Exobiology is frequently misunderstood. People who do know the phrase tend to think of it as the study of extraterrestrial life forms... in other words, a completely hypothetical science. But exobiology can refer to the biological study of living creatures that we know (like mice, or humans, or lettuce) in the environment of space, or on the moon, or on another planet.
Of course, we can also deal with more hypothetical questions, such as alien life forms. But here, exoscience is not a simple guessing game. Based on what we know about evolution, biology, and chemistry, we might be able to make some fairly reasonable guesses about what forms of life might evolve in different situations. Specifically, this branch of exobiology is usually termed xenobiology.
For a further discussion of this, please see the Alien Studies Page.