Here's why:

* Planets orbit stars: Planets are celestial bodies that orbit stars. The sun is the star at the center of our solar system, and the planets in our solar system (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) all orbit the sun.

* Stars produce light and heat: Stars are massive balls of hot gas that produce their own light and heat through nuclear fusion.

* Planets reflect light: Planets are much smaller and colder than stars. They don't produce their own light but reflect light from the star they orbit.

So, the sun is a star, not a planet.