Here's why:

* Kepler's Laws of Planetary Motion: Johannes Kepler, through his observations, discovered that planets move in elliptical orbits, with the Sun at one focus of the ellipse. This is known as Kepler's First Law.

* Gravity: The force of gravity between the Sun and a planet is not constant. As a planet gets closer to the Sun, the gravitational force is stronger. This variation in force causes the planet's path to deviate from a perfect circle, forming an ellipse.

Important Note: While the orbits are elliptical, most planets in our solar system have orbits that are nearly circular. This means the ellipses are not significantly elongated.