Looking For the City Lights of Alien Civilizations

Article written: 4 Nov , 2011
Updated: 24 Dec , 2015
by

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When most people think about the search for alien life, the first thing that usually pops into mind is SETI (Search for Extraterrestrial Intelligence). Primarily a search for extraterrestrial radio signals, another more recent facet of SETI is now looking for laser pulses as a conceivable means of communication across interstellar distances. But now, a third option has been presented: looking for sources of artificial light on the surfaces of exoplanets, like the lights of cities on Earth.

According to Avi Loeb at the Harvard-Smithsonian Center for Astrophysics, “Looking for alien cities would be a long shot, but wouldn’t require extra resources. And if we succeed, it would change our perception of our place in the universe.”

Like the other SETI initiatives, it relies on an assumption that an alien civilization would use technologies that are similar to ours or at least recognizable. That assumption itself has been the subject of contentious debate over the years. If an alien society was thousands or millions of years more advanced than us, would any of its technology even be recognizable to us?

That aside, how easy (or not) would it be to spot the signs of artificial lighting on an alien planet light-years away from us? The suggestion is to look at the changes in light from an exoplanet as it orbits its star. Artificial light would increase in brightness on the dark side of a planet as it orbits the star (as the planet goes through its phases, like our Moon or other planets in our own solar system), becoming more visible than any light that is reflected from the day side.

That type of discovery will require the next generation of telescopes, but today’s telescopes could test the idea, being able to find something similar as far out as the Kuiper Belt in our solar system, where Pluto and thousands of other small icy bodies reside. As noted by Edwin Turner at Princeton University, “It’s very unlikely that there are alien cities on the edge of our solar system, but the principle of science is to find a method to check. Before Galileo, it was conventional wisdom that heavier objects fall faster than light objects, but he tested the belief and found they actually fall at the same rate.”

The paper has been submitted to the journal Astrobiology and is available here.

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25 Responses

  1. Mike W says

    This is why people who believe we should be careful about the type of radio broadcasts we send into space are wasting their time. The reality is that our planet cannot be hidden from prying telescopic eyes no matter what we do. And even if we turned out all the lights, the presence of our civilization on Earth is betrayed by all the pollutants we pump into the atmosphere, and if telescopes can pick up city lights, they can certainly pick up the spectrum of the sunlight reflected off the Earth’s atmosphere.

    Get rid of all the pollution, and Earth still broadcasts the fact that it carries abundant life with it, having an oxygen as it does, so unless life-bearing planets are ten-a-penny in the Milky Way, Earth will certainly attract the attention of other civilizations scanning their skies for signs of life.

    The future of space exploration beyond our own Solar System is almost certainly going to be a long-range endeavor for hundreds of years, at least, so if we want to study planets in other solar systems, we are going to have to get very good at building telescopes capable of imaging them and analyzing them. That will probably mean, eventually, whole fleets of telescopes working together in the outer solar system, beyond the interference of the zodiacal light, Even though that will cost a bomb, it will be chicken feed compared to funding just one interstellar mission to the nearest star, and instead of studying just one system, they will be capable of studying thousands, if not millions.

    • calciu says

      sometimes i come across comments that just as interesting as the article itself. this is one of of them. Good job Mike . tnx

    • calciu says

      sometimes i come across comments that just as interesting as the article itself. this is one of of them. Good job Mike . tnx

    • calciu says

      sometimes i come across comments that just as interesting as the article itself. this is one of of them. Good job Mike . tnx

    • Anonymous says

      This is of course looking for an absolute needle in the haystack proposition. However, a survey might give an upper bound on the distribution of ETI in the universe. If we could image extrasolar planets the optical signatures of conditions there might suggest the existence of life. I think a very small percentage of such planets so identified will have intelligent life.

      LC

      • Torbjörn Larsson says

        But it is looking for it by lying in the straw bed instead of poking it here and there. (Ouch!)

    • Anonymous says

      This is of course looking for an absolute needle in the haystack proposition. However, a survey might give an upper bound on the distribution of ETI in the universe. If we could image extrasolar planets the optical signatures of conditions there might suggest the existence of life. I think a very small percentage of such planets so identified will have intelligent life.

      LC

    • Anonymous says

      This is of course looking for an absolute needle in the haystack proposition. However, a survey might give an upper bound on the distribution of ETI in the universe. If we could image extrasolar planets the optical signatures of conditions there might suggest the existence of life. I think a very small percentage of such planets so identified will have intelligent life.

      LC

  2. Mike W says

    This is why people who believe we should be careful about the type of radio broadcasts we send into space are wasting their time. The reality is that our planet cannot be hidden from prying telescopic eyes no matter what we do. And even if we turned out all the lights, the presence of our civilization on Earth is betrayed by all the pollutants we pump into the atmosphere, and if telescopes can pick up city lights, they can certainly pick up the spectrum of the sunlight reflected off the Earth’s atmosphere.

