We do not offer a separate Masters program, though some students obtain this degree in Planetary Sciences or another GPS option as part of their pursuit of a PhD. At the graduate level, Caltech is one of the pre-eminent groups in the world with a wide range of research opportunities and advanced course work. Some continue to span multiple research areas for their Ph.D. Admission is based on general excellence than on a close match to a particular research area, and it is possible to be admitted without a precise idea of which subfield is preferred. For the graduate program, a document outlining expectations for a typical Ph.D. trajectory is here.