Exactly as Kreator said, a workaround involves attaching the object (planet) to a parent object (concentric with the "Sun" but with visibility set to "off"). Rotate the central parent object, and the planet will orbit around it as if attached by an invisible stick. To create an eliptical orbit, animate the child object's proximity to the parent object following the vertical axis of a sine curve, prior to animating the parent object's rotation, with two oscillations per revolution of the parent object. Increasing the amplitude of the oscillation will tighten the elipse. This would be better accomplished with a script than with keyframe animation, but keyframing will still produce a good result.