It seems that aliens can be on any planet. Mars was recently discovered that it once had water on it. So, aliens could have lived on this planet. It does seem like there have been UFOs spotted in different places and sometimes people believe that the military has housed aliens who have crashed onto Earth.
I believe that if there are aliens or other beings living on planets that they would most likely live on Venus because that planet is closest to the way things are like on Earth. I know that only time will tell whether aliens are living on other planets. It seems like in the next century we may find out if there are other aliens on other planets.