# Are we alone? Intelligent aliens may be rare, new study suggests

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.

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.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

• Unclear Engineer
I don't disagree with what is in the article, but I don't think it does much more than update us to what we currently know.

At this point, I think we are close to finding out if life originates easily on many planets, or not. If it does, then the search for planets with intelligent life that has the potential to communicate with other planets would need to look for planets that have both water and rocky surfaces, etc. because a bunch of creatures confined to an ocean are not going to start transmitting radio signals, and probably don't understand fire.

So, the search for the necessary type of planet would be the next major issue.

Only with that information can we deduce whether the product of the probability that a intelligent and technologically communicative society will arise times its length of time in existence is rare or common, given the opportunity. And, we aren't likely to figure out that last pair of parameters separately unless we are able to find dead civilizations on other planets.

So, the existential question for humans on Earth is whether we can keep finding our way through the problems we create for ourselves, or not.

Even if we do survive as a technological society for the next few billion years, there is still the possibility that it is simply not possible for us (or any other technological society) to "spread across the galaxy" due to the limits imposed by physical reality of distance and speed limits. So, we can't use the apparent fact that no technological society has "spread across the galaxy" as evidence that there are no other technological societies in existence in the galaxy with very long periods of existence ("L" in Drake's Equation).
• ChrisA
THere is an easier way to figure this out. It also agrees with what we observe.

If we assume that Earth is not rare or unique and that there are millions of Earthlike planets in the galaxy then if this were true what should we expect? How can we calculate what we would see. Not "guess" but "calculate"

Lets build a simulated galaxy populated with exact copies of Earth but the key is each copy of Earth is made on a different year over the 4 billion years of Earth's existence. So our simulated galaxy has 4 billion earth-like planets and they all have different ages.

What would we see? Out of the 4 billion Earths
1) about 100 of them would be humans who know how to build radios. So radio technology would be very rare. with one
2) most of them would have only microscopic life
3) A fair fraction would have multi-cellular life
4) one in a million planets would have mammals, like mice and monkeys and such

Earth is the only data point we have but Earth has existed for 4 billion years, we can look at Earth one each of those billion years and get 4,000,000,000 data points

This is actually VERY disappointing from a SETI perspective because it means that even of something like 1 in 25 plants has an exact copy of Earth we can expect only 100 planets with radio and perhaps zero that can transmit a radio signal over interstellar distances.

So, bacteria-like life might be common, multi-cellular life would be rare, intelligent life would be a one-in-a-billion level rare and advanced technological life would be exactly zero (as we are not there yet.)

I think this is the ONLY method of prediction that does not use extrapolation or guessing. It makes an impossibly optimistic assumption and then concludes that we should expect to hear and see nothing even in a galaxy teaming with "life". In other words, this theory predicts what to observe.

If you want a theory that predicts that we will find ETs then you have to introduce guess and extrapolations like
1) High-tech societies do not destroy themselves be war or global warming or advanced AI.
2) As societies age they continue to care about the universe around them. We don't know. Perhaps they only play video games and live in simulations.
3) perhaps the biological people are peacefully replaced by some kind of hybrid AI and therefore required very small amount of resource for trillions of them to live and expansion is possible by simply building a one meter cube server room

We have no idea about 1,2 or 3 and any answer is a guess. But if you assume only what we 100% know, we should expect a silent galaxy that is filled with simple life.
• Unclear Engineer
At this time, I don't think we really have the data to tell us even that we should not be able to detect a signal from another technological species on another inhabited world even within the range of detection that we currently have.

We really don't know what planets are out there around relatively nearby stars. because we need transits or big planets close-in creating wobbles in their host stars. There are certainly more planets that remain undetected by us than the number we have detected so far.

And, we really just have a lot of speculation about what it takes to have indigenous lifeforms, and more importantly, why intelligence develops.

Do we really need a star similar to the Sun, with a guide planet similar to Jupiter, and an Earth-like planet with plate tectonics, a strong magnetic field, a large moon, an amount of water that creates oceans and dry lands, and a series of cataclysmic events that favor the development of intelligence over specialization to fixed environmental parameters?

If all of that set of conditions is necessary, then life at the technology level we have already created could be quite rare in the galaxy. IF so, then the lifetime of such technological civilizations could be quite long, and we might still never detect one.

