SETI, the organization that searches for extraterrestials using radio signals, says the likelihood of intelligent alien life is relatively rare throughout our Milky Way galaxy, with fewer than one in a million solar systems containing civilizations advanced enough to send out radio signals.
The new study was researched by a team that includes famed alien life advocate Jill Tarter --- the template for the Ellie Arroway character in the late Carl Sagan's book Contact later made into a film starring Jodie Foster --- and others from the Seti Institute.
"No signals of extraterrestial origin were found," the researchers conclude in the study, which was published in The Astrophysical Journal.
Space.com reports that in their search for alien radio signals, the team selected 86 stars using data from space agency NASA's Kepler space telescope, and also observed 19 stars that synchronously came into range as they searched the primary targets.
The researchers worked from a list of Kepler planetary candidates which at the time included 1,235 possible exoplanets --- that is, planets outside of the solar system. They are also referred to as extrasolar planets. That number has now reached 2,740, with 105 of them confirmed.
The team limited their search to stars that host five or six planet candidates, some of which reside within the habitable zone — the range of distances from a star where liquid water can exist on a world's surface. The selected stars are about 1,000 light-years to 1,500 light-years from Earth.
It's understood that radio signals in tight, focused bands are a possible indication of intelligent life, given that humans generate such signals on Earth, The Guardian notes.
Using the Robert C. Byrd Green Bank Telescope in West Virginia, the team searched each star system for five minutes between February and April 2011. Specifically, they examined the planets in a radio-frequency range of 1.1 to 1.9 GHz, which falls between the cell phone and television bands used on Earth.
Amazingly, this range of frequencies includes a so-called "watering hole" --- between 1.4 GHz and 1.7 GHz --- and is where hydrogen and hydroxyl (both components of water) emit signals from quantum processes detectable in radio telescopes.
"The analogy is, it's the water hole where animals go in the desert, so perhaps this band of frequencies is a common gathering place for E.T.," Andrew Siemion of the University of California, the study's lead author told Space.com.
After careful investigation of the planets, it was found that none of them were transmitting signals in a narrow band of 5 Hz --- i.e an unnatural source.
But all hopes of finding an alien civilization may not be lost.
There could still be millions of civilizations out there just waiting to be found, the scientists said, simply based on the fact that billions of Earth-like planets are thought to populate the Milky Way.
The researchers also think that searching for just one particular type of signal could have significantly reduced their odds of finding something.
"In particular, we can offer no argument that an advanced, intelligent civilization necessarily produces narrow-band radio emission, either intentional or otherwise," the study authors wrote. "Thus we are probing only a potential subset of such civilizations, where the size of the subset is difficult to estimate."
The team also said that that narrow radio signals are subject to interference from what is known as the interstellar medium — a thin gas floating between the stars — and the solar wind, which is a stream of particles coming from the sun.
The search for alien life will continue in the months ahead, using more refined methods. In particular, researchers will look at stars that have two planets aligning in relation to Earth.
Scientists plan to "listen in" as the planets communicate with each other, that is, if the planets are transmitting signals in the first place.
As well as the Green Bank telescope, future alien-hunting searches will also use more sensitive radio telescopes, such as the Square Kilometer Array, which may be able to pick up weaker radio signals.