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How many extraterrestrial intelligent civilizations are out there and when will one of them send us a message?
The answers, according to a paper published in The Astrophysical Journal are 42,777 and sometime in the next 2,000 years. It’s a decent explanation for the Fermi Paradox, which asks why we still haven’t received any messages from other civilizations despite there being a high probability of them existing.
It estimates the number of possible CETIs—communicating extraterrestrial intelligent civilizations—within our Milky Way galaxy. It also looks at how probable it is that one of them could contact us and when.
There are, of course, some huge unknowns behind these seemingly very precise estimates that if known would make a massive difference to the results:
- The probability of life appearing on rocky planets and eventually evolving into a civilization advanced enough to contact another.
- At what stage of their host star’s evolution such advanced civilizations would be born.
So the figure of 42,777—which has an error rate of a few hundred each side—is on the optimistic side. It’s based on an estimate that only 0.1% of civilizations could become advanced enough to contact another. This is where the Great Filter comes in.
It also takes into account the idea that any civilization would need to survive for about three million years, give or take, to reach that point.
Even if a message is ever sent to us from an advanced civilization elsewhere in the Milky Way the question remains as to whether humans can survive long enough to receive it. The authors suggest that we will need to wait as little as 2,000 years to receive one alien signal.
That’s the optimistic calculations. The authors’ pessimistic estimates are for just 0.001% of civilizations–about 111—to become advanced enough to contact another.
The upshot of that would be that humans would need to wait for 400,000 years to receive a message.
“The minimum value (0.001%) we take may also be overestimated,” write the authors, Wenjie Song and He Gao at Beijing Normal University’s Department of Astronomy. “If so, the number of CETIs would become even lower, and the opportunities for communication between CETIs would become extremely small.”
The only message ever received on Earth that could have come from an extraterrestrial intelligence is the Wow! Signal, which was received in 1977 at the Big Ear radio telescope, Ohio. It was heard for 72 seconds—the maximum possible at the time—and was never repeated.
The source of that signal remains unknown though a recent paper found only one Sun-like star (called 2MASS 19281982- 2640123) in a sample of 66 in the region of the night sky that Wow! came from. It’s 18,000 light-years away.
In 2012 a paper estimated that the closest civilization to the solar system could be 1,933 light years away.
So let’s send a reply? Sure—and why not, considering the chances of an advanced alien civilization being malicious are really low—though there is one problem. Any radio or laser transmission will travel at the speed of light, so would take 1,800 years to get there.
The authors note that astronomers did send the “Arecibo message” to the Great Hercules Globular Cluster (M13) in 1974 using the now collapse Arecibo radio telescope. However, it wasn’t much good. “If there are indeed CETIs in M13, their detection ability needs to be 21 orders of magnitude higher than ours to detect our signal,” write the authors. “Conversely, if they transmit a similar signal, we need to improve the detection ability by 21 orders of magnitude to detect it.”
Space is big—really big—and even in-galaxy messaging is completely impractical. Even if we’re not alone it’s doubtful we’ll ever find out.
But that doesn’t stop us looking for Earth-like exoplanets around 2MASS 19281982- 2640123.
Wishing you clear skies and wide eyes.
