Here's why:
* Gravity's influence: The planets are held in their orbits by the Sun's gravity. The force of gravity is not constant throughout the orbit; it's stronger when the planet is closer to the Sun and weaker when it's farther away.
* Orbital speed: Planets travel faster when they are closer to the Sun and slower when they are farther away. This variation in speed contributes to the elliptical shape.
While most orbits are slightly elliptical, they are close enough to circular that we often visualize them as circles for simplicity.
