A NASA scientist admits that it's entirely possible aliens have already visited Earth - and we simply never noticed. The space expert also noted that not all UFO sightings can be "explained or denied", and said scientists should be more open-minded about the possibility of alien visitors. Nasa has long been investing in SETI, the "search for extraterrestrial intelligence" - better known as aliens. And in a recently publisher paper on SETI, Professor Silvano P. Colombano suggested that alien life may have already visited us. He suggested that aliens could look so different from how we expect, and that they may be able to travel huge distances - because we simply can't comprehend their make-up or technology.