Which physical quantity remains conserved when planet revolves round the sun in an elliptical orbit?

When a planet revolves around the sun in an elliptical orbit, the physical quantity that remains conserved is angular momentum. This conservation occurs because the gravitational force between the planet and the sun acts as a central force, which does not do work on the planet and thus preserves its angular momentum. Additionally, the total mechanical energy of the planet-sun system is conserved, assuming no external forces act on it.