Aliens?
The idea of aliens is usually associated with science fiction but scientists have begun studying this more and more. They have been studying planets that could possibly be home to extra-terrestrial life forms, but these planets need to have very specific conditions in order to support this type of life. The planet would need to be a terrestrial planet which means that it has a solid surface. It would need to have a water source. It would also need to have an atmosphere and gravitational force much like that of the Earth.