For countless centuries people have looked up at the night sky and wondered. Some wondered about their gods, some tried to make sense of the patterns they saw and a few wondered if there were worlds out there inhabited by beings looking up and wondering is there anybody out there?
From Myth to Science
Whilst it is often pointed out that alien beings appear in the most ancient of human records, the true antiquity of the idea is rarely appreciated.
For example, the Sumerian civilization, which flourished more than five thousand years ago, believed that law, science, art and even morality were given to humans by alien teachers. In fact, many civilisations have founding myths that speak of beings from the sky imparting knowledge of giving laws to humankind.
There even is a thriving community of people who believe that these myths are in fact evidence of past alien visitation of our planet. However misguided and problematic the belief in ancient aliens might be, the point is that our ancient ancestors believed in the possibility that the heavens were the home to other beings, even if most were given the status of gods.
The rise of Christianity brought with it the widespread adoption of Aristotle’s geocentric view that the earth was the centre of the universe and all heavenly bodies revolve around it. For Aristotle and Christian doctrine which deified this geocentric view — the universe was the realm of god and talk of alien worlds was heresy.
However, the rise of scientific thinking reintroduced the possibility of alien worlds. The Victorians thought they had discovered canals on Mars and speculated that our own moon was covered in seas and that both might be home to otherworldly beings.
The question of ‘are we alone’ had returned and as our understanding of how the universe works has grown, we may be getting closer to providing an answer to that question.
Finding New Worlds
Ever since the launch of the Kepler telescope a decade ago, astronomers have been discovering thousands of new planets orbiting distant stars. It seems likely that every star we see (and can’t see) in the night sky has its own planetary system; from the strange hot Jupiters that orbit closer to their star than Mercury to Earth-like planets orbiting in the habitable zone, new planets are constantly being added to the catalogue.
Missions to the planets and moons in our own solar system have made scientists realise that there are far more warm and wet places where the conditions for life exist. Everywhere astronomers look they find organic compounds, on asteroids, on comets even in nebulas that cover vast expanses of space. All in all the universe is increasingly looking like it’s way more hospitable to life than scientists had initially thought.
Even before the confirmation of extra-solar planetary systems, scientists had speculated about the chances of life existing on other planets. Being scientists they then tried to put that speculation on the soundest scientific foundation possible.
The Drake Equation
In 1961 Dr Frank Drake, who was working at the National Radio Astronomy Observatory in Green Bank, West Virginia, conceived a way to scientifically estimate the number of technological civilizations that may exist in our galaxy.
Now called the Drake Equation, the formula involves looking at a number of variables, such as; the rate of star formation, the number of stars that have planets, the number of those stars where life starts, the proportion of those cases where intelligent life evolves, the number of those who communicate technologically and how long those civilisations last.
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Even though some of the variables in the equation remain unknown — some say questions like how long a civilisation lasts are unknowable, as the amount of data from astronomical surveys continues to pour in, scientists are beginning to provide more accurate estimates for some of the equation’s variables such as the rate of star formation and the number of stars that have planets.
As a result, astronomers periodically attempt to put the new data to work in trying to solve the equation. Depending on your optimism or pessimism, historically, the various answers so far have ranged from; none- we are truly alone in the universe — to there are potentially thousands of civilisations in our galaxy alone.
The Latest Best Guess
In research published in the Astrophysical Journal in 2020, Tom Westby, an engineer at the University of Nottingham in the UK and astrophysicist Christopher Conselice, have made a new attempt to solve Drake’s equation based on the best scientific data we currently have.
Their calculation combines current astronomical observations with one big assumption-that life on Earth is in no way special-to conclude that dozens of radio-capable civilizations may currently exist in the Milky Way, with 36 being the most probable number. They consider that figure to be a low-ball prediction but admit that they’re playing a highly uncertain and theoretical game
“The Drake Equation is our best and only tool for trying to tackle this age-old question, even though it’s based on things that are still wildly speculative,” says Conselice.
The two researchers concentrated the bulk of their analysis on what has been measured with decent accuracy, namely the number of stars and planets. They used observations of how quickly other galaxies generate stars, how many of those stars have the elements necessary to build planets, and they compared that with how many planets NASA’s Kepler telescope has spotted in our own galaxy.
However, their analysis is still subject to the difficulty of trying to account for those variables in the equation that are incredibly hard to pin down scientifically.
The Exception or the Rule?
For those variables in the equation for which there are no data — such as, the odds that life will arise if the conditions are right and the chances that if life arises then intelligence will evolve and that intelligent life would develop the ability to communicate with radio technology, they had to turn to philosophy.
They used a notion called the mediocrity principle, which states that humanity has nothing special going for it. Using this principle they presume that if a sufficiently Earth-like planet exists, a technologically capable life will inevitably evolve after about five billion years, just like it did on Earth.
The last variable of the equation, how long does a technical civilisation communicate before it destroys itself or is destroyed by a natural disaster is often considered the hardest variable to quantify. We only have ourselves to go by and we humans have been communicating with radio technology for around a hundred years. Based on this the researchers assumed that any technologically similar alien civilisation will survive for at least a hundred years once they start transmitting.
Westby agrees that these are very big assumptions, but he feels that this is a necessary evil if you want to try and solve the equation. “That’s the great problem with astrobiology, trying to extrapolate from one data point here on Earth, he says. “But if we are one of a sample, we should expect ourselves to be typical in most respects.”
This approach is not without criticism, chief of which is that we have no way of knowing if whether humanity is the exception or the rule. There are reasons to believe we are exceptional. The vast majority of stars in the universe are of the red dwarf type, which is nothing like our sun and most planetary solar systems aren’t like ours.
Even though life seems to have started on earth as soon as conditions allowed, it took many a couple of billion years for complex life to evolve on our planet and rather than being commonplace, it only happened once.
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Around 2 billion years ago one type of cell engulfed another and both learned to live together. One changed to become an organelle called mitochondria which now serves to supply cells with the energy they need to go about their business. All complex life on earth is descended from that chance encounter. As far as we can tell it never happened again.
Crunching the Numbers
If life on Earth didn’t hit the cosmic or evolutionary jackpot, and the mediocrity principle holds then it’s reasonable to expect that we may have company out there in our galaxy. So having crunched the numbers, using the toughest assumptions, Westby and Conselice calculated that somewhere between four and around 200 civilizations might be out there and broadcasting at this very moment in time.
For Westby, the likely number of 36 that emerged from the calculations is somewhat comforting. It’s not so huge that we should be deafened by countless alien radio stations but not so tiny that the reality is that we are likely alone.
“One of the most satisfying moments when I was throwing all the numbers together into the final simulation, that [final] number [of 36 civilizations] was relatively close to one,” he says, in comparison with less plausible estimates like a billion or 0.00000001. He adds that he considers the figure “a bare minimum based on our toughest assumptions,” such as “assuming we are near our own extinction.”
Westby and Conselice also ran the numbers with much less tough assumptions, like technological civilizations could arise much faster than the nearly five billion years it took for us to emerge, and the possibility that technological civilisations don’t destroy themselves quickly and so continue to broadcast for long periods of time – much more than the century that defines our broadcasting history. In these scenarios, they found that thousands of civilizations could be broadcasting at this very moment.
However, those optimists hoping to make contact with ET shouldn’t be holding their breath. Even under these more optimistic assumptions, which gave a figure of around three thousand broadcasting civilisations, our nearest neighbour would likely be around two thousand light-years away. Swapping a single round of messages with such a civilisation would be a four thousand year affair.