Our galaxy could have at least 36 intelligent alien civilizations, researchers say. It may take thousands of years to find them.

• The Milky Way should have at least 36 intelligent alien civilizations active right now, according to a new study.
• There could be hundreds of extraterrestrial civilizations active in the galaxy at this very moment, if life that can broadcast through space lasts longer than 100 years.
• Finding those aliens could tell us how long human civilization might last, but it could take thousands of years to communicate across space.

Our universe could be teeming with alien life, and some of it may theoretically have formed high-tech civilizations that could communicate across the stars.

There should be at least 36 such civilizations in our galaxy alone, according to a new study from researchers at the University of Nottingham, published Monday in The Astrophysical Journal.

In fact, 36 is the lower limit — an estimate based on the assumption that intelligent civilizations only survive long enough to broadcast through space for an average period of 100 years.

That's about how long humans have used radio communications like satellites and TV, which send waves radiating through space that aliens could potentially detect. If communicating civilizations can survive longer than that, hundreds of them could be active throughout the Milky Way.

The researchers assumed that it would take about 5 billion years for intelligent, communicating life to form on other planets, just as it did on Earth. Then they calculated how many such civilizations might arise in the galaxy, based on how many Earth-like planets orbit sun-like stars.

Intelligent societies could be constantly cropping up across the galaxy, bombarding space with radio signals for years before they die out and go silent. How many of them are active at any point in time depends on how long they survive, on average.

Finding intelligent alien populations, and learning how many of them are currently active throughout the galaxy, would clue us in to how long human civilization is likely to last.

"If we find that intelligent life is common, then this would reveal that our civilization could exist for much longer than a few hundred years," Christopher Conselice, a professor of astrophysics at the University of Nottingham and the study's lead researcher, said in a press release. "Alternatively, if we find that there are no active civilizations in our galaxy, it is a bad sign for our own long-term existence."

'This is one of the most challenging problems in science'

Scientists have been listening for radio waves from space for years, but have detected no signals from alien civilizations. If there are so many of them out there, why haven't we heard a whisper?

This question is known as the Fermi paradox, first posed by the physicist Enrico Fermi, who famously asked, "Where is everybody?"

Fermi was questioning the feasibility of travel between stars, but since then his query has come to represent doubts about the existence of extraterrestrials.

There are many explanations for this eerie silence in the cosmos, ranging from the aliens hibernating, to them communicating with a totally foreign technology, to a simple lack of interest in contacting us.

Conselice's team thinks that aliens may just be too far away to hear us. Their calculations suggested that these civilizations would be an average of 17,000 light-years away, making it "nearly impossible" to reach them or hear them with our current technology.

In order to communicate with such a distant civilization, and hear back from them, humans would have to keep up radio communications for 6,120 years, according to the researchers.

"If we do find things closer… then that would be a good indication that the lifespan of [communicating] civilizations is much longer than a hundred or a few hundred years, that an intelligent civilization can last for thousands or millions of years," Conselice told The Guardian. "The more we find nearby, the better it looks for the long-term survival of our own civilization."

It's still possible that we are entirely alone in the universe. Estimates like Conselice's come from rough calculations based on how long it took for life on Earth to emerge, how many Earth-like planets orbit sun-like stars, and how long life on Earth has been intelligent enough to send signals through space.

"From a statistical perspective, this is one of the most challenging problems in science," the study authors wrote. "All we can do is attempt to learn from a single known data point (ourselves)."

Finding an alien civilization would give us another data point and refine our understanding of how long our own civilization can truly last.

"By searching for extraterrestrial intelligent life — even if we find nothing – we are discovering our own future and fate," Conselice said.