. 'y', 'b' or [R G B], or structure

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   Outputs
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   hSurf    (1,1)     Handle to surface object

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