Start with Physics, Chemistry, and Biology, with a strong dose of math.

A planetary geologist needs to have at least a master's degree, and for most jobs a PhD is required. It's necessary to complete courses in geology, physics, chemistry, and other applicable sciences.

Planetary science is an interdisciplinary study, involving physics, geology, astronomy, environmental studies, and other fields, which tries to understand what planets are made of, how they operate, and differences between them in the known universe. This is done by analyzing the key features of planets and their systems.

Well, you've put the question badly, but it's either cosmology or stellar / planetary physics

William M. Kaula has written:

'An introduction to planetary physics' -- subject(s): Solar system, Planets