Astronomers hope to find alien signals within next 15 years

The recent rapid pace of discovery of "candidate planets" -- distant worlds that seem suitable for life -- make scientists engaged in the search for extra-terrestrial intelligence (SETI) hopeful that they could find alien signals within the next 15 years.

On Sept. 12, astronomers at the European Southern Observatory (ESO) in Chile announced the discovery of 16 new "super-Earths," planets orbiting distant stars that are smaller than gas giants, and most likely have rocky surfaces where life could gain traction. One of the newfound super-Earths, labeled HD 85512b, is the strongest candidate among them: It resides in its star's "habitable zone," where the temperature is just right to sustain liquid water -- the elixir of life as we know it.

The 16 new candidates join a list of 54 others discovered earlier this year by NASA's Kepler spacecraft.

On target

The SETI Institute in Mountain View, Calif., home of the best-known effort in the search for extraterrestrial life, was forced to shut down its radio telescopes in April due to funding shortages, but charitable donations are bringing the telescopes back online at this very moment. When the SETI team resumes its radio signal-scouring next week, the new super-Earths will be first on its telescope target list. [A Field Guide to Alien Planets]

"When the array is again operational we will go back to our exploration of exoplanets," Jill Tarter, director of the Center for SETI Research at the Institute, told Life's Little Mysteries. Unfortunately, HD 85512b is too far south to be targeted by the Allen Telescope Array, SETI's main detector. "We will, however, add those exoplanets reported by [ESO] that are visible to our list of targets. The more planets the better!"

After the scientists aim their telescope array toward a new exoplanet, it takes a few months to determine whether any of the radio signals coming from it are products of intelligent design -- that is, alien-made -- rather than just natural cosmic noise and planetary rumblings. "The results over all frequency bands [come in over the course of] months as we systematically go through the list," Doug Caldwell, Kepler instrument scientist at the SETI Institute, explained in an email.

Though SETI astronomers could find life on their very next try, doing so will probably require scouring the skies for years. Frank Drake, the SETI Institute's chairman emeritus and a pioneer in the field, compared searching for ET to playing the lottery. "It depends on the rate at which we search," Drake wrote in an email. "Discoveries of other civilizations are surely few and far between. You have to make many tries, just as winning the lottery requires the purchase of many tickets."

So how many tickets does the SETI team have to buy before they'll hear E.T. phoning home?

Fifteen to 20 years

Scientists use an equation formulated by Drake in the 1960s to estimate how long it will take to find intelligent life. The "Drake equation" determines the number of intelligent and signal-transmitting civilizations in our galaxy by multiplying a string of factors, including the number of stars, the fraction of those that have planets, the fraction of those that are habitable, the probability of life arising on such planets, the likelihood of that life becoming intelligent, and so on.

The values of many of these factors are highly speculative, but Drake himself estimated their product to be 10,000 -- as in, there are 10,000 civilizations transmitting signals in our galaxy at any given moment. Because there are 100 billion stars in the galaxy, the math says 1 in 10 million stars will be sending radio signals our way at any given time.

By making smart choices about which stars are likely to sustain life, we should be able to find someone or something by searching just 1 million stars, Drake originally estimated. That could take SETI, as it currently operates, as few as two decades 

However, that estimate predated the recent boom in exoplanet discoveries. "It was not long ago that we had only wild guesses about the number of stars with planets," said Gerald Harp, an astrophysicist at the SETI Institute. "I believe our earlier guesses were too low -- a large proportion of stars are now believed to host Earth-like planets. We now know that perhaps 1 percent of stars have planets where biological life may arise. We also know where some of those planets are."

Considering this new information, Harp thinks scientists will  have to search only 100,000 stars before detecting alien signals, and that they'll manage to examine that many in the next 15 years. "Fortunately for me, I expect to still be actively pursuing research in 15 years, so I will be around when it happens!" Harp said.

Not everyone is as willing to nail down a timeline, though they're all hopeful. "While I don't know if we will discover intelligent life -- or any life -- in the next 20 years, I'd hesitate to bet against it, given the amazing pace of development in the fields of astrobiology and extrasolar planets, both of which barely even existed 15 years ago," Caldwell wrote.

Similarly, Tarter wrote, "I'm not inclined to put a timetable on success. I don't bet on horses or football pools, either."