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The universe should either be teeming with life or barely host any life at all, according to a new study that reworks the Drake equation using probability logic.
A common axiom in the search for extraterrestrial intelligence (SETI) is that if we detect technologically advanced aliens, there are likely many, many cases of extraterrestrial life instead of there being just two cases (us and the new discovery), there are.
In a new paper, astronomers David Kipping of Columbia University in New York and Geraint Lewis of the University of Sydney describe how this logic works, based on a probability distribution first introduced by biologist and mathematician JBS Haldane in 1932. Let’s take a look at some Exoplanets that resemble Earthall with similar characteristics. Given their small differences, we would expect life to arise on all or none of them; there is no obvious reason why half of these nearly identical planets for example, half would support life and half would not.
We can then plot the different outcomes in a U-shaped graph, with the probability on the y-axis and the fraction of planets with life on the x-axis. The two points of the U-shape correspond to no or very few planets with life, and very many planets with life. The valley of the U-shape, which corresponds to a low probability, represents half of the planets with life.
Related: Drake Equation: Estimating the Chances of Finding ET
Now Kipping and Lewis have attributed Haldane’s logic to the famous Drake equation. Developed by astronomer Frank Drake before the very first SETI conference, on Green Bank Observatory In 1961, as a means of providing an agenda for the workshop, the Drake equation subsequently took on a life of its own, being used to estimate the number of technological life forms in the Milky Way Galaxy.
Drake’s equation is written as: N = R* x fp x ne x fl x fi x fc x L, where N is the number of civilizations, R* is the star formation rate, fp is the percentage of stars with planets, ne is the number of potentially habitable planets, fl is the percentage of those potentially habitable planets on which life emerges, fi is the percentage at which ‘intelligent’ life emerges, fc is the percentage at which communicative life emerges, and L is the average lifespan of civilizations.
Astronomers know the star formation rate (less than 10 solar masses per year in our galaxy) and the fraction of stars that have planets (almost every star has planets) are very good. The number of potentially habitable planets is less well known, but astronomers are learning more about it every day as they exoplanetary atmospheres with the James Webb Space Telescope and characterize those worlds. The values of the other four terms remain a complete mystery, making any attempt to use the Drake equation less than satisfactory, since so much of it is guesswork.
Kipping and Lewis point out, however, that the first six terms in the Drake equation describe the “birth” of what they call extraterrestrial technological instantiations, or ETI. In this way, they refer to technological extraterrestrial life, neatly avoiding terms such as “civilizations,” “species,” and “intelligence,” which have not only proven problematic (e.g., how do we define intelligence?) but may also be inaccurate when describing extraterrestrial life. Meanwhile, the final term, L, refers to the “death,” or otherwise disappearance, of ETI.
By splitting the terms of the Drake equation in this way, Kipping and Lewis were able to simplify the formula to: The time-averaged number of ETIs in the galaxy equals the birth rate of ETIs multiplied by their death rate.
“The beauty of our approach is that it’s completely general,” Kipping told Space.com. This means we don’t have to worry about terms of the Drake equation that we don’t know.
“We are not assuming any particular mechanism or means of birth,” Kipping added. “The births could be through spontaneous emergence, or panspermia sowing seeds, or building an empire or whatever you want – there’s just a birth rate.”
Kipping and Lewis start from what they call a steady-state Drake equation, where there is a roughly equal level of birth and death rates in an equilibrium that is inevitably reached once enough time has passed. The two astronomers then relate this back to Haldane’s prior (a “prior” is the name for a type of probability distribution, such as the U-shaped curve) via a property called the occupancy fraction, F. In the exoplanet example mentioned earlier in this article, a high value of F — close to 1 — would correspond to every planet harboring life, and a low value — close to or equal to 0 — would correspond to no planet harboring life.
The problem that SETI scientists face is that F, based on observations made so far, is probably not close to 1. Otherwise, we would have learned by now that we are not the only ones who assume that intelligent aliens can spread out well across the Milky Way and build megastructures like Dyson swarms and transmit radio signals. This means that if we really aren’t alone in the universe, the occupancy fraction must be closer to 0.5, putting it in that unlikely valley of the U-shaped curve. Based on that U-shape, it’s likely that we’re relatively alone — that technological life elsewhere in the universe is rare.
“These are examples of life that are evident, first by the signals they produce and then by their colonization, where they would be seen by megastructures,” Lewis told Space.com. “If such an ETI had originated in the life of the Milky Way, they could have colonized the entire galaxy in 10 million to 100 million years, and even after they fell, their debris would be around for a long time. The fact that we don’t see anything out there means that if they did exist, they disappeared long ago and their signatures have decayed, and we’re back to our original premise — ETIs appear to be rare in time and space.”
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Yet Kipping and Lewis are not advocating giving up on SETI. If we ignore the lack of evidence, the steady state Drake equation predicts a busy universe as likely as one in which we are lonely. For a crowded universe, the occupancy fraction must be close to 1, and perhaps under certain conditions this is still possible. Perhaps ETI stays in their own region, and our solar system happens to be in an area where no one has been before. That would mean the aliens are quite far away, and our strategy of looking for them around nearby stars is the wrong one. These inhabited areas might be more detectable in other galaxies. “I would certainly advocate for extragalactic SETI,” Kipping said.
Or maybe interstellar travel and megastructure-building are too difficult, or perhaps not even desired by an ETI living a more frugal, less colonial existence. And as for the lack of radio or optical signal detection, SETI has so far hardly had the resources to be particularly extensive in its search, and we would easily missed a signal.
It is also possible that while there is a lot of complex life, the development of technological life is rare.
There is also a chance that the birth and death rates of ETI have not yet reached a steady state, meaning that there is still time for new ETI to appear on the scene and increase the occupancy fraction. However, given the age of the universe and the finite lifespan of an ETI, this seems unlikely.
The research is currently available as a preprintand has been submitted to the International Journal of Astrobiology for publication with peer review.