The branch of science that studies planets and stars is astronomy.

Here's why:

* Astronomy is the study of celestial objects (like planets, stars, galaxies, etc.) and their properties.

* Astrophysics is a subfield of astronomy that focuses on the physical properties of these objects (like their composition, temperature, and evolution).

While other branches of science, like physics and geology, play a role in understanding the universe, astronomy is the primary discipline focused on planets and stars.