text: There are some theories (panspermia etc) that the earth is under constant bombardment by biological material from outer space that causes illnesses etc. Some claim that life on the earth was even brought to the earth from someplace else in the cosmos. \n\nBut you are probably not referring to this. You are thinking about the romantic visions one gets from science fiction books and movies. At this time, our understanding of physics precludes easy or possibly any visitation of the earth by intelligent alien beings travelling here in space craft. We do not yet find evidence for any other life, especially intelligent advanced life, in our solar system. The other stars are just too far away for some other alien civilization to go to the effort to come here.