    Get rid of all the pollution, and Earth still broadcasts the fact that it carries abundant life with it, having an oxygen as it does, so unless life-bearing planets are ten-a-penny in the Milky Way, Earth will certainly attract the attention of other civilizations scanning their skies for signs of life.

    The future of space exploration beyond our own Solar System is almost certainly going to be a long-range endeavor for hundreds of years, at least, so if we want to study planets in other solar systems, we are going to have to get very good at building telescopes capable of imaging them and analyzing them. That will probably mean, eventually, whole fleets of telescopes working together in the outer solar system, beyond the interference of the zodiacal light, Even though that will cost a bomb, it will be chicken feed compared to funding just one interstellar mission to the nearest star, and instead of studying just one system, they will be capable of studying thousands, if not millions.

  3. Mike W says

    This is why people who believe we should be careful about the type of radio broadcasts we send into space are wasting their time. The reality is that our planet cannot be hidden from prying telescopic eyes no matter what we do. And even if we turned out all the lights, the presence of our civilization on Earth is betrayed by all the pollutants we pump into the atmosphere, and if telescopes can pick up city lights, they can certainly pick up the spectrum of the sunlight reflected off the Earth’s atmosphere.

    Get rid of all the pollution, and Earth still broadcasts the fact that it carries abundant life with it, having an oxygen as it does, so unless life-bearing planets are ten-a-penny in the Milky Way, Earth will certainly attract the attention of other civilizations scanning their skies for signs of life.

    The future of space exploration beyond our own Solar System is almost certainly going to be a long-range endeavor for hundreds of years, at least, so if we want to study planets in other solar systems, we are going to have to get very good at building telescopes capable of imaging them and analyzing them. That will probably mean, eventually, whole fleets of telescopes working together in the outer solar system, beyond the interference of the zodiacal light, Even though that will cost a bomb, it will be chicken feed compared to funding just one interstellar mission to the nearest star, and instead of studying just one system, they will be capable of studying thousands, if not millions.

  4. Anonymous says

    I always thought we would be looking for worlds of light and life. Strange that we are only now discussing the possibility.

  5. Anonymous says

    I always thought we would be looking for worlds of light and life. Strange that we are only now discussing the possibility.

  6. Anonymous says

    I always thought we would be looking for worlds of light and life. Strange that we are only now discussing the possibility.

  7. Anonymous says

    “If an alien society was thousands or millions of years more advanced than us, would any of its technology even be recognizable to us?”
    There is little reason to assume “advancements” can be measured as any linear function of time, so the sentence above seems to make little sense.

  8. Anonymous says

    “If an alien society was thousands or millions of years more advanced than us, would any of its technology even be recognizable to us?”
    There is little reason to assume “advancements” can be measured as any linear function of time, so the sentence above seems to make little sense.

  9. Anonymous says

    “If an alien society was thousands or millions of years more advanced than us, would any of its technology even be recognizable to us?”
    There is little reason to assume “advancements” can be measured as any linear function of time, so the sentence above seems to make little sense.

    • Paul Scott Anderson says

      Well, even Carl Sagan postulated that the technology of a civilization that far ahead of us might be “indistinguishable from magic” from our perspective. Compare our modern society to ones just a few hundred or thousand years ago…

      • Anonymous says

        We should remember the last couples of centuries are in no way “typical”, but extreme. Sagans postulates may be – postulates.

      • squidgeny says

        I don’t think it’s unreasonable to measure technological advancements in years… though there are others, like number of patent applications, time is one that makes sense to most people.

      • Anonymous says

        Many people possibly think it makes sense to measure “advancement” or “progress” in years, but if it does is a different matter.
        Just ask more precisely what the real meaning of such an assumption could be. Is there any “law of progress” that says we will reach a certain point after so and so many years? If not, what then should make us believe it should beso on other planets? If we cannot both measure rates of “progress”, and say how much we “advance” each year the idea seems to be without any content.

      • squidgeny says

        Wasn’t that Arthur C Clarke who said that?

  10. Anonymous says

    i recall reading a transcript from one of the Apollo astronauts that city lights were not visable from the moon…But lightning was. Maybe looking for this might be easier..

    • Member
      Anonymous says

      You might have to wait for a ‘new earth’ condition, when the dark side of Earth faces the moon….. then try binocs? I believe the Earth was pretty well illuminated during the lunar missions…. a factor? REMEMBER: Don’t look directly at the sun! And be careful not to scratch your visor! Let me know how that works out? oTay?

  11. Anonymous says

    Assuming a civilization was technologically advanced would looking for Mercury spectral lines be possible. Of course they may be using incandescemt lights or sodium lamps which would be more difficult since the stars own spectrum would mask that. But an excess mercury sign might show up.

  12. Anonymous says

    Besides the obvious likely scarcity of ETI (see previous discussions on this). There is a chance that ETI’s might have developed more efficient lighting usage and tend to minimize how much gets wasted into the sky. Also, they may not use lighting, use sight, have reached the stone age or the light is masked by bio-lights.

    Still worth investigating.

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