Science can study almost anything, here are a few examples. Science can study the human body and plant life. They can study these things because of new technology and testing their theories, then they can tell the world.
Science can not study ALL of the planets in our solar system or science was not able to know about bacteria. They can study these things but not thoroughly we were not able to know about bacteria until microscopes but we still don't know everything about it. Now scientist know a little about the far away planets but not everything because we have not been their. Yet.