The place where such speculation seems to go off-the-rails of logical scientific conjecture is when it is supposed that an intelligent species will eventually develop the capability to travel throughout the galaxy if not killed off while doing that development. We have no logical or scientific basis for assuming that we or any other technological society will ever be able to develop even interstellar travel, much less trans-galactic travel. There could be a sizeable population of planets inhabited by disillusioned beings who have realized that they are never going to reach the stars that they can see.

But, they could certainly send signals farther than they, themselves, could ever travel.
• Galumph
A new interpretation of the famous Drake equation finds little reason to be optimistic about the search for extraterrestrial intelligence.

Are we alone? Intelligent aliens may be rare, new study suggests : Read more
Oh, the good ol' Drake Equation--one of the least useful predictive formulae that exist.

This particular analysis has at least some basis in mathematics/physics, in that the conclusion is based on the hypothetical scenario where we detect at least one other intelligent species in the galaxy.

Earth really isn't a data point we can use to form statistics about the likelihood of life arising under the right conditions, because there is a total observer bias. Without knowing how life arises or how likely it is to arise under ideal conditions, all we can say is that life can exist at least once throughout the observable universe and throughout time.

Without at least one other data point (e.g., confirming or ruling out life in ANY other star system)--The Drake Equation can only provide you with an estimate of the upper limit of how many other worlds have intelligent life. Anything in between 1 (representing life on Earth) and that upper limit number is just a wild guess because it's directly dependent on guessing the likelihood that life will arise under the right conditions. I.e., something we don't have usable data to determine as a result of not being able to actually check a single other star system for life.
• billslugg
In essence, the Drake Equation tells us the number of intelligent civilizations is equal to the number of planets there are times the percentage that have intelligent civilizations. It seems circular to me.
• Unclear Engineer
Agree, except that Drake did not intend his equation to be predictive. He introduced it to try to create some structure in the speculation about what it would take for there to be life that we can communicate with elsewhere in the galaxy.

Frankly, unless the planet is within a few dozen light years of Earth, it seems unlikely that we could hold a useful interstellar conversation, just due to signal transit times. So, even finding a more advanced life form on another planet might not be useful to us for learning anything other than that we are not alone.

Or, at least we weren't alone when they last transmitted. If their existence is as precarious as some of the pessimists on Earth say our own is, they might be gone before we get their last message.
• Classical Motion
If we ever find convincing evidence or even strong suspicion, it won’t matter. We will never be able to confirm it. And will always be in contention if it’s really evidence.

Especially if we only find ONE.
• Hildy
A new interpretation of the famous Drake equation finds little reason to be optimistic about the search for extraterrestrial intelligence.

Are we alone? Intelligent aliens may be rare, new study suggests : Read more
We already know, for certain, that we are not alone. Octopuses are certainly intelligent but would be undetectable on another planet. Their short life spans do present a barrier to civilizations but with a longer life an aquatic civilization is not out of the question.

Even if we limit it to civilizations that spew out radio waves, we have seen our own reduce its footprint steadily since clear channel stations have been shut down. If we build colonies on the Moon or Mars, communication will be via laser, not detectable by other civilizations.

Civilizations living in space habitats would be invisible to us.
• Unclear Engineer
And then there is the "theory" that the reason we can't detect 95% of the matter and energy in the universe is that the Klingons have it cloaked. ;)

But, seriously, good point about lasers as communication systems that we would not detect. An advanced civilization might need to actually want to be detected in order for them to emit something that we can readily detect.
• Manix
The problem here is us. Humans have this ego of they're the pinnacle of existance. There could be many millions or even billions of intelligent species out there whom have advenced passe dour primitive radio signals. They could also "mask" or cloak their planet from detection, to hide them from being observed. We might just be starting out, trying to discover other intelligent life forms out there, but it is possible several scenarios exist A: the don't to be detected; either because like Hawkins said they believe it could be dangerous to them; or they have already had that encounter and it didn't turn out well and now out of self preservation are hiding themselves even in plane sight. Or B: they are so far advanced that they simply see us, but we are so primitive they truly don't want to communicate with us. Their technology maybe so advanced we simply cannot detect it with our own primitive equipment as advanced as we think it is. Until we start thinking out of the box, I feel we're going to keep seeing the same results over and over again with no real conclusion.