Oxygen, Methane may be 'false positives' for Life on Exoplanets, Say Scientists

The apparent spectroscopic detection of a combination of oxygen and methane on an exoplanet planet may not be indicative of life, as has been supposed, three scientists say. They aver that a sterile world could have oxygen alone and its moon methane alone, and that the two could not be distinguished, leading to a false indication of life.
Either gas alone is apparently not considered sufficient evidence for life. Interesting, but I wonder if it's probable that an Earth sized planet in its star's habitable zone would have a moon with enough gravity to retain a substantial atmosphere. Even Earth's exceptionally large Moon doesn't. LInk to article on this, below:
phys.org...

originally posted by: Ross 54
The apparent spectroscopic detection of a combination of oxygen and methane on an exoplanet planet may not be indicative of life, as has been supposed, three scientists say. They aver that a sterile world could have oxygen alone and its moon methane alone, and that the two could not be distinguished, leading to a false indication of life.
Either gas alone is apparently not considered sufficient evidence for life. Interesting but I wonder if it's probable that an Earth sized planet in its star's habitable zone would have a moon with enough gravity to retain a substantial atmosphere. Even Earth's exceptionally large Moon doesn't.
phys.org...

They totally misunderstood this paper.

That's not what it says at all.

What it says basically is that we can't be sure that the oxygen or methane is coming from the planet or coming from an exomoon with current technology.

The false positive is that such a biosignature would be coming from the planet's moon rather than the planet. The oxygen biosignature would still be there. We just wouldn't be able to distinguish whether it came from the planet or the planet's moon.

Finding both Oxygen and Methane on a world would be exciting. What this paper tells us is that finding both doesn't mean they both come from the planet. One could come from the planet, the other from the moon.

However, what is not said in the article is that just finding the signature of oxygen in the first place would also be exciting.

Because there is nothing which replenishes oxygen in the atmosphere of an exoplanet which we know of other than life. Oxygen likes to bond with other elements so finding a lot of O2 (Oxygen) and/or O3 (Ozone) in spectrum like this will still tell us a planet probably has photosynthetic life (plants):



Methane (CH4) would be the result of bacterial and animal life if seen in combination with the Oxygen so finding both would be more exciting but finding a world with just Oxygen but not sure whether the Methane is from a moon would also be exciting.

If we find a world with a bunch of free oxygen in its spectra it's probably due to life.

Whether that spectra is a result of the planet or a habitable moon doesn't matter other than to us scientist who want to know where what we're looking at came from.

By the way, all of this gets easier with planets around lower mass (K and M) stars than our Sun (a G star). So what this says is that with current technology we should not limit ourselves to just looking for biosignatures on Earthlike planets around stars like our Sun but should focus on Red Dwarfs (M stars) and Orange (K) stars.

The idea advanced seems to be that either oxygen or methane can exist in chemical equilibrium in the absence of the other, and does not need to be replenished by life processes. Links, below, to similar remarks from Scientific American. Also, a suggestion of how an oxygen atmosphere may not be due to biology, from Science News:
blogs.scientificamerican.com...
www.sciencenews.org...

originally posted by: Ross 54
The idea advanced seems to be that either oxygen or methane can exist in chemical equilibrium in the absence of the other, and does not need to be replenished by life processes. Links, below, to similar remarks from Scientific American. Also, a suggestion of how an oxygen atmosphere may not be due to biology, from Science News:
blogs.scientificamerican.com...
www.sciencenews.org...

FIrst link is broken.

Second link is misunderstanding the fact that a planet whose ocean is being broken down by ultraviolet light would have a much different spectra than the one I posted above.

It would be a lot smoother with only a week O3 band or two and a strong H2O band present. Kinda like Mars's Ozone but with an H2O band:



That looks quite different than what we'd expect to see from a living breathing exoplanet.

It's good that there are people thinking about false positives because before anyone goes shouting "Aliens!" we want to be VERY sure that any planet (or moon) which we detect biosignatures from are due to biology.

That's why simulating different planetary atmospheres and the work being done at MIT and other places is very important. We'll want to know we found alien life and not just an alien ocean being vaporized.

First link now working. It seems to imply a similar doubt to the article I linked in the original post, about oxygen alone as a sufficient indicator of life.
As I said-- I think the problem, as originally stated, is overblown. It appears unlikely that the moon of a terrestrial planet, in its star's habitable zone, would retain an atmosphere abundant enough to matter.
If both oxygen and methane appear to be detected on an exoplanet, it probably means that both are actually present there, and that life is, too.

a reply to: Ross 54

Moon size has nothing to do with it. Look at Titan. It is smaller than earth, but bigger than the moon, and has an atmosphere.

I specified moons of terrestrial planets, inside what is understood to be the habitable zone. Methane is far less volatile in the cold reaches of the outer solar system than it is at Earth. Even a small body like Titan can hold onto it. See the link, below:
en.wikipedia.org...