Two possibilities exist: Either we are alone in the universe or we are not. Both are equally terrifying.
-Arthur C. Clarke
It is arrogant, and ignorant, to assume that human kind is the only intelligent life in this vast cosmos. With the fact that human beings have only existed for three million years, and the universe has existed for 13.7 billion years, it is hard to believe that other civilizations on other planets have not been able to evolve before humans did, and develop technology to explore the universe out of curiosity. We have only existed for 0.02% of total cosmic time, and only 0.06% of Earth's lifetime! Astrophysicists have found evidence that our sun, and the solar system that surrounds it, was formed by the remnants of an explosion of a nearby supernova -link. If this is how most solar systems are formed it is probable that planets formed, and evolved life, long before Earth was even created.
Sara Seager has contributed greatly to the blossoming field of extrasolar planetary science and astrophysics. Currently she is a professor at MIT, and has obtained a PhD from Harvard University. Her research and resulting papers have established a firm foundation for theories regarding life on other planets. There have been discoveries of extra solar planets such as Kepler-22b that have confirmed there are planets in other solar systems that are similar to our planet Earth in some ways, and could have lifeforms on it possibly. Dr. Seager has also proposed the idea that it is highly possible that if advanced aliens do exist, and they are looking for life on other planets, the detection of oxygen on Earth would be a clear indication to them that life has evolved in some way on this planet. Earth has 20% oxygen by volume. Without organisms such as plants, the oxygen content would be 10 billion time less. This scientific fact along with the radio and television signals that escape our atmosphere, most likely would be a beacon to an advanced alien civilization within range to observe them.
A thought provoking book titled UFOs: Generals, Pilots and Government Officials Go On the Record, has official records from international sources, military officer testimonials, and impressive documentation on UFO sightings, and details about interactions, that occured: http://www.amazon.com/UFOs-Generals-Pilots-Government-Officials/dp/0307717089/ref=sr_1_1?ie=UTF8&qid=1311995025&sr=8-1
On Wednesday, May 9th, 2001, the National Press Club hosted a large gathering of ex-high ranking officials including air traffic controllers, ex secret op. officers, commercial pilots, numerous military defense specialists with top secret clearance, people who had access to very sensitive documents, lieutenants, ex-commanders in the U.S. Airforce, astronauts,etc... all going before the national press club to discuss what their experiences have been regarding u.f.o's and all are willing to go before congress to testify under oath.. never before has such a group come forward.