The fact of human existence is evidence that technologically advanced life is possible in the Universe, yet all our searches for other civilisations have turned up exactly nothing. Since Earth harbours the only proof of life we have, it's difficult to know just how common - or rare - it is.
A new method for estimating the likelihood of life arising beyond Earth gives reason for hope, though. According to the parameters used, there should be at least 36 advanced civilisations capable of broadcasting communication signals in the Milky Way galaxy, although the number could be even higher.
"There should be at least a few dozen active civilisations in our galaxy under the assumption that it takes 5 billion years for intelligent life to form on other planets, as on Earth," explained astrophysicist Christopher Conselice from the University of Nottingham in the UK.
"The idea is looking at evolution, but on a cosmic scale. We call this calculation the Astrobiological Copernican Limit."
There are several parameters that can be used to estimate the number of intelligent civilisations in the galaxy. Conselice and his colleague, engineer Tom Westby, developed a scale of calculations, based on the number of parameters they included.
For the very weakest Astrobiological Copernican Limit, the team assumed that intelligent life arises wherever possible - on rocky planets in the habitable zones of host stars of sufficient age and metallicity - and persists for the entire life of the star.
This returned an estimate of tens of billions of potential habitats, but it's also extremely unlikely that intelligent life would actually arise in each of these instances.
For the other three limits, timeframes were set for when advanced civilisations could have emerged, based on Earth. It took around 4.5 billion years for advanced technological civilisation to spring up here; the first radio signal was only transmitted in 1895.
So, for the strongest Astrobiological Copernican Limit, the parameters included a star with Sun-like metallicity, and an age of between 4.5 billion years and 5.5 billion years. Assuming that the lifespan of a communicating civilisation is around 100 years (roughly how long humans have been transmitting signals), the lower limit on the number of civilisations is 36.
"The classic method for estimating the number of intelligent civilisations relies on making guesses of values relating to life, whereby opinions about such matters vary quite substantially," Westby said.
"Our new study simplifies these assumptions using new data, giving us a solid estimate of the number of civilisations in our galaxy."
So, if they're out there, transmitting electromagnetic signals into space, why haven't we found them, or they us? Well, the galaxy is a big place. Scatter 36 civilisations throughout its vast reaches, and you end up with an average distance of 17,000 light-years between each pair, according to the team's calculations.
Radio waves from our transmissions do propagate out into space, but there's a limit to how fast they can travel. Such signals are composed of electromagnetic radiation, meaning they can only travel as fast as the speed of light. Waves from that first radio broadcast in 1895 could only have travelled 125 light-years away from Earth by now.
In addition, many of the signals that emanate from Earth are probably garbled by the ionosphere. Even those that aren't – like Earth-space communications – would become so attenuated and weak by the time they've reached about 100 light-years away, they'd basically be undetectable anyway.
So, unless we can figure out how to build a radio megaphone, and unless we're able to survive and maintain our technological prowess for the next 17,000 years, we're not broadcasting nothin' to no-one, and vice versa.
But we don't just try to figure out the likelihood of alien civilisations because we want to make friends (which, yes, we do). We do it because it helps us to understand our own existence.
"Our new research suggests that searches for extraterrestrial intelligent civilisations not only reveal the existence of how life forms, but also gives us clues for how long our own civilisation will last," Conselice said.
"If we find that intelligent life is common then this would reveal that our civilisation could exist for much longer than a few hundred years," Conselice added.
"Alternatively if we find that there are no active civilisations in our galaxy, it is a bad sign for our own long-term existence. By searching for extraterrestrial intelligent life - even if we find nothing - we are discovering our own future and fate."
We're definitely Team Aliens.
The research has been published in The Astrophysical Journal.