That's the minimum number of intelligent, communicating alien civilizations two astronomers at the University of Nottingham believe could exist in the Milky Way, our home galaxy.
A new study, published in The Astrophysical Journal on Monday, provides an updated estimate of the likely number of alien civilizations that could exist in the Milky Way. The analysis, performed by astronomers Tom Westby and Christopher Conselice, starts with revising the Drake equation, a formula proposed by Frank Drake in 1962 to estimate how many worlds are likely to harbor intelligent life in our galaxy. The equation relies on a variety of factors, including how often sun-like stars form in the galaxy, how many stars are orbited by planets and how often life evolves and becomes intelligent enough for us to detect it.
But as it is, the Drake equation is fundamentally "unsolvable" and contains a major variable we can't know until we find intelligent life: What's the average length of time alien civilizations are detectable?
"The classic method for estimating the number of intelligent civilizations relies on making guesses of values relating to life," said Westby in a press release. "Our new study simplifies these assumptions using new data, giving us a solid estimate of the number of civilizations in our Galaxy."
Westby and Conselice started with the Drake equation but approached the search for so-called "Communicating Extra-Terrestrial Intelligent" civilizations in a slightly different way. They built a key assumption in to their estimate: Life on another planet will arise in a similar way to how it did on Earth.
In effect, this means their revised equation takes into account the idea that a planet must exist for around 5 billion years in the habitable zone around a star before it can develop intelligent life with the capability to communicate across the universe. The duo placed three different sets of limits on these "suitable planets" harboring life with weak, moderate and strong categories with different time frames for life to arise.
The weakest limits allowed them to make estimates on a time frame of greater than 5 billion years, while the strongest limit only assessed worlds between 4.5 and 5.5 billion years old.
When plugging the strongest limits and numbers into their complex new equation, which they dubbed the CETI Equation, the data reveals there could be a minimum of eight CETI civilizations within the Milky Way. Such an estimate is relatively close to the figure of 10 that famed astronomer Carl Sagan came up with when discussing the Drake Equation on the '80s science show Cosmos.
There is a catch: Those worlds are at least 7,000 light-years away -- making it almost impossible for us to contact them. The team estimated we would need to be actively searching for signals from space for around 6,300 years before we might receive messages from another civilization.
"It is clear that the lifetime of a communicating civilization is the key aspect within this problem," the authors write.
On the other hand, using weaker limits, Westby and Conselice suggest there could be as many as 2,900 worlds where life has found a way that means we may be able to detect them sooner.
"This calculation highlights that the requisite conditions for life are commonplace throughout the Galaxy," notes Daniel Price, an astronomer at Swinburne University in Australia who was not affiliated with the study.
What the study says about humans is kind of grim, though: In our cosmic game of hide-and-seek with other life, we've only just shouted "ready or not, here I come!" and we've taken a few tentative steps in our search. But if we're going to win the game, civilization will have to survive a long time to uncover the hiding spots of other life in the universe. And if we can't find intelligent life close by, that might not bode well for our survival in the long term.
"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," said Conselice. "Alternatively, if we find that there are no active civilizations in our Galaxy, it's a bad sign for our own long-term existence."
But hearing a signal isn't as simple as listening out. The other intelligent civilization needs to be broadcasting, too.
"The detectability of a technosignature depends upon both our technological capability and theirs," says Price. "The Square Kilometre Array, due to start construction next year, will be able to detect radio transmissions at levels similar to our own from thousands of nearby stars."
Though an interesting new way to examine an age-old question, the work relies on a lot of assumptions. The authors make it clear there is only one data point for intelligent, communicating life and that is humanity. Using us as the basis for other life in the cosmos may itself be flawed because the truth is we simply don't know what other intelligent life might look like or where it might thrive.
Still, as with the Drake equation, the CETI equation provides a framework for examining the chances of life in our galaxy and debating the likelihood we might hear from aliens sometime soon. "The only way we can know for sure that we're not alone is by an unequivocal detection," Price says.
"By searching for extraterrestrial intelligent life -- even if we find nothing -- we are discovering our own future and fate," said Conselice.