It is possible, at least speculatively, to calculate the possibility of other intelligent life in the universe. Since the 1960s, a general equation for doing so has existed, and is known as the Drake Equation. The Drake Equation tells us what variables will probably affect the chances of other life existing among the stars, but we still do not yet know precisely what numbers to plug into the formula. As a result, the “product” of the Drake Equation can vary from a very small number of intelligent civilizations to a fairly high number, depending on the optimism of the person or group performing the equation.
– Life vs. Intelligent Life –
A distinction must be drawn, in the beginning, between the mere existence of other life in the universe, and the existence of intelligent life elsewhere in the universe. The first is taken virtually as a given by most astronomers. Assuming that life on Earth was the product of evolution, it is almost certain that there are many other places in our galaxy alone where at least primitive life similar to our single-celled organisms (bacteria) could develop. Actually, there are several places in our own solar system which theoretically might fit those conditions today, may have fit those conditions in the past, or might fit them in the future. These include Mars, Titan, and Europa, as well as several less likely candidates.
Intelligent life, however, is almost certain to be far more rare. It takes special conditions for advanced species like humans to develop, even beyond just the conditions for life itself. Probably hundreds of millions of years are required for the evolutionary process to work – and this might be shorter or much longer, depending on whether we humans took an average amount of time to evolve, were the slow-growing runts of the galactic litter, or are child prodigies.
– The Drake Equation –
The basic variables which have to turn out right – all at once – for intelligent life to emerge are listed in a mathematical formula known as the Drake Equation. In its original version, delivered to a Green Bank conference by Frank Drake in 1961, the equation multiplies seven basic variables to come up with a result equal to the probable number of civilizations in our galaxy: the average number of stars which form per year, the percentage of stars with planets around them, the average proportion of planets capable of supporting life (i.e. with the right climate and size), the percentage of those planets on which life will evolve independently, the percentage of life which will go on to become intelligent, the percentage of intelligent species which form civilizations capable of interstellar communication, and the length of time such civilizations survive.
Unfortunately, we can actually identify only a handful of these variables. According to the Max Planck Institute, about one new star is born in our middle-aged galaxy per year (but the number was far higher when the galaxy was young). We simply don’t know how many of these stars have planets, mostly because we can’t see most planets from great distances, but probably at least one-third of stars have planets, and maybe almost all stars have planets. We also don’t know how many planets can support life. Even if our own solar system is average, that figure could range from one in eight (Earth) to four in eight (Earth, Mars, one of Jupiter’s moons, and one of Saturn’s moons). We definitely don’t know much about the biological and sociological variables; unhelpfully, some astronomers tend to assume that the last of these are equal to 100% (i.e. once intelligent life emerges, sooner or later it will usually become intelligent and spacefaring, even though there is no real evidence to believe this other than our own history).
Ultimately, then, the outcome of the Drake Equation can fluctuate dramatically. It is easy for a pessimist to fill in the formula in such a way that at best there are only three or four other intelligent civilizations in our entire galaxy – but it is nearly as easy for an optimist to fill in the formula in such a way that there are dozens. And in a universe filled with hundreds of billions of galaxies, it is almost certain that there are hundreds of billions, or even trillions, of intelligent civilizations.
– So, Where Are They? –
There is one problem with completing the Drake Equation in such a way that there are many other intelligent civilizations in the universe: we should have seen some of them by now. We have only been searching for the radio signals from other civilizations for a few decades. However, assuming that the alien civilization would progress technologically at the same rate ours appears to be, one would expect them to be far more visible. A civilization even a few million years older than ours (which is easy to imagine in a galaxy billions of years old) might even have colonized most of the galaxy by now.
This problem – that if many civilizations exist, we should have found them by now – is known as the Fermi Paradox. There are a number of solutions to the Paradox. Perhaps we simply have not been looking long enough. Perhaps intelligent civilizations destroy themselves through nuclear war or climate change (both real risks to humanity here on Earth) – or are actively destroyed by a very small number of interstellar predator species. Perhaps we severely underestimated some of the special features of our own solar system, like Jupiter’s role in clearing the inner solar system of asteroids, or the Moon’s influence on the tides, or the ancient impact that tilted Earth on its axis and gave us our seasons. Perhaps we really are the only ones. For the moment, there is simply no way to know.