If you only need to render the surface of the planet, then you can use a Sphere object instead of the planet object. The Sphere is found under the Add Object > Displaceable Object menu. It has Scale parameters, so you can adjust the xyz values to shape the sphere as desired.
However, the atmosphere is limited to a spherical shape and tied to the planet object, so it can't be deformed as the Sphere object can. We're looking at adding this feature to the atmosphere in Terragen soon.
