Here's why:
* Gravity: The sun's gravity pulls on the planet, keeping it in orbit.
* Inertia: The planet also wants to move in a straight line due to inertia (resistance to changes in motion).
* Balance: The combination of gravity and inertia results in an elliptical path, where the planet is sometimes closer to the sun (perihelion) and sometimes further away (aphelion).
It's important to note that while the orbits are elliptical, they're close to circular in most cases. For example, Earth's orbit is slightly elliptical but is often depicted as a circle for simplicity.
