The numbers game

New research claims that are at least 36 communicating species in the Galaxy, but do the numbers add up?

For sixty years, modern efforts in the search for extraterrestrial intelligence have listened and looked to the stars for signs of life beyond our Solar System, but without success. The optimists say we’ve just not looked long or hard enough. The pessimists say that all the signs suggest that there’s no one out there. The trouble for SETI is that we just don’t know – there could be thousands of technological extraterrestrial life-forms, or just a few, or none at all. If we just had some idea as to our chances of success, it would be a great help in at least managing our expectations of SETI.

Now, two scientists at the University of Nottingham, Tom Westby and Christopher Conselice, claim to have solved this quandary in a new research paper published in The Astrophysical Journal (you can read the pre-print here). 

They bring good news and bad news. The good news first? They calculate that we are indeed not alone, and that at minimum there are 36 communicating species (± 17532) out there in the Milky Way that we could potentially detect right now. The bad news is that without a huge stroke of luck, at our current speed of progress in SETI, it will take 3,060 years to find just one of them among the 150–200 billion stars that inhabit our Galaxy. 

Their research seems to have struck a chord with the media, with the claims of 36 civilisations splashed across headlines on various news websites. 

The media have gone to town with the conclusion that there are at least 36 communicating civilisations in the Galaxy.

Westby and Conselice are not the first to try to calculate the number of planets with intelligent life, and efforts in this regard date all the way back to the Drake Equation. It’s certainly fun to try and model different scenarios and see what number you come up with. Yet SETI is always going to be hamstrung by a lack of information, and it’s tempting to roll my eyes each time someone tries to estimate how populated the Galaxy is. So does Westby and Conselice’s work fall into that eye-rolling category, or is there more to it?

A new equation

The key to what they’ve done is their remodelling of the Drake Equation. Actually, it’s less a remodelling and more of a ‘throw-it-away-and-start-over’ technique. Frank Drake concocted his famous equation to help summarise the agenda for the first ever SETI conference, which was held at Green Bank radio observatory in the United States in 1961. Drake wasn’t trying to derive a mathematical equation that would give an absolute answer, it was just a clever way of summing up some of the main factors that he thought could help point us in the right direction of whether technologically intelligent, communicative life is common in the Universe or not. What the Drake Equation tells us is that we are largely ignorant of many of those factors, such as the fraction of planets that develop life, the fraction that develop intelligent, and then technological, life, and how long that life communicates for. These factors aren’t astrophysical, they are astrobiological, evolutionary, and societal, and we only have a sample of one – ourselves – to base them on. 

Frank Drake, writing his eponymous equation that attempts to estimate the number of civilisations in the Milky Way Galaxy. Image: SETI Institute.

So any solutions to the Drake equation are, by the nature of the equation, just guesses. So Westby and Conselice have tried to sidestep the issue of our ignorance by creating their own equation based on astrophysical properties that we do know. It looks like this:

N = N* ⨉ fL ⨉ fsp ⨉ (L/𝞃’)

Okay, so if you’re not mathematically inclined, it might look a little scary, but it’s not, I promise – it’s just one long multiplication with a little bit of division at the end. Each letter, or group of letters, refers in shorthand to one of the factors of the equation. N, for example, refers to the number of ‘intelligent’ civilisations/societies/species in the Galaxy – this is the value the equation is attempting to calculate. 

Continuing through the equation, N* is the total number of stars in the Galaxy, fL is the fraction of those stars older than five billion years, fsp is the fraction of those stars that has a suitable planet for life, L is the average lifespan of a civilisation/society/species during which they are communicative, and 𝞃’ is the average amount of time during which ‘intelligent’ life could have evolved on a suitable planet. You can think of the fraction (L/𝞃’) as being the probability that we exist at the same time as the signals from another civilisation (they might have actually gone extinct, but their signals could still be propagating through space towards us, such is the distance they have to travel).

Furthermore, Westby and Conselice define fsp as being the product of fm, which is the fraction of stars with the right amount of heavy elements (‘metals’, in astronomical parlance) heavier than hydrogen and helium needed to build planets and life, and fHZ, which is the fraction of stars in the Galactic Habitable Zone. This refers to the region, or regions, in our Galaxy where those elements are abundant and where the supernova rate is not too high to constantly be sterilising worlds.

Life needs a suitable planet in the habitable zone. Image: NASA.

The assumption of mediocrity

It’s at this stage that they make their first big assumption – one that they freely admit to. They assume the principle of mediocrity, which is an extension of Copernicanism, stating that the Earth is nothing special in the context of the Universe, and therefore the life that is on the Earth is also not unique.

Now it would be disingenuous to criticise this approach too much; after all, the principle of mediocrity is one of the unspoken foundations of SETI. That said, the Universe is the way it is, not the way we want it to be or even think it might be. The principle of mediocrity might simply be a delusion of the technological apes occupying the third planet around the Sun, in an effort to ward off the oppressive feeling of cosmic loneliness that would otherwise be true. The truth is, we don’t know whether Earth is special and life here is a fluke, or whether habitable, and indeed inhabited, planets are ten-a-penny. Westby and Conselice’s assumption – and, by extension, SETI’s assumption – might be wrong.

