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The Earth, itself, is the source for gravitational force. The hot, molten rock inside the core of the earth makes a magnetic field around it(the earth) which's force or pull that ensures all the objects of the earth are well stuck to it; a ball, if kicked, always come down to the ground only because of gravitational force. This is the gravitational force of the earth.
The earth is the source and that discover was made by newton but the sun holds the earth and the rest of planets in the solar system with its gravity