Here's why:

* Kepler's Laws of Planetary Motion: Johannes Kepler, through meticulous observation, formulated laws describing planetary motion. His first law states that planets move in elliptical orbits with the Sun at one focus.

* Gravity: The force of gravity between the Sun and a planet is not constant. It's stronger when the planet is closer to the Sun and weaker when it's farther away. This variation in force causes the planet's path to curve into an ellipse.

While a perfect circle is a special case of an ellipse, most orbits in our solar system are slightly elliptical.