After several centuries or millennia of speculating and several decades of searching, humans have failed to find irrefutable evidence of any form of exobiology, or life outside of the confines of the planet Earth. It would therefore be foolish to make any confident, positive claims concerning the existence of life on other worlds. There simply is no reason to adhere to the belief that there is anything “out there.”
However, it is equally fallacious to argue for certain that humans are not alone in the universe. This is for two reasons. First, concretely disproving such a proposition would require exhaustive knowledge of the universe. It would be impossible to overturn every rock on every planet around every star in every galaxy looking for life, so it is impossible to say for certain that no extraterrestrial life exists. In addition it is not even really valid to say that alien life is particularly unlikely. This is true for the simple reason that humans have not even begun to search their own local galaxy comprehensively. There simply is not enough data to make any sort of claim about the existence of alien life at all.
Neil deGrasse Tyson, one of the world’s most famous astrophysicists and popularizers of science, captures this idea rather well in a metaphor he often uses when speaking. The Milky Way galaxy is on the order of 100,000 light years in diameter. SETI, the Search for Extraterrestrial Intelligence, relies principally on the transmission and reception of radio waves to try to find exobiots. To use a nicely round number, humans have been producing and listening to radio waves for approximately a century. Because radio waves travel at the speed of light, by the present day the Earth is encompassed in an ever-expanding bubble of radio waves, with a radius of 100 light years in any direction. On the outside of this bubble are the earliest transmissions ever sent. About 70% closer to Earth is the point at which signals from SETI could be heard. This means that in a galaxy with a radius of 50,000 light years, humans have only searched an area with a radius of 100 light years. That’s about a fifth of a percent of the galaxy that has even been touched with any radio signals from Earth, let alone by the tiny minority sent by SETI that might actually elicit a response. And that’s just one of the hundreds of billions of galaxies that exist.
Making claims about extraterrestrial life at this moment in history, Tyson points out, is much like wading into the ocean, dipping in a Dixie cup, and making a claim about the existence of whales based on whether or not the cup contains when it is pulled out of the water. It’s ludicrous.