In the world of Dune, human civilization about 10,000 years in the future had colonized the galaxy, with an empire spanning a “million” worlds. There were no aliens to get in our way, so once we had the technology for interstellar travel, we spread out. The first Dune novel was published 15 years after Fermi made his famous observation – essentially, if aliens exist, where are they? Why haven’t they colonized the galaxy like the humans in Dune?

More than half a century later, the Fermi Paradox remains a hotly debated mystery. One might also invoke the Drake Equation in this discussion. I often hear the Drake Equation dismissed as pseudoscience, but it is just a thought experiment. It is an equation that can be used to calculate the number of technological civilizations in the universe by plugging in all the relevant variables – number of stars, then planets, then planets with life, than life that evolved intelligence, etc.  The only thing that can be considered pseudoscience is plugging in numbers that are just guesses and pretending they are scientific estimates.

The Drake Equation

We have started to make progress informing the variables in the Drake Equation, however. Astronomers have a pretty good handle on how many stars there are in the observable universe. Current estimates are that there are about 2 trillion galaxies in the observable universe, with an average of about 100 million stars per galaxy – 200 quintillion stars. We could be off by an order or magnitude or two, either way that is a massive number.

However, many people are interested in the subquestion – how common is intelligent life in the Milky Way galaxy? Therefore, how many stars are there in the Milky Way?  Estimates vary there as well, but it is on the order of 100 billion (could be several hundred billion).

After estimating the number of stars, further estimates in the Drake Equation get immediately dodgy. What we need to know next is how many stars have planets, and what is the typical distribution of planets by size and distance from their host star. In other words, how many stars have planets that can potentially host life? This is further complicated by the fact that we can only guess at how adaptable “life” is. Can life develop under the ice of large moons of gas giants? How about in the upper atmospheres of those gas giants?

It is easier, therefore, to answer the question – how many Earth-like planets are out there?

With the exoplanet project we are starting to get a handle on that question. As of right now there are 3708 confirmed exoplanets, with 4496 candidates that need to be confirmed. We have discovered exoplanets in 2763 star systems. Of the planets we have confirmed, 927 of them appear to be terrestrial (small rocky worlds like Earth). While that is a lot more information than we had 20 years ago, it is still not a thorough or even random survey. We cannot generalize too much from this data, because what exoplanets we can find are biased by the methods we use. But still, we are starting to get a picture of what planets are out there. We can at least say that exoplanets are common, and that terrestrial planets are also fairly common.

Astronomers are poised to start working on the next variable in the Drake equation – how many of those terrestrial planets have signs of life?  The Transiting Exoplanet Survey Satellite (TESS) is a NASA mission due for launch on 16 April. The purpose of TESS is to locate Earth-like planets close to home. They want to identify at least 50 such worlds. Then, when the James Webb Space Telescope (JWST) launches in 2020, they will point it at these 50 worlds and examine them for signs of life. That will be accomplished by examining the spectrum of starlight going through the atmosphere of transiting worlds. If they contain oxygen, it’s a good bet they contain life.

This would still put us a long way from filling in the Drake equation with any confidence. But at least we will have some way to calculate how common life is likely to be in the galaxy.

The Fermi Paradox

Now back to Fermi – where are all the aliens? We can ask, why haven’t they made their presence known on Earth, and also why hasn’t SETI found alien radio signals? No one knows, of course, but here is a quick list of possibilities.

• They are here, but are hiding. Aliens, or their probes, may by watching the Earth right now, but there are rules against interfering with a developing civilization.
• Life may be rare in the universe. Perhaps the next planet with life is a galaxy over. I think this answer is extremely unlikely. Everything we know right now indicates that life is probably common. It formed quickly once the Earth cooled, for example. But we may have much more information on this question within a decade.
• Life may be common, but intelligent life is rare. I suspect this may be true. Only one tiny branch of evolution’s tree developed a technological intelligence. It may simply be an extremely rare adaptation. Related to this – many forms of intelligence may not be compatible with technology. Perhaps most “intelligent” species are aquatic, or don’t have the combination of intelligence and appendages that can be adapted to tool use.
• Most technological civilizations are short-lived on astronomical time scales. They may destroy themselves in all sorts of ways, or fall victim to natural calamity, such as asteroid impacts or gamma ray bursts. Even if on average technological civilizations survive for a million years, that is a tiny blip on the over 13 billion year lifespan of the universe.
• There may simply be no way around the vast distances of space. It may never become economical to engage in interstellar travel, so most civilizations do not venture beyond their own system in any significant way.
• But – for those civilizations confined to their own system, why aren’t they sending out radio waves to say “hi”? Perhaps humans are the exceptions in our desire to contact other possible civilizations. Perhaps the window of radio technology is extremely brief, and most civilizations hit upon another better form of communication we haven’t discovered yet.
• Perhaps most civilizations turn inward and live their lives in a virtual world.

Of course, any combination of the above factors can be at work. Perhaps each factor knocks down the number of potential technological civilizations who might engage with us in some way. The ultimate density of civilizations trying to communicate with us may be less than one per galaxy – too far away to produce signals we can detect.

But again, we don’t have enough data. SETI has only explored a small percentage of the sky in a small percentage of frequencies. We have only just started to look for exoplanets. It may take us centuries to have a real handle on the answers – assuming we aren’t contacted in the meantime. Everything would instantly change the moment we make contact, but until then we can only make more and more refined estimates.

In the meantime it is best to assume that human civilization is something rare and precious. Hopefully we will provide a positive example in terms of estimating the survivability of technological species.