Planetary orbits are not circles...quite

Graphic - planetorbits
A circular planetary orbit is consistent with Newton’s 2nd Law. But the orbit doesn’t have to be circular. A full solution of the equation of motion shows that the orbit is, in general, an ellipse (Kepler’s 1st Law). In fact most planets do have near-circular orbits. Pluto’s is one notable exception. And comets (shown as wedges in the figure) often have highly eccentric orbits.