In a paper to be published later this year, UC Berkeley researchers say that less than 1 percent of Milky Way planets have intelligent civilizations advanced enough to transmit radio signals.

Andrew Siemion, who was a UC Berkeley doctoral candidate in astronomy during the study, said he was specifically seeking intelligent extraterrestrial civilizations that might communicate through high-frequency radio transmissions.

Although Siemion was unable to detect extraterrestrial signals, his research narrowed down the number of potential communicative systems scientists are aware of. He said he hopes his paper will provide a basis for his future research in the Search for Extraterrestrial Intelligence (SETI) field.

“A common question that you’ll get is why we expect that aliens would use radio signals,” said Siemion. “The fact of the matter is electromagnetic radiation remains the best way to communicate over interstellar distances.”

After narrowing down the number of testable systems, Siemion and his team searched for and analyzed the radio transmissions within 86 star systems in hopes of finding signals produced by intelligent civilizations.

“So there’s the analogy of the drunk looking for his keys under the street lamp and the question is why does the drunk look for his keys underneath the street lamp?” Siemion said. “Because that’s the only place where there’s light, it’s the only place he can look.”

According to Siemion, radio signals can readily travel through the spaces between solar systems and are relatively low-energy to emit.

“We liken SETI to looking for a needle in the haystack,” said Dan Werthimer, who, in addition to being one of Siemion’s advisors in the study, is the co-founder and chief scientist of [email protected], the campus-based research institution searching for extraterrestrial intelligence. “We’re only looking at a corner of the haystack. We’re going to do much more, and the capabilities of SETI will increase.”

Eric Korpela, another of Siemion’s advisors and a project scientist at [email protected], said the results of the research were not surprising.

“It’s really nothing to be discouraged about,” Korpela said. “There’s less than 200,000 civilizations that are broadcasting in our galaxy, and that’s actually a better limit than we had.”

Siemion said that a major obstacle in his research was differentiating between terrestrial, or Earth-based, signals and extraterrestrial ones, as well as analyzing the complex frequencies he detected.

As for the future, Siemion is currently involved in a new research project building on his most recent findings, this time looking for signs of communication between planets within the same system. He said he intends to continue the search for extraterrestrial intelligence.

“I think understanding our place in the universe is why many people study astronomy or just science in general,” Siemion said. “For me, the most direct way to answer that question is looking for other intelligent beings.”