NASA Refines Criteria for Life Signs on Exoplanets

NASA scientists have been running a large number of computer simulations of possible exoplanet atmospheres. They now conclude that the detection of any one atmospheric gas is inadequate to establish the presence of life on a planet. Such gases may arise due to non-living processes. Combinations of certain gases, such as oxygen and methane will be necessary to be certain that extraterrestrial life has been discovered. Details are given in the article, linked below:
phys.org/news/2014-09-nasa-guideline-future-alien-life.html

a reply to: Ross 54

Problem with this is, that they are still assuming that life needs to be carbon dependent lifeforms.

If we assume that oxygen+carbohydrates = carbondioxide + water. is the key to life on earth. This only explains how several "life" species can support each other. it does not explain consciousness, nor life itself. And their research, only explores the possibility of another planet to support our life. But not weather life exists there.

What if a different microbial lifeform exists, that is not part of the oxygen+carbon cycle. And our contact with it, would kill us ...

Agreed, once again it all boils down to life as we know it. With a universe as vast (or possibly infinite) such as ours the possibilities for different types of life are endless, there could be life out there which is so different to what we know that we wouldn't even recognise it as life.

We understand our own sort of life well enough to search for it on other likely planets. That does not appear to be the case with alternate biochemistries. There are speculations about life using different chemicals and producing other chemical signs of its existence, on planets very different from our own.
Supposing that this actually occurs, how would we distinguish biologically produced hydrogen and ammonia, for example, from these same gases produced by non-living processes? I'm not suggesting that this will ultimately be an impossible task, merely doubting that we currently have sufficient knowledge to do so.
If anyone can suggest similar detection criteria to what NASA has now produced for life as we know it, but for alternate biochemistries, I would be very interested in hearing of it, and I'm sure others would be, too.

originally posted by: longy9999
Agreed, once again it all boils down to life as we know it. With a universe as vast (or possibly infinite) such as ours the possibilities for different types of life are endless, there could be life out there which is so different to what we know that we wouldn't even recognise it as life.

And this is a bit of a conundrum because, as you point out, "life as we don't know it" would be difficult to find. We can look for the markers and telltale evidence that "life as we know it" can display, and create tests that can detect that life; HOWEVER, if we don't know what markers and telltales that "life as we DON'T know it" displays, then that makes it difficult to find that life.

The problem is this: how do we create the tests to look for "life as we don't know it" if we don't even know what to test for?

originally posted by: Ross 54
NASA scientists have been running a large number of computer simulations of possible exoplanet atmospheres. They now conclude that the detection of any one atmospheric gas is inadequate to establish the presence of life on a planet. Such gases may arise due to non-living processes. Combinations of certain gases, such as oxygen and methane will be necessary to be certain that extraterrestrial life has been discovered. Details are given in the article, linked below:
phys.org/news/2014-09-nasa-guideline-future-alien-life.html

Yes, it's the same as one of my answers on ATS about how we could detect alien life on an exoplanet. Just finding free oxygen isn't going to be that convincing but if the telescope is powerful and sophisticated enough we'd get detailed spectra which would be able to show a good amount of other gasses in an planet's atmosphere.

Oxygen + Methane would be a dead giveaway for our type of life currently but for a good portion of Earth's history (over half) Earth didn't have much oxygen at all yet life was here in the form of microbial life. Learning to detect the life on a nearby planet which has a composition like our ancient Earth is very important to detecting life in the galaxy and was the subject of this discussion:

originally posted by: longy9999
Agreed, once again it all boils down to life as we know it. With a universe as vast (or possibly infinite) such as ours the possibilities for different types of life are endless, there could be life out there which is so different to what we know that we wouldn't even recognise it as life.

While true, we expect there to be plenty of life out there which we would recognize because 1) Carbon based organic molecules (ie amino acids) are fairly common in the universe and have been detected at interstellar distances already. 2) We know what to look for to find ourselves and our type of life. The same could not be said of "life as we don't know it".

So look at this as looking for the low-hanging fruit of life like ours in the galaxy before approaching the more difficult search for stuff we are not even sure exists.

We know we exist and know how to find life like our Earth's past, present and perhaps future life. The same can not be said for "life as we don't know it" and there is nothing which would indicate that other types of life are in fact possible anyway.

Water, carbon, hydrogen, nitrogen, oxygen just works and are among the most common molecules in the universe.

Nature tends to use what is commonly available. It's hard to find anything which could be capable of producing complex molecules that is as common as those elements. Silicon while common doesn't seem an easy way to the complexity of life.

a reply to: JadeStar

Very well said, Jade Star.

Carbon can form a virtually infinite number of different chemical bonds. This makes it the most versatile atomic basis for life. The next most versatile atom is silicon, which can form 9 different chemical bonds.

It may be that the universe simply is not old enough for silicon-based chemistry (or some other process) to have stumbled across a molecule that can self-replicate.

At any rate, you are correct. Just because we can't search for everything, it doesn't mean we shouldn't search for anything.