Here's why:

* Newton's Law of Universal Gravitation: Planets are held in orbit by the gravitational force of the sun. This force is inversely proportional to the square of the distance between the planet and the sun. This means that the force gets weaker as the planet moves further away from the sun.

* Kepler's Laws of Planetary Motion: Kepler's First Law states that planets move in elliptical orbits with the sun at one focus of the ellipse.

While a circular orbit is a special case of an ellipse, in reality, most planets in our solar system have slightly elliptical orbits.