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I think the point is that when you move to an interstellar scale, the "extremes" on Earth are actually a very, very narrow band of possible environments. Even just within our solar system the differences in temperature alone are many orders of magnitude beyond those found on Earth. That's ignoring the differences in gravity, pressure, chemical makeup, etc.

Disregarding the possibility of life elsewhere that couldn't possible survive on Earth discounts most of the planets we know about.

There are some relatively clear 'bands' where we expect life to be possible. Anywhere from -273 right up to 1,000 degrees Celcius. That's a 'narrow range' by the range available within the solar system but lower you won't go and higher has some interesting problems associated with stability of the vast majority of materials that could be your building blocks. Gravity and pressure are less of a consideration though those would 'shape' life much as life on earth has adapted to the pressure gradients available. Gravity is even less of a concern. Chemical makeup is important, it determines the available building blocks which is one reason we are concentrating on second generation stars because they have enough complex building materials lying around. First generation star systems are chemically too simple to support life as we can imagine it.

So if survival of those life-forms depends on gravity, pressure or chemistry then we definitely should not rule out places where those are different than on earth, in fact that is to be expected. But temperature is a very important factor and the make-up of the star itself is also very important.

So it makes sense to check the likely places first before spending time and effort on much more unlikely places.

I was going to comment that I thought 100C was the upper bound for life that relies on water, since any higher than that and it boils (although in retrospect that only happens at standard pressure). But I googled a bit and found a BBC article on the Uzon caldera in Siberia [1], which hosts microbes that can thrive up to 122C! (1000C is still far out of reach, though).

Some of these organisms also have novel ways of acquiring energy, so it seems like research like this is probably our best near bet for understanding how life can thrive in extreme conditions.

[1] http://www.bbc.com/earth/story/20160209-this-is-how-to-survi...

I would suggest that something like a virus can probably "survive" far higher temperatures (though to my knowledge none that do are isolated atm) but they would require leaving that environment (for example via the water steam) to replicate.

I'd put the lower bound for life at closer to -100C, as you go below 0C there just isn't enough energy available for life.

It won't be fast, that's for sure!

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