Question

why earth is a planet and sun is a star?

Answer 1

Do you know their definitions?

Answer 2

Stars are those who have own light and create them by nuclear fusion inside it.
and Planets have not own light they glow by other star's light e.g. venus(we called it the morning star).Planet also moves around the star
As The Sun has own light and create its own light so it is a star
Earth has no own light and moves the sun So it is a planet