Here's why:

* The Nebular Hypothesis: The prevailing theory of solar system formation is the Nebular Hypothesis. This theory states that the Sun and the planets formed from a giant cloud of gas and dust called a nebula.

* Sun Formation: The collapse of this nebula under its own gravity led to the formation of the Sun. The center of the nebula became hot and dense enough to start nuclear fusion, which is the process that powers stars like our Sun.

* Planet Formation: The remaining material in the nebula then began to clump together to form the planets, including Earth. This happened after the Sun had already formed.

So, the Sun came first, and Earth formed later from the leftover material that was orbiting the newly born star.