This was originally composed as a writing piece for ComSciCon 2015, and is now published in Universe Today Magazine.
As an astronomer, the question people ask me most often is: “Do aliens exist?” Most people believe that astronomers possess some secret insight into this age-old question that we are simply hiding from the public. In reality, there is no consensus among scientists. Astronomers generally settle into two camps; those who believe that life is extremely rare, and we may be the only intelligent life in the universe, and those who believe that life is common, and the only reason we haven’t heard from E.T. is the vast distance between habitable worlds. Let’s examine both arguments.
Life on Earth
The story of life on Earth began 4.6 billion years ago when the Earth first formed out of bits of dust in orbit around a medium-mass, medium-brightness star. As it so happened, the particular bits of dust that would go on to form the Earth happened to lie in a narrow slice of Solar System real estate known as the “habitable zone.” The habitable zone of a star is the distance from the star that a planet needs to be in order to sustain the liquid water on its surface. Since liquid water is a necessary ingredient for life as we know it, we are very lucky that Earth formed in this so-called “Goldilocks zone.”
Over billions of years, life slowly found a foothold on Earth, but it had a major threat to its existence to contend with: mass extinctions. We know of several such events which have occurred throughout Earth’s history, including the asteroid impact which famously killed off the dinosaurs. In fact, over 99% of all species that have ever existed on Earth are now extinct. The current inhabitants of this planet are the survivors. While mass extinctions almost wiped out all life, they also paved the way for mammals to dominate the planet and develop intelligence. If not for these extinction events, you would not be here, reading this article.
So far, we have reviewed the history of intelligent life on Earth until modern times. But what we really want to know is: when did we first began broadcasting our existence into space? This is the moment that intelligent life on Earth became detectable to alien species. The first broadcast from Earthlings into space with the intention of reaching alien species was in 1974 from the Arecibo Radio Telescope in Puerto Rico.
To summarize, let’s imagine that the Earth has a 10 billion year lifetime, and scale that down to a 100 year human lifetime. On this scale, the Moon formed when Earth was just 1 year old. Simple life formed when Earth was 8 years old, and it wasn’t until Earth was 26 years old that it developed complex life. The largest extinction event in history occurred when Earth was 43 years old, and it was 45 when the dinosaurs died off. By this scale, humans arrived on the scene just 7 days ago. We have only been beaming signals into space for the last 13 seconds!
Life in the Milky Way Galaxy
Now that we have determined just how difficult it was for intelligent life as we know it to develop on Earth, how likely it is that it could have developed elsewhere? To answer this question, we have to rely on our limited observations of the universe to date. Still, let’s crunch the numbers!
Our sun is a rather boring star - medium mass, medium brightness, and currently middle-aged. Astronomers now believe that there are about 400 billion stars in the Milky Way galaxy, about 10 billion of which have masses and brightnesses similar to the sun. Of those 10 billion sun-like stars, we would like to know how many have Earth-like planets, preferably in the habitable zone of their host star. By observing sun-like stars in the solar neighborhood, astronomers have already discovered over 1,000 confirmed exoplanets. We now estimate that 1 out of every 5 stars like the sun host Earth-like planets in the habitable zone, leaving us with 2 billion habitable worlds in our galaxy alone.
Just because a planet is in the habitable zone, that doesn’t mean that life will necessarily arise. Even if life were lucky enough to get started, that doesn’t necessarily mean that it will evolve into intelligent life. Even if intelligent life arises, it will not last for the entire lifetime of the planet itself, either due to the depletion of resources, or something more catastrophic such as nuclear warfare. This is the point where our best guesses come into play, and things get much muddier. It’s time to make some assumptions!
Let’s assume, conservatively, that 50% of all habitable planets actually develop life. But how many of those worlds host intelligent life? To answer that question, you would have to decide whether you believe intelligence is a rare fluke of nature, or whether it is an evolutionary inevitability. Astronomers and biologists are split on this issue. Let’s assume, then, that 50% of life forms that develop on habitable worlds will be intelligent. That leaves us with 500 million intelligent civilizations that, at one point or another, lived in our galaxy. But then, we really only care about those civilizations that we can communicate with. For two-way communication, both civilizations would need to be transmitting signals into space at the same time. Assuming that intelligent, broadcasting civilizations last 10,000 years, we then find that there are 500 advanced alien civilizations in our galaxy that we could contact right now.
Life in the Universe
So far, we have only discussed the probability of intelligent life in our own Milky Way. We must now consider the fact that there are over 100 billion galaxies in the observable universe. That means that there are more stars in the observable universe than there are grains of sand on every beach on Earth. Of course, just because we cannot observe the entire universe, doesn’t mean that the universe itself is finite. It may be that the universe stretches on infinitely, or that ours is just one of many universes. These are questions that are not yet answered by astronomers (but I’m told the cosmologists are working on it).
So, when people ask me if I believe that aliens exist, I always answer “yes, of course.” When faced with the seemingly insurmountable odds of creating life out of random bits of gas and dust, the universe answers with an infinite (or near-infinite) number of opportunities to do so. So, what do you believe?