Planetary orbits are not circles...quite
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.