When I was a child there were no extra-solar planets known about. People assumed they may exist but we had no evidence that they did. This allowed us to continue to believe that there may be something special about our solar system. Indeed I remember arguments of the form that there may be no other solar systems, and this was an argument for the Earth as a special case in a special solar system.
What boggles is me is that the since the first planet was confirmed in 1992 is the speed with which this argument has been effortly demolished!
There are now over 464 confirmed extrasolar planets according to Wikipedia, and more possibles. This boggles my mind. What is amazing is the variety that we are able to determine.
In December 2009 GJ 1214 b was confirmed as “the first of a new class of planets with small size and relatively low density”, which could be a large rocky planet and seems likely to have water. A very significant fact as water means that life would be possible. The presence of liquid water remains one of the special features of Earth. Of course at the moment we don’t have direct evidence of water yet, but it looks likely.
Given the speed at which different types of planets are being found and in different ways, including by visual observation (with the Hubbles space telescope).
This is a real revolution in our knowledge about our place in the universe since 1992, but it has gone by relatively quietly. The chances of our solar system being unique before the first planets was real, now it seems unlikely in the extreme.
It struck me that the appeal to exceptionality was a really appealing argument intuitively, it feels like a strong argument but has a big problem. If the exceptionality is lost the argument falls. However it was always a weak argument: This has convenient special properties does not mean there are reasons for the special properties.
The fact that “why is our planet capable of intelligent life” can only be asked by creatures living on planets capable of intelligent life, given the scale of our galaxy, and the number of planets found in a tiny fraction of it, it seems highly like that there is another planet with people making similar arguments to this. Better arguments are needed.