Canon 500mm L f/4 is excellent lens for DS.... (I have 400mm Lf/2.8, stepped down to f/4 by external aperture mask).
However, planets (and Moon) are different - for that you will need telescope and good barlow.. and camera capable of taking video (which can be stacked with help of Registax, for example).
Fantastic work on planets was done by some members of this forum, using 8~12" Nextonians, see
here.