Although it is highly likely that LIFE of some form DOES exist somewhere else in the universe if not in our own Galaxy , (the Milky Way),and maybe even within our own solar system, there is absolutely NO evidence that the Earth has ever been visited by aliens !!!

Because aliens from a different star system would be composed of a different isotopic mixture of the common elements, it would be VERY easy to prove or disprove the authenticity of any material (living or inanimate) from another starsystem. And despite the hundreds of reports of UFO's and aliens, NO ONE has ever produced any MATERIAL that has an unusual mix of isotopes.
It is very easy to fake a picture and also to mistake a weather ballon or aircraft for a UFO!!!

Scientists are trained to be skeptical!!! We know what to look for and NO ONE has ever produced any material that has an anomalous mixture of isotopes.

We do get inanimate objects that come to earth from space all the time; these are called meteorites and they originate with our own solar system.

check out this site on the web to learn more about something that is REAL !!! the science of meteorites !!!

http://www.meteoritecentral.com/

I would like to very much believe that aliens do live, but as of today's date, no actual physical evidence has been shown to verify this belief.I think that aliens do exists because of the statistical probability: so many stars means so many planets--and the fact that life seems to find a way, but none have been to earth and left physical proof. If there are alien civilizations out there, then our best hope is to listen for them (via radio telescopes). We should be able to pick up some form of transmission--since radio waves have a tendency to travel long distances. Look for a program called SETI on the internet . They haven't found anything yet, but they are looking.