Intelligent Life in the Milky Way

Over the past few years astronomers have been making considerable progress on estimating the number of Earth-like planets we should expect within our own galaxy. The most recent study by a team of astronomers at the University of Auckland claim that there are approximately 100 billion. This estimate increases the likely number of Earth-like planets by 82 billion. A study published by astronomers from Harvard Smithsonian Center for Astrophysics in January posited that there were approximately 17 billion Earth-like planets. In my opinion, I think the 100 billion estimate is probably more reliable. The study produced by the Harvard Smithsonian Center for Astrophysics collected data from measuring dimming of planets orbital periods, whereas the study produced from the University of Auckland collected data from gravitational microlensing. Gravitational microlensing is a more effective method for finding smaller planets with Earth-like orbital periods.

However, whether there are approximately 17 or 100 billion Earth-like planets in the Milky Way (or more) the important point is that astronomers now have reason to believe that there are a ton of Earth-like planets. This definitely puts a previously contentious issue to rest in the world of astronomy. In the 1990s and 2000s we had very little to no idea how common Earth-like planets would be. Obviously, these data have tremendous implications for our search of extraterrestrial intelligent life. As a result, these data raise some of the biggest questions we can ask as a species. And in this post, I would like to analyze them.

In short: if we now know that there are several billions of Earth-like planets in our galaxy alone, why is it so quiet out there?

Possibility 1. We are the first

Our universe is 13.82 billion years old. That is a long time. However, our universe has not been habitable for this period of time. The universe has gone through stages of development that astronomers and cosmologists understand fairly well. Throughout the early stages of this development life could not have existed. Furthermore, the first galaxies and star systems were composed of stars that were much larger, less stable, and had shorter lifespans than second and third generation galaxies and star systems. This has important implications for the search for intelligent life because although we have only one case example of how life evolves (that would be us!) I still feel that it is a reliable indicator that biological evolutionary processes require billions of years to produce highly complex life forms. If that is a valid assumption of the time scales required for biological evolution to produce complex life forms then the first galaxies and star systems would not have been ideal candidates for the development of intelligent life. Many of the early stars lasted for 10-100 million years. Complex life requires stable star systems that last for billions of years.

Our Milky Way galaxy is approximately 8 billion years old. If life requires at least 2-4 billion years to produce highly complex life it is still plausible that many planets in our galaxy possess complex life forms and healthy biospheres. However, we also know from our one known case example of life that intelligent life is very rare. Trillions of multi-cellular species have inhabited our planet. Only 1 has evolved meta-awareness and the ability to understand the processes that allowed for its existence. Again, if this is characteristic of biological evolution we should expect most Earth-like planets (that remain stable for more than 1 billion years) to produce a biosphere with no self-aware intelligent species. If this is the case, it is definitely plausible that we are the first (at least in the Milky Way).

How plausible do I think this situation is? I actually think it is highly plausible. Selection for high intelligence is rare in nature. Brains are the most expensive organs. The Milky Way may have billions of biospheres, but only 1 biosphere with an intelligent civilization.

Possibility 2. Intelligent Civilizations have a short life span

Our civilization is very young on the scale of deep time. To accurately conceptualize how young we must turn to the Cosmic Calendar. If the entire history of the universe were conceptualized within one calendar year, modern, sedentary, agricultural human civilization would arrive 13 seconds before New Year’s Eve on December 31st. In this astonishingly short period of time we have completely transformed our planet, landed on another celestial object, and started to explore our solar system with robots. This pace of change is accelerating. If the cultural and technological evolutionary processes that have enabled us to do this are characteristic of intelligent species, we should suspect intelligent civilizations to develop very quickly on galactic scales. We should also suspect them to be very loud. Already, within barely a century of using global communications technology our radio waves have reached hundreds of other star systems (check out a this brilliant atlas depicting the extent of our radio emissions).

