Answer:
Gravitational attraction of the sun.
Step-by-step explanation:
Gravity is an attractive force. Any two masses will exert an attractive force on the other according to Newton's law of universal gravitation. The more massive the objects, the stronger the force. The sun, as you can probably guess, is pretty massive - 330,000 times more than Earth, and 1,048 time more than Jupiter, our solar system's largest planet. Just like man-made satellites around Earth, the planets in our solar system are constant process of "falling" around the sun, locked in their orbits by its mass, but slowing dramatically in their orbital velocity the further away they are.