The question asks to identify the term for a person who studies stars and planets. Let's examine the options:
Biologist: A scientist who studies living organisms. This does not relate to stars and planets.
Geologist: A scientist who studies the Earth, including its materials and processes. This also does not pertain to stars and planets.
Astronomer: A scientist who studies celestial bodies, including stars and planets. This aligns with the description provided in the question.
Physicist: A scientist who studies matter, energy, and the fundamental forces of nature. While some physicists may study phenomena related to stars and planets, this term is broader and not as specific as 'Astronomer.'
Based on this analysis, the correct answer is: Astronomer.