I am trying to emphasize an important point here. If intelligent civilizations are common and develop on many Earth-like planets, they must have short life spans because we have not heard them yet. As Ross Anderson of Aeon Magazine has pointed out: “no impressive feats of macro-engineering shine out from our galaxy’s depths.” But if intelligent civilizations develop often and have long life spans (on scales of deep time) we should expect to see such feats of macro-engineering. Could it be that intelligent civilizations have very short life spans?

Robin Hanson of The Future of Humanity Institute believes that there must be a “great filter” between the development of life and a galactic-sized civilization. He suspects that if microbial life is very common in the universe (which it probably is), then the great filter must be between a civilization like ours and a larger-scale multiple star-system civilization.

How plausible do I think this situation is? Actually, I don’t think this is the most plausible situation. I think that once a civilization like ours exists it would take an extreme catastrophe to eradicate it entirely. Almost all potential natural disasters that could erase a civilization like our own would not cause complete extinction. And complete extinction is what would be necessary to prevent further development of our species on the scale of deep time. Perhaps I am being naive regarding this assertion. There may be some great filter and maybe it is the development of nuclear arms. Maybe it is the development of advanced nanotechnology and A.I. Maybe it is something that will exist in a century or two. However, at the moment I think it is more plausible to suspect that intelligent civilizations are rare with long life spans, as opposed to common with short life spans.

Possibility 3. Space Expansion Hypothesis incorrect; Transcension Hypothesis correct

For a long-time many astronomers, cosmologists, and futurists assumed that the natural trajectory for an intelligent civilization was expansion into space. In my opinion this is a foolish assumption. Of course it is possible (in fact plausible) that expansion is the natural tendency for intelligent civilizations like our own. However, we cannot discount the possibility that intelligent civilizations do not expand; they transcend. This hypothesis posits that the reason we do not see any “impressive feats of macro-engineering” in space is because intelligent civilizations turn inwards. Intelligent civilizations may start to compress space, time, energy, and matter (STEM compression) to the point that virtual minds inhabit nano-scales (as opposed to minds inhabiting the macro-scale). Eventually this compression should lead to the ability to exploit the extra-dimensions of space, and perhaps allow intelligent civilizations to escape this universe into a different (or neighbouring) one. If you would like to read an article by the futurist who proposed transcension: read this. If you would like a quick video explaining the idea: watch this.

At the moment the Transcension Hypothesis is quite controversial and untested. To many the ideas seems ludicrous. But many ideas seem radical when they are first proposed. John Smart, who proposed the hypothesis, believes that if Transcension is the fate of intelligent civilizations, we should suspect mini black holes in the habitable rings of spiral galaxies. These mini black holes would be the remnants of “transcended civilizations.” If true, this would certainly explain Fermi’s Paradox and account for the eerie silence.

How plausible do I think this scenario is? To me, this is the most difficult one to make a firm conclusion on. At the moment, I am still convinced that the Space Expansion Hypothesis is correct. But I do not want to assume that it is correct. In the future we may find out more about how intelligent civilizations evolve. If the technological singularity is a “thing” (which I’m pretty sure it is) then we have no clue what our civilization will look like in 100, 200, or 500 years. We may explode into space, or we may explode into the nanoscale. Either way, at the moment I’m going to say that we should suspect robotic expansion to be the expected trajectory of intelligent civilizations. So this would make both possibilities 1 and 2 more plausible.

Possibility 4. Intelligent Life Ignores Us

Arthur C. Clarke famously stated that: “any sufficiently advanced technology is indistinguishable from magic.” Certainly that is true. It would literally be unimaginable for someone 200 years ago to conceive of a smart phone (for example). If there are intelligent civilizations out there (perhaps Type II or III or even IV level civilizations), then they would certainly possess technologies and produce patterns that were impossible for us to imagine. This raises two possibilities:

A) If they wished to remain invisible to us then they could certainly achieve that goal.

B) They could produce patterns that we are unable to detect or recognize.

In situation A) they could know we exist and not care. Or they could know we exist and wish to just observe us from a distance and see what we do. Whatever a civilization (Types II-IV) like this wanted to do they could.

