Hello!!

no external torque→angular momentum conservedno external force→linear momentum conservedno external torque→angular momentum conservedno external force→linear momentum conserved

There is no external torque about the sun since the force of the sun and position vector are always at an angle 180180 since τ¯=r¯×F¯τ¯=r¯×F¯ , so angular momentum is conserved.

But since the path in not circular but elliptical , the position vector is not perpendicular to direction of motion hence some work is done, which changes the momentum indirectly by changing magnitude of velocity directly.

Thanks

Hii,

Angular momentum is conserved or constant because the force of gravitational attraction between the planet and the Sun exerts zero torque on the planet.

Hope this helps.