Well you might consider making the sun larger. The sun after all is many times larger than our earth. You throw in distance and the sun should show along most if not all of the edge of the planet as it rises. The sun is no small body. You could also give the sun some more flare by adding an acctual light. A visible volumetric spot seems to make the most brilliant looking light beems. Could create a cool effect across the edge of the planet. Just some thoughts.