We know now that there are planets out there.
Lots and lots of planets.
We are still pinning down the exact incidence over all stellar populations, and we are barely at the point where we can directly confirm the presence of terrestrial planets, but if parameter space is smooth and the universe does not conspire against us, then terrestrial planets must be quite common.
10% incidence would not be a bad conservative guess, but I would not be surprised if the incidence is 30-50%.
We will know for sure soon.
We don’t know how life starts.
We have some well founded suspicions, and every year the gap in the evidence shrinks as we tighten our knowledge of the transition from complex self-catalytic chemical systems to minimal living systems.
It is, still, conceivable that life is a near unique contingent event, but that is looking unlikely.
It would not be unreasonable to infer that life will appear, and quite rapidly, wherever suitable mild and stable conditions persist.
We do understand quite well know how evolution progresses, and I use that word with some deliberation.
Evolution is an emergent property of self-replicating systems, and will emerge in any such system where information is duplicated and transmitted.
Selection also occurs, and there is, in a broad range of circumstances, selection that favours the ability to efficiently process information, lots of information and fast.
So, there ought to be in many circumstances selection pressure to evolve intelligence on suitably long time scales.
We don’t know the characteristic time scale for the evolution of intelligence comparable to ours, the one data point we have is inadequate, though one may infer that there was independent evolution of not insignificant progress towards intelligence in multi-cellular animals on Earth.
Some steps in the progress to get to that point may be low probability choke points, though less so as we learn more.
eg. multi-cellular life could, in principle, be a major hurdle, but current research suggests it comes about relatively easily.
There is still a lot of contingency in the emergence of intelligent life – both astronomical, geological and internal fail points.
The Earth could have been sterilized repeatedly: slightly larger asteroidal impactors; deeper snowball phases; self-pollution.
We also don’t know if our particular implementation of life is uniquely possible, or if there are many systems which could implement life. The latter is looking more likely as we understand more.
So, where are they?
The Fermi Paradox has been hashed out many times, and we need data, the speculation is probably mostly complete.
Even if intelligent life is rare, there are a lot of stellar systems out there; some 100 billion or so in our galaxy, and there are trillions of galaxies.
It seems unlikely life on Earth is unique, and it seems plausible that somewhere life achieved intelligence, and technology, and that technological capabilities can expand to physical limits.
We are at the technological point where we ought to be able to see sufficiently advanced civilizations. Say anything at Kardashev level 1.7 or higher.
If they are reasonably abundant.
At roughly incidence of 10-4 we ought to be able to see our neighbours.
Unless they are very good at hiding, and have a reason to hide.
I don’t know how we will see them, but it will probably not be the way we now think we will, but we are pretty good at looking.
As we get better at looking, we can squeeze both the incidence rate of arbitrarily advanced civilizations, and their level of advancement, crudely measured in terms of level of power under their control.
If they are not there, we need to seriously think about why that might be.
If they are there, we have a whole lot of new thinking to do.