According to a NASA scientist, an intelligent alien life could have visited Earth and people have missed it.
Silvano P. Colombano, a member of NASA's Intelligent Systems Division, wrote a paper titled "New Assumptions to Lead SETI Intelligence. "And presented it at the" Decoding Alien Intelligence Workshop. Colombano:
Recent discoveries from the Kepler Project have identified a planetary system of 10.4 gyr (Kepler-10)   and 11.2 gyr (Kepler-444)  As the age of our solar system is about 4.5 Gyr, Earth-like planets that are 6 Gyr and older than our own could exist, given the fact that technological development in our civilization began about 10,000 years ago, and scientific methods did not begin Over the past 500 years, we can surmise that we could have a real problem predicting technological development for the next thousand years, let alone 6 million times that amount!
Considering that The technological development of our civilization began only about 10,000 years ago and only in the last 500 years scientific methods emerged suggest that we could have a real problem in predicting technological development even for the next thousand years, let alone 6 million times that amount!
Given these numbers, I think we must even revisit our most cherished assumptions: interstellar travel is impossible or highly unlikely … Radio waves are still the most important form of communication for thousands or millions of years … Intelligent civilizations One has to base oneself on carbon life … We were and will not be visited ….
Given the notion that extraterrestrial civilizations may not be based on carbon life, Columbano stated:
Just me I would like to point out the fact that the information we could find and which we might find (if not already happened) may not be generated by carbon organisms like us. How could that change the above assumptions about interstellar travel? Our typical lifespan would not be limited anymore (though this could also be related to multi-generational missions or interrupted animations), and the size of the explorer could be that of an extremely small, over-intelligent entity. And how could this change our assumptions about openness or the desire to communicate with other civilizations?
Columbano said that it was perfectly reasonable to speculate that extraterrestrial life had visited Earth, and wrote:
It seems to me SETI has the potential relevance of UFO phenomena (at least officially) for three reasons ignored: 1) The assumption of extremely low probability of interstellar travel, 2) The very high probability of jokes, false perceptions or even psychotic events in UFO phenomena, and 3) The general avoidance of the topic by the scientific community.
I think the approach that the scientific community might follow is very similar to what SETI has done so far: finding the signal in the noise. In the very large amount of "noise" in UFO reporting, there may be "small" signals that indicate some phenomena that can not be explained or denied. If we adopt a new set of assumptions about what forms of higher intelligence and technology we might find, some of these phenomena might fit with certain hypotheses, and we could start a serious investigation.