Here's why:

* Kepler's Laws: Johannes Kepler, a German astronomer, discovered three laws of planetary motion. His first law states that planets orbit the sun in ellipses, not perfect circles.

* Gravity: The sun's gravitational pull is the force that keeps planets in their orbits. This pull is strongest at the closest point in the orbit (perihelion) and weakest at the farthest point (aphelion). This variation in gravitational force is what creates the elliptical shape.

While we often depict planetary orbits as circular, the truth is they are slightly oval-shaped, with the sun located at one of the focal points of the ellipse.