Planetary fields

The field lines of a spherical mass are always directed towards the centre, so the field pattern is just the same as for a point mass are always directed towards the centre, so the field pattern is just the same as for a point mass.

For a spherical mass M of radius R, the force of attraction on another mass (m) at distance r from the centre of M is the same as if mass M was concentrated at its centre. Therefore, the force of attraction between m and M,

F= GMm/r²

Therefore, the magnitude of the gravitational field strength at distance r is given by g=F/m= GMm/r², provided distance r is greater than or equal to the radius R of the sphere.