if you don't want to move'em anywhere then you can just use
turn object left,planet,step_value
where planet is your object number and step_value is kinda how fast it'll turn. Higher numbers faster.
if you animate it in a modelling package then i reccomend putting a "handle" inside of it, which you can link the planet to.
the handle will never be seen s it's usually like a blank 6 poly cube.
then you can animate the planet in your package, set object speed in db, loop object in db, and when you want to move it grab it by the "handle" which stays oriented get it?
ZX Spectrum 48k Issue 3, Radio shack Tape drive, Rank arena 12" T.V. set.