The solar system is a many-body system. But as far as any individual planet is concerned, its motion can be described (to a good approximation) in terms of just itself and the sun, because the sun has well over 90% of the total mass in the solar system.

So we say that planets orbit the sun in elliptical orbits, or, if the eccentricity is small enough, circular orbits.

But this is only a good first order approximation. What is the effect, over millions of orbital cycles, of perturbations due to other planets, or drag due to the very sparse interplanetary medium, or outward pressure due to the solar wind? I do not know about the detail of perturbation theory solutions for many-body systems of the solar system type, but I suspect that one property of the accumulated perturbations would be to gradually reduce the eccentricity of planetary orbits.