Neil deGrasse Tyson is one of the (if not the) most well-known astrophysicists. He is also an accomplished author and science communicator/entertainer, and the Frederick P. Rose Director of the Hayden Planetarium at the Rose Center for Earth and Space in New York City. Also, he doesn't think that we are alone in the universe.
Appearing on C-SPAN's In Depth, he explained how the chemistry of life makes it extremely unlikely that Earth is the only place in the universe where life has formed. He said, "Whatever happened on Earth, it's not likely to be rare or unique. Because carbon chemistry, on which life is based, is the most fertile kind of chemistry there is. And carbon is abundant across the universe."
However, Dr. Tyson does separate the questions of whether there is life elsewhere in the universe, and if intelligent life exists, from if we have been visited by any intelligent life. "What the UFO community puts forth as evidence is weak on a level that, in any scientific circle, would be kicked out of the lab room."
The basis of this argument boils down to the foundations of the scientific method. Eyewitness testimony is nowhere near enough evidence to support a claim as fantastical as alien visitors. Current evidence of chemistry is enough to allow scientists to assert that there is life on other planets, but that evidence does not extend as far as intelligent life.