Effect of planetary layer densities on trajectories of a satellite

[Tahmeed]

According to Cornell university's ask an astronomer site, we figure out the layers and their density of planets by checking the trajectory of a satellite /space probe or something like that, near the planets.
But planets are quite spherical. then their density profile shouldn't be effecting the gravitational field at all.

 

[tony873004]

The trajectory gives us mass. The visual size gives us volume. Together this gives density. If the surface is ice, but the overall density is that of rock, that tells us a little something about the layering.