The Center for Astrobiology at the University of Colorado

There is no formal undergraduate major in astrobiology. Rather, we feel that undergraduates should be well trained in a core discipline and, at the same time, exposed to the breadth of disciplines that make up astrobiology. Therefore, we recommend that undergraduates with an interest in astrobiology choose a major that resonates with them; examples might include geological sciences, physics, astrophysics, biology, chemistry, and so on. We recommend that a student take courses at all levels that cover a wide variety of disciplines that make up astrobiology. In addition to the junior-level course in "Extraterrestrial Life" (GEOL 3300/APS 3300), there are a variety of courses in the various departments that deal with the nature of planets, the diversity of life on Earth, and evolution.
You should make an effort to take courses outside of your discipline; for example, biology majors should take some related courses in the physical sciences, and vice versa. And you should not ignore the humanities--there are undergraduate courses in the philosophy of science, for example, as well as the nature of intelligence and the mind or the evolution of human thought.