In 1950, Nobel Prize-winning physicist Enrico Fermi — who led the team that created the first-ever nuclear reactor, the inadequately named Chicago Pile-1 — and a few of his colleagues were discussing UFOs during their lunch break. The conversation prompted Fermi to ask his companions, “Where is everybody?” [13 Ways to Hunt Intelligent Aliens]
Fermi meant that the lack of visits by ET is distinctly odd. The Milky Way harbors hundreds of billions of stars and is about 13 billion years old, so there has been plenty of time and opportunity for alien civilizations to rise and spread throughout the galaxy. By some estimates, a colonization-minded species with propulsion technology not much more advanced than our own could island-hop its way to every corner of the Milky Way in just a few million years.
The physicist’s simple question is enshrined now as Fermi’s paradox — one of the two coolest paradoxes of all time, along with the crocodile paradox — and it continues to puzzle scientists to this day. Indeed, the mystery has deepened considerably over the years. For one thing, we’re not just talking about the lack of visitation anymore. In 1960, 6 years after Fermi’s death, astronomer Frank Drake pointed a radio telescope at West Virginia’s Green Bank Observatory at the nearby sun-like stars Tau Ceti and Epsilon Eridani, kicking off the search for extraterrestrial intelligence (SETI).Nearly 60 years later, SETI scientists are still hunting for the first confirmed peep from ET.
Then there’s the exoplanet revolution. Alien worlds were purely hypothetical objects in Fermi’s day and for decades afterward; scientists didn’t announce the first confirmed detection of a planet beyond the solar system until 1992. But in the last decade or so, NASA’s Kepler space telescope and other instruments have revealed that the cosmos is teeming with possibly life-supporting worlds. Kepler’s discoveries suggest that about 20 percent of the Milky Way’s sun-like stars host an Earth-sized world in the “habitable zone” — that just-right range of orbital distances that would allow you to walk around in flip-flops pretty much year-round. The proportion appears to be similar for red dwarfs, the small, dim stars that dominate our galaxy. (About 75 percent of Milky Way stars are red dwarfs, whereas just 10 percent or so are similar to our sun.)