The force responsible for the shape of planets in orbit is gravity. This fundamental force arises from the mass of celestial bodies and their mutual attraction, dictating the movement and structure of planetary systems. According to Newton's law of universal gravitation, larger masses exert stronger gravitational pulls, resulting in elliptical orbits for planets around stars. Gravity plays a critical role in maintaining the stability of celestial structures, influencing phenomena such as tidal forces and the formation of galaxies.