I really came across it completely by mistake before I started my undergraduate studies. I was looking at where I might want to go to graduate school and what I would be most interested in for biology and I found University of Washington's Astrobiology page. The idea of that being a topic took me by surprise so I began looking into it and became pretty hooked on the concepts. Its a fairly interdisciplinary field and, being in a college that only had interdisciplinary courses, it seemed like a natural fit for me.
There aren't any astrobiology programs by themselves at this point. Usually it is topics studied within microbiology (extremophiles), Earth Science (Early Earth evolution, planetary surfaces), Atmospheric Sciences, and Astronomy (planet detection). My top school actually has a program to get a dual-title PhD in my department and astrobiology. It requires you take course in another department and have at least one professor from another department on your dissertation committee.
And yeah, I keep telling myself that I wanted to only apply to my top programs this year and spread out next year if I don't get in. Still it gets hard when you are looking at statistics on admissions and waiting for the decision to be made. Rationality has no place in this purgatory that is grad school admissions.