Moments of inertia of planets
The planets are very, very close to spherical (the Earth has a variaton of about 0.1% between polar and equatorial radii).  

The rotational inertial for a solid sphere is

$$I=\frac{2}{5}mr^2$$

If you give us details about why you might be interested in looking at the rotational inertia of planets in greater detail, there might be more motivation for putting effort into finding an answer for you.

Just saying.