Answer:
Gravitation.
Explanation:
The space where the planets are is absolutely empty, so there is no friction. The planets may have some magnetism, but magnetic forces are negligible when compared to the Gravitational force over the planets due to the Sun mass. The equation which describes the Sun's force is: F = [tex]\frac{G.msun.mplanet}{r^{2} }[/tex] which is the famous Newton's gravitational law. Radiation from other stars doesn't contribute to the planets' motion, which is elliptical trajectory.