In the context of SETI, I think it’s a fair, if not necessarily correct, assumption to make. The problem is that although Westby and Conselice clearly clarify in their research paper that they have made this assumption, in the resulting media coverage it is not always apparent. What Westby and Conselice are really saying is, “if we assume that life on Earth is not a fluke, then we can expect at least 36 extraterrestrial civilisations/societies/species to be communicating in the Galaxy right now.” And that’s a huge assumption, because until we learn more about the origin of life, we cannot say either way, so bear that in mind when considering any calculations of this kind.

What we do know is that the Earth is 4.54 billion years old, and the earliest good indications of life on our planet are 3.8-billion-year-old stromatolites. There’s even more controversial microbial fossil evidence dating back 4.28 billion years, and it is quite possible that life originated on Earth even earlier than that.

Again, assuming the principle of mediocrity, which would imply that Earth developed life just as quickly as any other planet, we might then conclude that it takes about five billion years on average (rounding up from 4.54 billion) for ‘intelligent’, communicative life to evolve on a planet like Earth. Westby and Conselice use this deduction to introduce us to their Astrobiological Copernican Scenario, of which their is a ‘Strong’ version and a ‘Weak’ version.

Are Earth-like planets with life rare, or does the principle of mediocrity come into play? Image: NASA.

The aliens 17,000 light years away

In the Strong Astrobiological Copernican scenario, ‘intelligent’ life evolves on a planet within 4.5 and 5.5 billion years of that planet forming. On the other hand, the Weak Astrobiological Copernican scenario is one in which it takes longer than 5.5 billion years for ‘intelligent’ life to evolve, on average. 

At which point they set about calculating the number of stars in the Milky Way Galaxy that are about five billion years old, or older. According to our Galaxy’s star-forming history, the peak period of star production was about 10 billion years ago. If it takes five billion years on average for ‘intelligent’ life to evolve (there’s a reason I keep putting ‘intelligent’ in single quotes, which I’ll explain shortly) then most communicative species would have existed about five billion years ago, just when the cloud of molecular gas that produced the Sun was begin to swirl into action. In we’re late to the party: SETI might have been much easier five billion years ago.

Anyway, they plug the numbers for the various factors into their equation, and come out with a range of estimates, bracketed by the constraints of the Strong and Weak Astrobiological Copernican scenarios. If we assume the Strong scenario, then there are at least 36 ‘intelligent’ societies in our Galaxy that can currently be detected, and that if they are evenly spread across the Milky Way, then we would find the closest one to be 17,000 light years away, likely orbiting an M-dwarf star. These are the most common type of star, cool and long-lived, but they can be prone to bursts of radiation, especially when still young, which could potentially sterilise an orbiting planet. However, should a planet have a thick enough atmosphere and a strong enough magnetic field, it could ward off the worst of the radiation. The kicker is that if we continue with SETI at its current rate of progress, Westby and Conselice calculate that it would take up to 3,060 years before we find just one of these 36 communicating species (of course, if we’re extremely lucky, we could find them tomorrow, it’s just a matter of chance).

If we relax our assumptions and opt for the weak scenario, then things become a little better, with an estimated 928 communicating civilisations that we could currently potentially detect, with the nearest being 3,320 light years away, and a discovery time of up to 1,030 years. Increasing the value of L, which the lifetime of the communicating species (Westby and Conselice assumed an average value of 100 years, based on the fact that this is how long humans have been radiating electromagnetic signals into space), increases the number of civilisations and reduces both the distance and the discovery time.

Tom Westby and Christopher Conselice estimate that it could take 3,060 years to find ET if their Strong Astrobiological Copernican scenario is correct. Image: NRAO.

Intelligence without technology

This is all well and good, but I fear that calculations such as this ignore hugely important facets in the story of life. We’ve already covered the principle of mediocrity and the origin of life, but it’s worth reiterating: if life on Earth is a fluke occurrence, then life on Earth might be alone in the Universe, or at least life elsewhere may be extremely rare – far more rare than 36 extraterrestrial species – yet in their assumption of mediocrity, Westby and Conselice completely gloss over this. 

Another subtlety that they miss is how they are defining intelligence. SETI has this horrible tendency to interpret intelligence as meaning only technological intelligence. However, in The Contact Paradox I go to great lengths to show that, while homo sapiens might be the smartest species on our planet, we are not the only intelligent species on Earth. There are different kinds of intelligence, from non-technological dolphin intelligence, to the hive-minds of insects. The Universe may well be full of intelligence, but if it’s all dolphin-like intelligence, then with the best will in the world it’s not going to be building radio transmitters with flippers, so SETI will never find them. Intelligence does not just mean technology, and there are potentially many different kinds of intelligence. Hence why I’ve been using the single quotes.

