Collisions in jPCT are always bounding "object" (ray, sphere, ellipsoid) agains actual geometry. In your case, the planet is the actual geometry. There's no need to make it "solid" or anything. It already is. The sphere/ellipsoid that you define in the method call is the bouding object for the colliding entity (a moving ship, the camera etc.(, not the collision's target (your planet).
Then you call the collision methods (in your case, most likely ship.checkCollisionEllipsoid(...)) and you should be fine. You might have to adjust the collideOffset setting in Config, because it usually isn't suitable for large scales as it is by default.