Gravity on round object ';..;'

I assume Thomas means he's made a 3D sphere (the planet) and wants his object to be attacted to the center of the sphere no matter where it is on the surface, like a giant walking around a small moon. Much like the games Populous 3 or the mini game in the latest Ratchet and Clank game.

I too would like to get some tips on how this could be done :-)