Scientists don't understand why a big planet is orbiting a small planet. When Newton wrote F=MA, his calculus actually takes the "mass" and "calculates the curve of an object or volume", so it's really Mass divided by Volume (or calculus) which is density. It's the density of an object that makes mass/volume. So in the case of the small planet, it has more mass than volume. The larger planet has more volume, but less mass. So the moral of the story is to use both mass/volume when calculating any object, or you will be like Einstein with half a formula E=MC2 and it should be E=M/VC2