What actually are aliens? Are they organisms from another planet, or are they humans visiting from the future?

They are anyone not from America who came to America from somewhere else. Some of them might come from space. If they don’t have a visa they are just going to have to leave. That’s the law.
Before you ask this question about extraterrestrials, are you really sure anyone has visited Earth from another planet or maybe from the future? What is your evidence based science to back up that assumption?