In science it’s often helpful to ask about extreme cases or limits to phenomena. For example, you’ve got an estimate of the masses of planets orbiting another star, but just how big could these worlds be without precipitating dynamical chaos? Or you’ve found a new hardy organism that seems to be playing some fancy biochemical games to resist extreme environments. Just how far can it go?

These kinds of test are also really helpful in the absence of detailed knowledge, when we’re feeling our way in the dark. 

The question of life in the universe pretty much falls into that category. Here too, the limits can be useful. For example, a couple of years ago Adam Frank and Woodruff Sullivan wrote a provocative little paper called ‘A New Empirical Constraint on the Prevalence of Technological Species in the Universe’. In this work the authors point out that since we have constraints on how many stars and planets there are in the observable universe we can place a lower bound on the probability of a world producing a ‘biotechnical’ species. How do we do this? Well, we basically assume the worst-case scenario where we are the only such species to have occurred in the observable universe.

According to Frank & Sullivan that yields a lower bound probability of about 2.5 x 10-24 for a planet in the nominal habitable zone of its star to ever evolve such a species. As they’re quick to point out, this is the most pessimistic value possible – but the neat thing is that it is a number we can now write down. To put this another way. If we ever come up with a first principles theory of life and intelligence and that theory yields a probability less than this value, we’ve probably got the theory wrong.

Maybe there is more that we can do with this kind of ‘just how bad can it be?’ approach. Take another factor in the quest to find technological life in the cosmos. Let’s suppose that the farthest we’ve ever gone towards interstellar space – currently about 16.25 light hours (Voyager 1) – is actually about as good as it ever gets for any species. 

What would that mean? Well, this could mean that wide binary star systems – where each star has some planets orbiting it – could become occupied if biotechnical life emerges around one or the other star. In places like the core of globular clusters, where star-star separations can be as little as perhaps 30-40 light hours, there’s a slim chance of proper star-hopping. The core of the Milky Way has stars pretty closely packed, but typically still separated by light-weeks. And there might be other problems for life down there – such as occasional outbursts of energy from the central supermassive black hole or supernovae explosions. 

Of course, stars move around, so opportunities to hop across to other planetary systems might exist. But if you have to wait for a star to come within a light-day of you, chances are that the presence of that mass will do some very nasty things to the long-term stability of your planetary system.

In other words, in this particular worst-case scenario, there’s not going to be interstellar dispersal of species. You could slot in a number of other barriers here too – from the challenges of sustaining organisms during long interstellar flight to just keeping machines functioning. 

The option perhaps remains for Breakthrough Starshot-like nanocraft to be pushed to other stars with big laser propulsion systems. But such exploration might never lead to transferal of a species (unless you send an awful lot of tiny machines and start printing out cellular life at the other end).

What this worst possible case does suggest is that SETI should be targeting star systems where there’re a lot of intrinsic resources: multiple temperate worlds, lots of nice asteroid resources, long-lived stellar parents, and perhaps widely-spaced (but not too far apart) binary star systems where there are options for sitting out existential challenges in your neighbor’s backyard.

Another question is to do with the very origins of life itself. The basic building blocks of life occur in abundance around the cosmos. The geochemical mix that seemingly primes an environment for the first chemical networks and anabolic metabolisms doesn’t appear to be particularly special. Any freshly formed rocky planet with some water is going to have this stuff, at least for a while.

The worst-case question to ask is just how bad do things have to be for the first steps towards life to not happen at all? Instead of asking how life starts, we might learn from asking how life can be best prevented in young planetary systems.

And on a grander scale, just how useless does the universe have to be to not generate more than one example of a planet with thinking life? This is of course a domain where the idea of an anthropic principle, and the fine-tuning of physics come to play. I don’t want to go into that in detail; except to say that there seems to be an implicit assumption in many such lines of reasoning that living systems are acutely dependent on a ‘just so’ set of physical constants and cosmic properties. In other words; the universe has to be perfect for life or else life won’t happen.

However, I don’t think we know that to be true at all. This could be a lousy universe for life. Others, like the physicist and astronomer Fred Adams have thought about this, with intriguing results. 

Indeed, we might learn much more by asking what the worst universe for life (where life nonetheless manages to occur at least once) would be like.