The universe should either be crowded with life or harbor hardly any life at all, according to a new study that revamps the Drake equation using probabilistic logic.
A common axiom in the search for extraterrestrial intelligence (SETI) is that if we do detect technologically advanced aliens, there are probably many, many instances of alien life out there rather than there just being two cases (us and the new discovery).
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 the biologist and mathematician J. B. S. Haldane in 1932. Let's imagine a bunch of Earth-like exoplanets, all with similar characteristics. Given their minor differences, we would expect life to arise either on all of them or on none of them; there's no obvious reason why half of these near-identical planets would support life and half wouldn't, for example.
We can then display the various 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 prongs of the U-shape correspond to none or very few planets with life, and lots of planets with life. The valley of the U-shape, which corresponds to a low likelihood, represents half the planets having life.
Related: Drake Equation: Estimating the odds of finding E.T.
Now Kipping and Lewis have ascribed Haldane's logic to the famous Drake equation. Developed by astronomer Frank Drake prior to the first-ever SETI conference, at Green Bank Observatory in 1961, as a means of providing the workshop with an agenda, the Drake equation has subsequently taken on a life of its own, being used to estimate the number of technological lifeforms in the Milky Way galaxy.
The Drake 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 fraction of stars that have planets, ne is the number of planets that are potentially habitable, fl is the fraction of those potentially habitable planets that evolve life, fi is the fraction that develop "intelligent" life, fc is the fraction that have communicative life, and L is the average lifetime 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) very well. The number of potentially habitable planets is less well known, but astronomers are learning more about them every day as they probe exoplanetary atmospheres with the James Webb Space Telescope and characterize those worlds. The values of the other four terms remain a complete mystery, which renders any attempts to use the Drake equation less than satisfactory because so much of it is guesswork.
However, Kipping and Lewis point out that the first six terms in the Drake equation describe the "birth" of what they call extraterrestrial technological instantiations, or ETI. This is how they refer to technological alien life, neatly sidestepping terms such as "civilizations," "species" and "intelligence," which have not only proven problematic (for example, how do we define intelligence?) but may also be inaccurate when describing alien life. Meanwhile, the final term, L, relates to the "death," or otherwise the disappearance, of ETI.
Splitting the terms of the Drake equation this way has allowed Kipping and Lewis to simplify the formula, to read: 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 is totally general," Kipping told Space.com. This means that there is no need to have to worry about the terms of the Drake equation that we don't know.
"We are not assuming any particular mechanism or means of birth," added Kipping. "The births could occur via spontaneous emergence, or panspermia seeding, or empire building or whatever else you want — there simply is a birth rate."
Kipping and Lewis assume 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) by way of a characteristic called the occupation fraction, F. In the exoplanet example mentioned earlier in this article, a high value of F — close to 1 — would correspond to every planet having life, and a low value — close to or equal to 0 — would relate to no planets having life.
The problem facing SETI scientists is that, based on observations so far, F probably is not near 1; otherwise, we would have noticed by now that we are not alone, assuming that intelligent aliens are proficient at spreading across the galaxy, building megastructures such as Dyson swarms and beaming out radio signals. This means that, if we really are not alone in the universe, then the occupation fraction must be closer to 0.5, placing it in that unlikely valley of the U-shaped curve. Based on that U-shape, it is likely that we are relatively alone — that technological life elsewhere in the universe is rare.
"These are instances of life who become obvious, firstly through the signals they produce and then through their colonization where they would be seen through megastructures," Lewis told Space.com. "If such an ETI had arisen in the life of the Milky Way, then they could have colonized the entire galaxy in 10 million to 100 million years, and even after they fall, then 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 vanished long ago and their signatures have decayed away and we are back to our original premise — ETIs appear to be rare in time and space."
Related: The search for alien life
Yet Kipping and Lewis don't advocate giving up on SETI. If we ignore the lack of evidence for a moment, the steady state Drake equation predicts a crowded universe as being equally likely as one in which we are lonely. For a crowded universe, the occupation fraction must be close to 1, and perhaps this is still possible under certain circumstances. Maybe ETI stays in their own region, and our solar system just happens to be in a region that no one has spread into yet. That would mean the aliens are quite far away, and our strategy of searching for them around stars close by is the wrong one. These inhabited regions might be more clearly detected in other galaxies. "I certainly would advocate for extragalactic SETI," said Kipping.
Or perhaps interstellar travel and megastructure-building are too difficult, or maybe they are not even desired by an ETI living a more frugal, less colonial, existence. And with regards to a lack of a radio or optical signal detection, SETI has hardly had the resources to be particularly comprehensive in its search so far, and we could easily have missed a signal.
It's also possible that there is plenty of complex life, but that the development of technological life is rare.
There's also a chance that the birth and death rates of ETI have not reached a steady state after all, meaning that there would be still time for new ETI to arrive on the scene and increase the occupation fraction. Given the age of the universe and the finite lifespan of an ETI, however, this seems unlikely.
The research is currently available as a pre-print, and has been submitted to the International Journal of Astrobiology for peer-reviewed publication.