In situation B) organizations like SETI are just unable to detect the types of signals or recognize the types of patterns produced by advanced civilizations. This is an intriguing possibility to me. Consider the fact that we do not know what most of the universe is composed of (e.g., dark matter and energy). If we can’t even understand all natural patterns and phenomena in the universe, why should we suspect to be able to detect patterns and phenomena of advanced galactic civilizations?

How likely do I think these scenarios are? I think that any opinion on these scenarios can only be made by a gut reaction or perhaps a marginally educated guess. I personally think both situations are unlikely. I will say that I think situation A) is more unlikely than situation B). I think it is more likely that advanced civilizations are doing things that we can’t detect. I don’t think there is some advanced Milky Way federation of civilizations that are observing us from a distance. Finally, I will add that I think possibilities 1, 2, and 3 are more likely than these possibilities.

Possibility 5. Faster-than-light travel is impossible

The final possibility is that the universe has a speed limit and there is no way to get around this speed limit. If this is the case then expansion into space makes very little sense and intelligent civilizations would give up on this idea. Instead they simply continue to inhabit their home planet until global catastrophe eradicates them (whether that be self-inflicted or natural).

Of course, light travels very quickly. In fact, light travels so quickly that it can circle our planet 7 times in 1 second. However, even if an intelligent civilization developed a space craft that could travel this quickly, it would still take 4 years to travel to the nearest star system. Developing a civilization connected over these distances would not be feasible, especially when you consider that our galaxy is 100,000 light years across. However, I suppose it would be possible under this scenario for civilizations to expand and disperse. There could be rings of civilizations that diffuse outwards to new planets and then remain disconnected from one another (or very loosely connected — perhaps tweeting back and forth every couple hundred years).

How plausible do I think this situation is? I actually think this is the least likely of all the scenarios. We are not even a Type I civilization and we have already proposed several theoretical models that explore the possibility of faster-than-light speed travel. A few of these ideas include wormholes and the Alcubierre drive. Wikipedia has great articles on both if you want to learn more about them. My point is simply that just because we currently have a poor (or limited) understanding of how to circumvent the speed-of-light we should not expect a more advanced civilization to find this problem insurmountable. I would not be so foolish as to claim it impossible that speed-of-light really is an impossible speed limit to pass. However, I think it is unlikely to be. I think it is a problem that an intelligent civilization could solve if given enough time.

Implications of 100 Billion

The implications of 100 billion Earth-like planets in our galaxy is profound (it may have even more profound implications for the universe as a whole). The knowledge that Earth-like planets are abundant forces us to confront big questions about extraterrestrial life. The point of this article was to explore these questions with some level of depth. I believe that everyone’s opinion on what the implications are will be slightly different. In my opinion, current evidence presents us with five potential scenarios for advanced intelligent life. In this article I have tried to rank them from 1-5 (with 1 being the most probable and 5 being the least probable).

  1. We are the first (at least in our galaxy)
  2. Intelligent civilizations have a short span
  3. Space Expansion Hypothesis incorrect; Transcension Hypothesis correct
  4. Intelligent life ignores us
  5. Faster-than-light travel is impossible

What do you think?

Did you like this article?  Let Cadell know on Twitter!

Related Advanced Apes content:

Another Earth

Life on Europa

Dear Universe, Sincerely Earth


About Cadell Last
I am a science educator, freelance science writer, and founder of The Advanced Apes based in Toronto, Ontario. In the past my academic research focused on the evolution, ecology, and behaviour of non-human primates (i.e., chimpanzees, gorillas, ring-tailed lemurs). Currently, my official blog, The Ratchet, can be found via The Advanced Apes and Svbtle. I enjoy exploring recent research in human evolutionary sciences, as well as biology, ecology, astronomy, physics, and computer science. My work has been featured in Scientific American, American Humanist, Richard Dawkins Foundation for Reason and Science, and Jane Goodall Institute of Canada. I am also exploring science popularization in new mediums in collaboration with PBS Digital Studios with an animated YouTube channel. You can contact me on Twitter (@cadelllast) or via email:

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