Quote:
Originally Posted by photon
That depends, it's complex, but is there reason to think that the situation is different elsewhere?
We do have a larger data set, we know the size of the universe and we know the properties of other galaxies and how it compares to our own, and the properties of other regions of our own galaxy and how that compares to our own region.
I appreciate your point from a philosophical point of view; we can't say there is an earth like planet out there somewhere until we actually find one, but it would be surprising if there weren't given what we know so far. If we are the only one there would likely be additional things that are known that would account for the seeming defiance of things like the cosmological principle.
We lack the hard data of all the planets in the universe, but we can make a prediction based on other known things.
If I find a hoof print in my yard, it could be a horse, a zebra, or a unicorn, and I can reasonably guess what it probably is. There is no certainty, but I don't really need it if I'm willing to change my mind when new info comes to light.
The information going into the probably is far more than a couple of quick observations in the case of earth like planets.
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I think the bigger problem with the penny example is natural vs. unnatural. With a penny you know somebody must have left it there and then another close by, but that doesn't give you much to work with.
OTOH if I walked out my door and found a dandelion, then walked twenty feet and found another, I would feel more justified in extrapolating. Yes conditions may vary for the viability of dandelions in different locales, but I can be relatively certain that these things developed in situ and given reasonably similar conditions could extrapolate that the conditions lend themselves to the development of dandelions. I recognize that there are weaknesses with my analogy too (seeds, for one) but I think the penny thing is a red herring unless you're a creationist. (By which I mean that a creationist might see the existence of two habitable worlds the same way I see the existence of two pennies a few feet apart, evidence that someone put them both there. As an interesting aside, what if the atmospheric makeup, size and other characteristics of Gliese were sufficiently similar to Earth, would that be evidence for Intelligent Design?)