The search for intelligent life in the universe is still on.
Despite the absence of interstellar tourists to date, astronomers at the SETI Institute (Search for Extraterrestrial Intelligence) are hoping that we are not alone.
And with new spacecraft to locate planets circling nearby stars, as well as more effective listening devices here at home, scientists have more tools at their disposal to find Earth-like planets or signs of other life forms.
But the possibility of intelligent life is what interests scientists at SETI. Using SETI's 42-antenna Allen Telescope Array in Northern California, they can listen in many directions for unusual radio signals coming from space.
According to institute astronomer Seth Shostak, Carl Sagan posited that more than a million civilizations might be capable of broadcasting signals. Scientist and author Isaac Asimov hypothesized that the number might be half that. SETI astronomer Frank Drake has estimated the number might be closer to 10,000.
These represent the more optimistic calculations that have been put forward, but if these scientists are even in the ballpark, Shostak believes the Allen Telescope Array may well find extraterrestrial life in the next two dozen years.
Meanwhile, NASA's Kepler telescope is collecting and transmitting data on a targeted band of stars up to 3,000 light-years from our sun, looking for planets that might be capable of hosting life as we know it. What's more, scientists say any evidence for or against interstellar company probably will tell us even more about ourselves.
"Finding extraterrestrial life will provide crucial insights concerning the origins of life," says astrophysicist Mario Livio of the Space Telescope Science Institute (STScI).
"The ultimate mind-changing idea is, 'What if we find something?' " says SETI astronomer Jill Tarter, who was the inspiration for Jodie Foster's alien-hunting character in the 1997 movie Contact and who was a consultant on the film. "That 'something' is going to be so different from us. It would hold up a mirror to the entire planet and trivialize the differences among us that we find so incredibly divisive."
But if SETI does nothing but change our perspective as humans, she says, it will still qualify as "one of the most profound endeavors in history."
An ancient concept
Around 300 B.C., the Greek philosopher Epicurus wrote that the universe is full of other worlds where life might exist.
But some contemporary skepticism about extraterrestrials comes out of a lunchtime chat at Los Alamos National Laboratory in New Mexico in 1950, when physicist Enrico Fermi famously asked, "Where is everybody?" If there were Earth-like planets and other forms of life, Fermi suggested, we should have been visited by extraterrestrials long ago.
A decade later, Cornell University astronomer Frank Drake performed the first SETI experiment, Project Ozma, listening for unusual radio signals from other solar systems. In 1961, he came up with the "Drake equation," a formula designed to calculate how many intelligent life forms we might be able to contact based on estimates of the number of stars in the galaxy, the likelihood of habitable planets, the odds of intelligent life arising there, and the chances that life will send detectable signals into space.
But with the exception of a lone signal recorded at an observatory outside Delaware, Ohio, in 1977 and never found again, no extraordinary emissions have been confirmed.