well
as i see this equation, and i remember it as F = GMm/d^2 (d=distance), if the object is at the center of the planet, the distance will be 0, 0^2 = 0, soo F=Something/0, there fore F=INFINITIVE
The formula only applies to masses centered at points, and when you enter the sphere, you have forces that undermine each other, so you should perform partial integration on each axis, on the sphere.
btw, x/0 is undefined, not infinite.