DHM wrote:
Actually it's looking more special than it ever did. We are coming to the conclusion that the creation of "life" only happened once. All life existing now or that we can track through fossil records is basically the same biologically. There are common components that can be found in all forms of life - even virus. The conclusions that can be drawn from that are that either only one form of life survived or can survive on Earth or that only one life creating event actually happened in the entire history of the planet.
Or we are the end result of the interactions between myriad life creating events. Or, or, or ...
As for what scientist mean by Earth type planets, the criteria that is thought to be of critical importance is liquid water. Temperature, atmosphere, Gravity etc all become factors.
I think the above is completely unscientific. We don't even know how life started on this planet and yet somehow it's possible to rule out it beginning in ALL the possible permutations of environments other than our own and those similar? No chance.
I mean, there's strong possibility we may find fundamentally different lifeforms on one or more of our near neighbours in the solar system. Granted, the chances of sentience are slim. But here again - we aren't even sure what "sentience" is - so it's a bit rich to make definitive claims about it.
I think it's highly unlikely that life as we know it does not exist on other planets, but there is a chance we could be alone and the biggest fluke in the universe. If you think like that then the way we behave and the value we place in each other is a disgrace.
Look on the bright side. Plenty of room for expansion.