I am really glad to know that you want to pursue your career in Astrobiology. Astrobiology as you know is all about understanding the origin, evolution, distribution and future of life in the Universe. The origin of life is still one of the biggest mysteries of Science and probably would require help from every discipline to crack this mega mystery. I therefore think anybody with a Science major can become an Astrobiologist as every single contribution counts when it comes to solving problem as big as origin of life. You can be a chemistry major trying to understand the chemical origin of life or you can be a microbiologist studying extremophiles. You can be again a mathematician who is creating a mathematical model to understand non-equilibrium systems required for the formation of self-replicating systems. The list is huge. What I mean is that you can have any degree in Science and still you can contribute to Astrobiology.