Answer: The gravitational force that exists between the planet and the star.
Step-by-step explanation:
Any two mass-containing objects pull toward one another, according to Newton's law of universal gravitation.
When a planet orbits a star, the gravitational pull of the star works as the centripetal force to maintain the planet's curved course around the star. The necessary inward push to keep the planet's elliptical or circular orbit is provided by the gravitational pull between it and the star.