On the gravitational potential of an inhomogeneous ellipsoid of   revolution (spheroid)
  It is shown that the gravitational potential outside an inhomogeneous ellipsoid of revolution (spheroid) whose isodensity surfaces are confocal spheroids is identical to the gravitational potential of a homogeneous spheroid of the same mass. 
V.V.Gvaramadze & J.G.Lominadze