Of course, SETI concerns itself only with technological intelligence, and Westby and Conselice have covered themselves by referring to communicative species, but by ignoring the spectrum of other types of intelligence, SETI is blinding itself to what might be out there, and Westby and Conselice’s calculations take no account of this.

Even if we’re just concerning ourselves with technological species, another assumption that Westby and Conselice is that they are all communicative. We’re not talking about TV and radio leakage, which would be far too weak to detect at 17,000 light years (see section 4 of this paper by Jim Benford and John Billingham), but deliberate transmissions. However, embarking on a dedicated programme of interstellar messaging introduces economic and societal concerns – would a species want to devote the necessary time and resources to constantly transmitting interstellar signals (as this paper from Jim, Greg and Dominic Benford shows, such messaging is expensive)? Would its culture even be interested in life elsewhere? We don’t know – as our experience on Earth shows, not everybody is interested in life beyond Earth. So there’s no guarantee that every technological species in the Galaxy is going to be communicative, but this is not reflected in Westby and Conselice’s results.

SETI is multi-disciplinary 

The point I’m getting that is that if we are to seriously consider the likelihood of technological, communicative life elsewhere, then we have to accept that it’s a multi-disciplinary project. I had hoped that the SETI community was starting to make progress in that direction, but perhaps not.

“With new and better data on our Galaxy’s star-formation history and a better knowledge of the characteristics of exoplanets, we can now make a solid attempt to answer the question of the likelihood of intelligent life elsewhere,” Westby and Conselice write in their paper. Also, “We make the assumptions that if a planet could potentially support life, then it will inevitably develop a CETI [communicating extraterrestrial intelligence] but no earlier than 5 Gyr [five billion years]. To determine this, we use the star-formation history of the Galaxy and the Initial Mass Function [i.e, the distribution of stellar masses when they are born].”

They are approaching the problem of SETI from a purely astrophysical standpoint, but a planet existing in the habitable zone of a star does not tell us how life originates, or whether intelligence or technological intelligence is a convergent (i.e. it evolves time and time again) or contingent (i.e. a one-off niche solution) property of evolution. And once that life has formed, we don’t know whether it will choose to think seriously about life beyond its planet, or try to communicate with that life. 

SETI isn’t just about astrophysics, and it’s a problem because it’s mostly astrophysicists who do SETI, and so they often only think about it in those terms. There are evolutionary, societal, economic and environmental factors at play too in terms of the development of a technologically-intelligent society that wants to communicate with the Universe. We have to consider them as important factors when trying to estimate how many communicative species are out there. We can’t just ignore them because they are difficult to attribute a number to in an equation.

SETI needs to bring in as many disciplines under its roof as possible, and get everyone working together, because each discipline has its own unique insight that, on its own might be too narrow, but when combined with other disciplines could form the foundations of a theory of extraterrestrial societies or civilisations. I’d love to see an analogue of the Drake Equation written by, for example, an evolutionary biologist or an anthropologist.

The long search

All that said, there are still many things that I like about Westby and Conselice’s research paper. The low number of 36 certainly emphasises how hard it is going to be for SETI to become successful, which will help lower our expectations, and lowering expectations in SETI can be a good thing. We’ve seen how people have grown frustrated that we haven’t found ET yet after sixty years, or how people assume that because we’ve not found anything yet, there must be no one out there. But we’ve really only searched a tiny portion of the Galaxy so far, and we need to be prepared for a long and difficult search. Westby and Conselice’s findings certainly provide context to that.

A schematic of our Galaxy. Are there regions within the Milky Way that form a Galactic Habitable Zone? Image: NASA/JPL–Caltech/R. Hurt.

I also like that they have identified the Galactic Habitable Zone as being important. There are some differences in opinion as to where the Galactic Habitable Zone is. The traditional view is that it’s out in the galactic suburbs, a band around the Galaxy ranging between 22,800 and 29,500 light years from the galactic centre (the Solar System, at 26,000 light years from the centre, falls nicely in the middle of this range), where the abundance of heavy elements is high, but the density of stars not too great that we’re likely to be caught in a supernova blast too often. An alternative view of the Galactic Habitable Zone is that nearer the galactic centre is better, because although there are lots of supernovae there, there are so many stars that some are sure to come through unscathed. The irony there would be that the Solar System would not even be in the Galactic Habitable Zone. Whatever the case, it’s good to see Westby and Conselice seriously accounting for it in their calculations.

They also recognise the importance of the L-factor, and how coming up with as accurate a number as possible for it can inform us about our own future too. In their calculations, they went with a lower limit of 100 years for L, because that’s roughly how long we humans have been spilling weak electromagnetic signals into space. Let’s hope L is much longer than that, both for increasing SETI’s chances and for our own existential future. If L is a 1,000 years, or 100,000 years, or a million years, it changes their calculation substantially.

Are there 36 communicating species in the Galaxy? I don’t know. Westby and Conselice have nailed the astrophysical factors, but there’s still so much else we have to understand before we can even begin to come up with a reasonably accurate estimate.

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