NASA scientist creates expanded list of habitability possibilities for other worlds

June 10, 2014 by Bob Yirka, report
This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star. Credit: NASA/Ames/JPL-Caltech

( —A NASA scientist based at Ames Research Center has compiled a checklist of habitability possibilities for planets or other bodies in the solar system or beyond. In his paper published in Proceedings of the National Academy of Sciences, Christopher McKay outlines ways that life could be possible on other planets, moons, or even other bodies.

Research here on Earth, McKay notes, has led to findings that show that life can exist under what would previously have been considered impossible environmental conditions. Thus, it seems logical that what we define as the conditions possible for life existing in other places should expand as well. Not all life forms need the same requirements as most of the life we see around us, some can survive or even thrive in very .

He notes that some types of microorganisms, for example, have been found to live in environments that are consistently well below freezing or well above the . Thus, it would not make sense to rule out a planet simply because it's too cold or too hot.

He also notes that not all require as much water as was once thought. Some algae, for instance has been found living inside of rocks, where very, very little water is available. Not unlike the water that is trapped in rocks on the moon, as just one example.

The need for light or some other form of energy source might have been overstated as well. Creatures have been found living in the sea, for example, at depths almost beyond where sunlight can penetrate. Might that mean that some have been wrongly excluded as possible life holders, simply because they are too far away from their star? No one knows, but perhaps we should start including more of them on our list of possibilities.

There's also the problem of radiation—too much of course and life should not be able to survive—but what about those microbes that have been found living inside of nuclear reactors? Perhaps we've been too narrowly focused in this respect as well.

And finally, being creatures that need a lot of oxygen to survive, it would seem only natural that we would expect other habitable worlds to have it as well. But research has shown that it too isn't always necessary and is sometimes even fatal to some forms of life, such as a type of bacteria living in soil. Nitrogen, on the other hand seems to be far more critical. Perhaps it should be one of our primary clues.

In short, McKay is reminding us that we maybe ought to be more careful in what we exclude when looking for life elsewhere, perhaps now more so than before as our technology improves to the point where we might finally have what we need to actually prove that does exist out there, somewhere—even if it's in a form we never might have imagined was possible before.

Explore further: Life on other planets could be far more widespread, study finds

More information: Requirements and limits for life in the context of exoplanets, Christopher P. McKay, PNAS, DOI: 10.1073/pnas.1304212111

The requirements for life on Earth, its elemental composition, and its environmental limits provide a way to assess the habitability of exoplanets. Temperature is key both because of its influence on liquid water and because it can be directly estimated from orbital and climate models of exoplanetary systems. Life can grow and reproduce at temperatures as low as −15 °C, and as high as 122 °C. Studies of life in extreme deserts show that on a dry world, even a small amount of rain, fog, snow, and even atmospheric humidity can be adequate for photosynthetic production producing a small but detectable microbial community. Life is able to use light at levels less than 10−5 of the solar flux at Earth. UV or ionizing radiation can be tolerated by many microorganisms at very high levels and is unlikely to be life limiting on an exoplanet. Biologically available nitrogen may limit habitability. Levels of O2 over a few percent on an exoplanet would be consistent with the presence of multicellular organisms and high levels of O2 on Earth-like worlds indicate oxygenic photosynthesis. Other factors such as pH and salinity are likely to vary and not limit life over an entire planet or moon.

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5 / 5 (10) Jun 10, 2014
Finally someone that puts it in a paper. I hope that all this 'habitable zone' nonsense will come to an end and we keep an open mind (and an open eye) for lif ein places we have currently dismissed without bothering to check.
4 / 5 (3) Jun 10, 2014
I think that if we find a place that is habitable for life, but find none, we should put it there!

Think of it as a hedge for DNA.
4 / 5 (3) Jun 10, 2014
The habitable zone is not "nonsense" but an established tool to more economically and faster filter out planets that should be looked at first for inhabitation or not. (Given observational constraints, of course.)

There are known cases of putative biosphere types that won't fit within the various forms of current HZs (surface, tidal and crustal foremost), such as ejected icy nomads, Pluto & Ceres. So that criticism isn't well directed. Recently, HZs have been suggested as "too optimistic", but that is a filter tuning & public relations problem. (McKay's counter here amounts to that they are "too pessimistic".)

McKay's suggestion on the other hand goes against the distinction between biospheres and environments conducive for life emergence. Those are _known_ to be different, e.g. life arose on a different Earth than the current rather bioproductive one. There will be planets that are habitable for some forms of life, yet uninhabited.
not rated yet Jun 10, 2014
And finally, being creatures that need a lot of oxygen to survive, it would seem only natural that we would expect other habitable worlds to have it as well.

It is only natural to have that expectation if you are ignorant and arrogant enough to think that all other life in the universe must be like us with the same requirements for life.

The moment that we close the door on a possibility is the moment that we lose that possibility to our blindness.
not rated yet Jun 10, 2014
The interesting exoplanets are not the "habitable" ones in general, the interesting ones are the habitable planets where live could be detected by us someday. If we find an exo-moon similar to Europa in a far away system we could never confirm if it has live or not in a theoretical subsurface ocean, (or at least not before other live easier to detect had been detected long time before that one, as to make that moon almost irrelevant ) so putting the "habitable" label to it just because "live is not impossible there" does not mean anything. Many types of live could exist, but the interesting "habitable" category for us is in the kind of live that could produce detectable signs in the surface or atmosphere of a planet. Or better said, "live with potential to become the first detection when technology permit it".
5 / 5 (2) Jun 10, 2014
And you STILL haven't proven life exist on any one of them. We can play these academic games all day with defining the "zones" but the reality is you need some fracking data.
5 / 5 (2) Jun 10, 2014
Personally I think when we finally get technologically capable of actually getting to some other stars and planets we are going to find life based on all kinds of stuff... carbon, silicon, metals, etc... It's a big universe out there and thinking that all like will be like up is pure hubris on our part.
1 / 5 (1) Jun 11, 2014
And you STILL haven't proven life exist on any one of them. We can play these academic games all day with defining the "zones" but the reality is you need some fracking data.

Don't have to prove nothing. Don't have a space ship so this is the best you get.
not rated yet Jun 13, 2014
I think that if we find a place that is habitable for life, but find none, we should put it there!
I have long thought the two main jobs for humankind (if we even need a purpose) are to propagate life to other worlds, and to acquire and codify of knowledge, that being the only true currency we can pass to our descendants. As unlikely as it is that we are the only life in the Universe, we should act as though we are until we learn differently, and the eventual development and distribution of various simple microbial life to other worlds and other star systems seems like a pretty good service to provide for the Universe. Not quite yet, though.

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