We know the answers now, and they are pretty encouraging. There's around one new star annually, most stars have planets, and about one star in five has a planet with liquid water on the surface. That means that around a hundred billion planets in this galaxy alone can support life, but that's just a start ...
There are around a hundred billion galaxies in the universe, so the total number of potentially life-supporting planets potentially able to support life is about 10,000,000,000,000,000,000,000 (10 billion trillion).
What Adam Frank has done, in his recent book Light of the Stars: Alien Worlds and the Fate of the Earth, is to point out that there must therefore have been a lot of "exo-civilisations".
Make your assumptions about first life and then intelligence emerging on any given planet as pessimistic as you like, and there will still be a lot.
What Frank really wants to know is how many of those civilisations made it through the bottleneck - and for that he doesn't need to know anything specific about those unknown exo-civilisations.
He only needs to know that all civilisations use large amounts of energy, and that there is a strictly limited number of ways that a young technological civilisation like ours can access energy.
There will be fossil fuels on some planets, but not on others. There's hydro, wind and tides. There's solar, geothermal and nuclear. That's it. Using energy always produces waste, but some of these modes produce far less heat, carbon-dioxide, and toxic chemicals than others.
So put different original mixes of these energy sources into your experimental models, put in different planetary conditions as well, and run a few thousand of these models through your computer. It turns out that most of the models see runaway population growth, followed at a distance by growing pressures on the planet's environment that lowers the "population-carrying capacity".
At some point the alarmed population switches to lower-impact energy sources. There is still a steep die-back (up to 70 per cent) in the population, but then a steady state emerges and the civilisation survives.
In other models, the planet's people (creatures? beings?) delay switching the energy sources for too long, and the late movers don't make it. The population starts to fall, then appears to stabilise for a while, then rushes downward to extinction.
So where does our own planetary civilisation fall on this spectrum of possible behaviours? I don't know, but this just in - oil production is at an all-time high of 100 million barrels a day, and the Organisation of Petroleum-Exporting Countries has just predicted that it will reach 112 mbd in the next 20 years. That's definitely the wrong direction.
