Here's why:

* Planets orbit stars: Planets revolve around stars, while stars are massive balls of hot gas that produce their own light and heat through nuclear fusion.

* The sun is the center of our solar system: All the planets in our solar system orbit the sun.

* The sun is much larger and hotter than planets: The sun is significantly larger and hotter than any of the planets in our solar system.

So, while the sun is an important part of our solar system, it's not a planet but a star.