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Comment by ceejayoz | Hacker News Reader

This is why the term "habitable zone" can be a bit laughable.

So far as has been observed, life does require liquid water. Some life can cease activity when dry and then restart when water resumes, but no life has been found that can live in an entirely dry environment. If this proves to be a requirement for life, that does validate the notion of a habitable zone.

The liquid water requirement tells us a life-bearing planet can't be too hot, but finding life in the deepest parts of the ocean suggests that the water may not need to be on the surface. The nutrition raining down from the surface ultimately links back to photosynthesis, though, so it's not entirely convincing evidence.

The "habitable zone" concept is still useful when it comes to actually searching for signs of life. I'm guessing a planet with organic life on the surface would have stronger signatures than one with a thriving ocean under 500 km of solid rock and ice.

> The nutrition raining down from the surface ultimately links back to photosynthesis

This is a key point. The trench lives on the spillover from the surface. If there was no habitable surface full of life, there would be no life in the trench.

Have a read of http://en.wikipedia.org/wiki/Whale_fall - the deep ocean floor may host ecosystems, but this does not mean that planets where the best conditions are like the deep ocean floor are "habitable".

Similarly, the presence of mosses and lichens in Antarctic dry valleys is interesting, but does not prove beyond doubt that such organisms could evolve if the best conditions were like that.

> So far as has been observed, life does require liquid water.

That's a bit like a newborn baby saying "so far as has been observed, there are only a couple people in existence".

That strikes me as a reasonable assumption to hold, pending evidence to the contrary.

A baby doesn't have evidence that there's stuff outside his/her perceptions. We do - knowing that we keep finding extremophiles in hot springs and underground rocks and water in unexpected places like Mars and Europa, we should be a lot more careful than a baby when we say "this is how it is".

But then again, this is an universal counterargument and can be used to dismiss any kind of selection as hubris. We need to act to the best the information available allows us and learn from mistakes.

Thus is the nature of knowledge and of discovery? This is why the Scientific Method was invented, correct?

Part of the scientific method is knowing when to say "we don't know yet". We don't know how to define "habitable" yet, so we should stop saying it.

You quoted "So far as has been observed". How much more hedging do you need? Basically you're arguing for something useless; that no definition be made until there is 100% certainty, regardless of caveats. Such a high bar gives us no utility.

But liquid water can be found in as far away places as Saturn's moon Enceladus and Jupiter's moon Europa.

This particular discovery doesn't back that assertion up very much, as it's an ocean floor biome that is dependent on nutrients from the surface, but there are deep ocean environments which are not. We know that it's possible for food chains supporting even complex multi-cellular life to be founded on chemosynthetic organisms subsisting on hydrothermal sulfide emissions or serpentinization reactions. Those sorts of conditions are likely to be fairly common throughout the Universe on a wide variety of bodies. Whether or not life can be bootstrapped and evolve into complex varieties within the constraints of such environments remains to be seen, but it's a tantalizing possibility.

Indeed, we find life practically everywhere on Earth and there's no telling what kinds of adaptations alien life might have to all sorts of seemingly inhospitable environments.

Preexisting life being able to adapt to inhospitable environments does not also mean that they can develop there. We know very little about under what conditions abiogenesis can occur.

>We know very little about under what conditions abiogenesis can occur.

I think that's ceejayoz's point though. The less we know about something, the less confident we should be about declaring limits to when that something can occur.

In response to this, I will offer the suggestion that the life forms that live in the extreme climates lived in less extreme climates first. It is possible that some creatures started in "hard mode", but most likely, they progressively adapted to the extreme environments from more palatable climates.

Hard is relative. For example, the filling of the atmosphere with a powerful oxidizer was a major extinction event (the "Oxygen Catastrophe"), yet we do not find it extreme because we are adapted to it.

Even environments that are energy-poor, or cold enough as to inhibit chemical reactions, may support rich forms of life that operate on larger time scales.

Everywhere on Earth is still within the habitable zone of the solar system.

The "habitable zone" isn't so well defined, though. Europa's believed to have an ocean of liquid water and heat from tidal forces. Mars may well have liquid water at times, and who knows what we'd find miles down underground - we've found all sorts of extremophile life in mines we never expected to.

Well Mars is inside the traditionally defined habitable zone (which extends halfway into the asteroid belt), but Europa is a point. More generally one can imagine e.g. larger rocky planets whose active core keeps them warm, rather than just their sun.

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