Camera angle has a lot to do with the planets getting squished, especially as they appear around the fringes of the image. Instead of being up close with wide angle, back off a bit and zoom. This helps. In the camera attributes box there's a FOV value. 60 is the default. Try a lower number. The human eye has about a 55 FOV so 60 is actually slightly wide angle. The actual FOV number that you use to stop the squishing is dependent on how far away the object is from the center line of the camera. Corners in the image vs. center of the image.