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The argument seems to stand on the greater risk to life on a planet with relative axial rotation. So one without is more likely to have "sweet spots."

As an earthling this is probably the first thing you think about when you read about Mars - too hot for life in the day, too cold for life at night. But that makes way too many assumptions relative to _human_ life. It assumes nothing could be rigid enough to adapt to day and night extremes greater than Earth's.

It seems to me the greatest challenge to thinking about potential life outside this planet is being mentally bound to the constraints of life on this planet.

>It seems to me the greatest challenge to thinking about potential life outside this planet is being mentally bound to the constraints of life on this planet.

I always felt the same way about astronomers linking liquid water with life but then again where do you start if you don't use the only known instances of life in the universe as a template? Maybe it happens that there's a viable evolutionary path for sentient Nitrogen clouds but how would we know that?

Furthermore it doesn't sound too absurd that the very complex chemical constructs necessary for life would have a greater chance to stabilize in less extreme environments with a smaller temperature amplitude. Especially if you're looking for complex life and not merely microbes (which tend to be a lot more resilient).

The rationale is because carbon chemistry is the richest chemistry out there. Other elements are not nearly as flexible to sustain complex molecules while maintaining the possibility of reversability in chemical reactions. Thats actually a very sound argument.

I always thought that most likely place to have life in solar system (except for Earth) is Venus. Also hardest to discover for us.

Most likely because looking at extremophiles on Earth, you just need somewhat stable energy gradient and you'll find some life that draws energy from that gradient using weird chemistry.

Hardest to find for us because most of our chemistry research is geared towards carbon compounds around 20deg C, not sulfur compounds around 500deg C that dissolve most of our equipment in minutes.

liquid water acts is a catalyst for a large number of chemical reactions. life on earth is complex in a way that only carbon base molecules with liquid water can support. Maybe life can exist where average temperatures are 90C (but this in itself changes the possible chemical reactions)

Life on this planet doesn't seem all that constrained. There is life in the deepest spots in the ocean and in the highest reaches of the atmosphere and everywhere in between.

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!

"It seems to me the greatest challenge to thinking about potential life outside this planet is being mentally bound to the constraints of life on this planet."

I'm having a hard time finding all the people stodgily insisting that only life exactly like Earth's is possible through the hordes of people screaming about how it might not be. Everybody already knows that life might not be exactly like Earth life. Any illusions to the contrary have been shattered by Earth life itself and the concrete existence of extremophiles, which are themselves already not what most people imagined "Earth life" to be.

I'll make that claim. Even extremophiles are very earth like life forms: the have similar chemistry behind them.

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