Below are two pictures, one is a nasa photo of jupiter, the other is a reference for light on form from proko.com.
Note how on the planet, the light transitions from halftones to just shadow and it stays black then. There is no core shadow on the planet because there is nothing in space out there that would reflect light back onto the side of the planet lying in shadow.
Unless a planet is lit from the back, there should be no rim lighting. So to make planets appear spherical, pick the light source in a way that you draw between 50% and 60% of the planet in light and halftones along a nice curve(which should be half of an ellipse == a circle in perspective(which cuts the sphere in half, perpendicular to the direction of light)) and then leave the remaining 50% to 40% of the planet black, no rim light on the black side.
