When it comes to aliens, everyone has an opinion despite no one having much of anything concrete to go on. Famed physicist Enrico Fermi famously looked up into the deafening silence of the night sky above Los Alamos National Laboratory in the 1940s and demanded to know where the hell everybody was.
If even one of the greatest minds of the 20th century was stumped, it's a good bet that we're not going to do much better at answering the question, "where are all the aliens?" There is still a lot to say about them though, and the idea of alien species on other worlds or in different realities is about as old as human thought.
For whatever reason, aliens have a hold of our imagination, and they frankly always have. So what do we mean when we talk about aliens? What are our best guesses on their appearance if they do exist? And, honestly, what are the odds that they're actually out there, and why should we care so much?
Do aliens exist?
That is the million-dollar question, isn't it? We've discovered thousands of exoplanets in the past 30 years, and we haven't heard a peep one way or the other.
If we just look at the question of probabilities, then it seems like madness to doubt the existence of aliens. There are about 400 billion star systems in the Milky Way galaxy alone, and each of those is almost guaranteed to have at least one exoplanet. Most systems we've looked at in detail have half a dozen exoplanets, with two or three in the "habitable zone" of their star—the range of distance from the star where liquid water can exist on its surface at least for a substantial portion of the year.
The Search for Extra-Terrestrial Intelligence Institute has been scouring the skies for radio signals from intelligent civilizations for decades now. While there have been plenty of false positives, we've yet to intercept or otherwise receive so much as a "Hello World". We've even put many of our species' significant highlights on a golden record and shot it into interstellar space—twice—in the hope that it bumps into someone up there who will then get on the 'phone' and call us to let us know that we're not alone.
But in addition, many of our next-generation science instruments, like the James Webb Space Telescope or the Nancy Roman Telescope, are specifically designed, or at least have it in their remit, to look for alien life. It's unquestionable that if there is alien life out there capable of being detected, we're closer to making contact than we've ever been.
But that would be true whether we were days away from getting an interstellar email or we've got decades ahead of us before any kind of contact is made, and so all most of us can really do is look up at the night sky like Enrico Fermi and ask the big questions while we wait for an answer, one way or the other.
What do aliens look like?
Aliens could and will look like just about anything you can imagine, given the nature of their evolution and development (should they exist). Even on Earth, we are constantly surprised by the kinds of utterly bizarre flora and fauna that live 1 kilometer below the ocean surface, and we are far closer to a barreleye fish (see above) than we'd be to Alpha Centaurians.
Unless the exact opposite were true.
There is a theory in evolutionary biology called convergent evolution. According to this idea, geographically isolated species are likely to adopt the same evolutionary adaptations due to their lived environment.
All those movies with humanoid aliens may be a more accurate representation of our future alien relationships than anything out of Independence Day. Suppose the aliens we're talking about evolved primarily on land and on a planet similar to ours. In that case, they'll have many of the same physiological developments that we do, even if there are some more unique configurations.
We're far more likely to recognize ourselves in land-evolved aliens than we are for anything that evolved in the oceans, however, so that is definitely something to think about as we look out into the cosmos for evidence of intelligent life—and also reminds us that there are plenty of aliens to be discovered nearer to home than many people realize.
What about the Drake Equation?
The Drake Equation was introduced by the astronomer Frank Drake in 1961 as a starting point for discussion at the first meeting of astronomers working on the subject of the search for extraterrestrial intelligence, and it has since taken on something of a life of its own.
According to the SETI Institute, the Drake Equation "is generally agreed to be the 'second most-famous equation in science (after E= mc2),' and you can find it in nearly every astronomy textbook." That isn't that much of a stretch, and the Drake Equation has influenced the conversation around alien life for the past 60-plus years.
The Drake Equation looks at several factors to determine the probability of alien life in our galaxy. The Drake Equation is defined as
N = R* • fp • ne • fl • fi • fc • L
with the terms in the Drake Equation being:
- N: The number of civilizations in our galaxy whose electromagnetic radiation is detectable
- R*: the rate of star formation in the galaxy that produces suitable conditions for the development of life
- fp: The fraction of those stars that have planetary systems
- ne: The mean number of planets in a star system that can support life
- fl: The fraction of such planets where life, in fact, develops
- fi: The fraction of life-sustaining planets where intelligent life develops
- fc: The fraction of those planets which have civilizations capable of producing detectable electromagnetic signals, like radio waves
- L: The average length of time that a civilization can produce such signals, in years
What we're looking for is N, namely the probability that an alien civilization is alive and broadcasting at a point in time when we can hear them. Take that probability and multiply it against the number of stars in our galaxy (400 billion). In theory, you can come up with a rough estimate of the number of active alien civilizations currently inhabiting our galaxy.
If you've read those variables closely, though, it should be fairly obvious to you that this is not an equation in a traditional sense like, say, E=mc2 or a2 + b2 = c2.
The Drake Equation is, ultimately, a probabilistic one, giving you a result between zero and one, and which tells you the odds of a particular outcome, much like the flip of a coin or the roll of a die, only with many more sides and each weighted very differently from the others.
If the number of life-sustaining planets in a planetary system is one or two planets greater than expected, the impact on the result can be substantial. The time a civilization can exist while being capable of producing electromagnetic waves can likewise produce a galaxy buzzing with activity, or, it can turn the Milky Way into a cosmic crypt with a single inhabitant—us—that is destined to take its place beside the rest of the dead alien species that we won't ever even know existed.
Because the variables in the Drake Equation are so fluid, it's not like math equations produced by thinkers like Einstein or Euclid. Those are meant to describe something concrete in an abstract way. Meanwhile, the Drake Equation is much fuzzier, and each variable is open to interpretation (at least for now), and serves a different purpose than mere math.
In many ways, the Drake Equation is aspirational and is more an expression of hope in the existence of advanced extraterrestrial life than it is science (which is one of the principal criticisms leveled against it). Hope is not science, and in many ways is antithetical to sound science, at least in that a good scientist shouldn't go out in search of evidence to support a conclusion, but should rather see whether neutral (or as close to neutral as possible) observation or experimentation supports a hypothesis.
But there is a place for something like the Drake Equation, taken in its proper context. When looking out into the cosmic void of hundreds of billions of stars in our galaxy alone, confronting the apparent silence of it all can easily lead one to write off SETI as a fool's errand.
In cosmic terms, we've only had the capacity to detect aliens on other worlds for just over a hundred years, at most (assuming that rudimentary radio receivers on Earth could somehow pick up radio broadcasts from thousands of light-years away, not that we'd understand what we were hearing necessarily). That's not long at all, even in terms of the lifetime of our species, much less the age of life on Earth or the galaxy as a whole, which is thought to have taken on its current form about 9 billion years ago.
In short, we've only just turned on the radio, and we haven't even had a chance to hear the song. Given that our individual lives are so short, cosmically, it would be absurd to say that there is no alien life out there because we haven't heard from them. That is what makes the Drake Equation a critical point of discussion from a scientific perspective because it gives scientists some perspective.
Yes, that is an enormous haystack we have in front of us, and after some searching, we've yet to find any needles. Some scientists might be tempted to throw their hands up and say the search is pointless, but the Drake Equation reminds us that there might still be needles in there, even tens of thousands of needles. We just have to keep searching if we ever hope to find them. Whether we continue to do so is entirely up to us.