On November 16, visitors to Columbia University’s Pupin Hall were treated to a far-ranging lecture by Dr. David Kipping on the detection of alien civilizations. Yes, this was about technological civilizations, not just simple signs of life, and as such, poses a separate set of questions and potential detection methods.
Dr. Kipping is a professor at Columbia’s astronomy department, where his principle area of research is exoplanets and exomoons. His teaching includes courses in exoplanets, astrobiology, and astrostatistics. He also leads the university’s Cool Worlds Lab, which focuses on the detection and analysis of worlds found at longer orbital periods https://www.youtube.com/coolworldslab. You also may recognize him from his television appearance on Science Channel’s How the Universe Works. During his talk, he gave us a glimpse at some of the fascinating research he has been conducting.
The lecture began with a bit of history: specifically, the 1593 trial of the Dominican friar Giordano Bruno. Following on the then-recent work of Copernicus, Bruno was one of the first to propose that the distant stars were surrounded by planets, some of which might be inhabited by beings like us. For this and other heresies, among which the denial of the virginity of Mary, he was burned at the stake!
Thanks to the revelations of the Kepler survey, Bruno’s claim about a “plurality of worlds” is now undeniable. Dr. Kipping ran us through some simple arithmetic: with 200 billion stars in the Milky Way, of which 75% lie outside the galactic core (which is likely unfriendly to life), and among which 3/4 are M-type dwarfs with 1/6 of those hosting a potentially habitable planet, this yields a possible 20 billion habitable worlds in our galaxy alone. If you prefer stars more like our Sun, the calculation is similar for F-, G-, or K-dwarfs, with 3% of them having a potentially habitable planet, yielding a total of about 1 billion habitable worlds around such stars in our own galaxy. The above does not even take into account the possibility of habitable exomoons.
This raises the issue of the Fermi paradox: if there are aliens all around us, why haven’t we encountered them? We have been listening for extraterrestrial radio signals for five decades now, without any convincing signals. But as Dr. Kipping pointed out, SETI efforts historically have been extremely limited by minimal funding and lack of search completeness, and our technology is not sensitive to accidental “leakage” of radio signals from other worlds. The classic analogy for this is filling a glass with seawater at a beach, and concluding from its examination that there are no fish in the ocean.
But is it even reasonable to expect aliens to use radio technology? Professor Kipping noted that the Earth is becoming increasingly radio-quiet now after only one century of broadcasting. There are a large number of ways we could in theory communicate with other civilizations; but since we can’t speak to them just yet, how do we agree on a common medium for communication? As he put it, the problem is akin to trying to meet someone in Manhattan without being able to speak to them to decide on a meeting place.
This problem is precisely the one researched in 1958 by the economist Thomas Schelling. Professor Kipping then discussed the science of Schelling Points, that is, a solution which you might expect the largest number of other people to select in the absence of communication. There are many examples of such behavior; in the case of meeting in Manhattan, this turned out to be Grand Central Station. But what would be a Schelling Point for communicating with aliens?
Referencing a 2016 paper he co-authored with graduate student Alex Teachey, Dr. Kipping suggested that planetary transits in front of their stars may be interstellar Schelling Points. After all, thanks to the Kepler survey, such transits are our most successful means to learn about exoplanets. If other civilizations build Kepler-like satellites as well, it would be during Earth’s transit in front of the Sun as seen from their location that they would be most likely to observe us.
He then turned to the problem of how to encode information in the transit signal seen from afar. One way to achieve this would be via shadow imaging: build large structures which obscure part of the signal in a telltale way. In another paper co-authored this year with graduate student Emily Sandford, Kipping describes how the light curve produced by an occulter can be analyzed to reveal the occulter’s shape. As he puts it, if we find a light curve which reveals an occulter shape that does not occur in nature (for example, a triangle or a gate), “then we’re in business”!
Of course, the above is a recipe for detecting intentional signals from space. What about unintentional leakage of infrared radiation given off by alien mega-structures, such as a Dyson sphere? We are not capable of building these and won’t be for centuries at the least, but maybe someone else is. In that context, Dr. Kipping turned to a discussion of Kardashev civilizations; a Type II
Kardashev civilization would be capable of building such a mega-structure, which we could detect by its infrared glow.
The celebrated Tabby’s Star provides an example of such analysis. Upon studying the light curve from this famous star, Kipping and Sandford found that the “occulter” is dusty, rather than a solid object.
Dr. Kipping finished by noting that although to date, limited searching via radio, structures in transit, and waste infrared energy has failed to detect any civilizations, imagine how much deeper we could search if we decided to devote serious resources to the project. Although we might eventually find that we are indeed alone, imagine how much we could learn if we discovered that we were not!
 In 1964 Nikolai Kardashev proposed a scale for measuring the technological level of a civilization based on the amount of energy it could use and store. A Type I civilization can harness the energy of its entire planet; a Type II can do the same for a star; and a Type III civilization could control the energy of an entire galaxy.