According to General theory of relativity, gravitational force is produced due to stretching of space-time fabric by mass of a heavy body. It is also dependent on the density of the object. If mass of two objects is equal but density is different, then gravitational force on the surface of object having higher density will be higher. Thus gravitational force on the surface of planets varies from one planet to another according to their mass and density. A more massive and denser planet has higher gravitational force than a planet with less mass and less density.