Oxygen is not definitive evidence of life on habitable extrasolar planets

September 10, 2015, National Institutes of Natural Sciences
Abiotic oxygen can be produced from water in the presence of titanium oxide and an electron acceptor under UV light. Our report suggests that this photocatalytic reaction can supply significant amount of abiotic oxygen on habitable extrasolar planets. Credit: National Institutes of Natural Sciences (NINS)

The Earth's atmosphere contains oxygen because plants continuously produce it through photosynthesis. This abundant supply of oxygen allows life forms like animals to flourish. Therefore, oxygen had been thought to be an essential biomarker for life on extrasolar planets. But now, a research assistant professor Norio Narita of the Astrobiology Center of National Institutes of Natural Sciences (NINS), which was founded in April 2015, and an associate professor Shigeyuki Masaoka, of the Institute of Molecular Science of NINS, have presented a novel hypothesis that it could be possible for planets to have large quantities of abiotic (non-biologically produced) oxygen. This study is a good example of interdisciplinary studies that combine knowledge from different fields of science to promote astrobiology in the search for life on extrasolar planets. The study is published in Scientific Reports on Sep 10, 2015.

Until now, it had been thought that if a planet has oxygen, that must mean that some form of plants are producing it through photosynthesis. Therefore, it had been assumed that when searching for signs of life on habitable extrasolar planets, the presence of oxygen in the atmosphere could be considered a definitive biomarker. However, non-biological chemical reactions can also affect atmospheric compositions of extrasolar planets. Now, the research team led by Dr. Narita has shown that, abiotic oxygen produced by the photocatalytic reaction of titanium oxide, which is known to be abundant on the surfaces of , meteorolites, and the Moon in the Solar System, cannot be discounted.

For a planet with an environment similar to the Sun-Earth system, continuous photocatalytic reaction of titanium oxide on about 0.05 % of the planetary surface could produce the amount of oxygen found in the current Earth's atmosphere. In addition, the team estimated the amount of possible oxygen production for habitable planets around other types of host stars with various masses and temperatures. They found that even in the least efficient production case of a low-temperature star, the photocatalytic reaction of the on about 3% of the planetary surface could maintain this level of atmospheric oxygen through abiotic processes. In other words, it is possible that a habitable extrasolar planet could maintain an atmosphere with Earth-like oxygen, even without organisms to perform .

Dr. Narita said, "To search for life on through astronomical observation, we need to combine the knowledge from various scientific fields and to promote astrobiology researches to establish the decisive signs of . Although oxygen is still one of possible biomarkers, it becomes necessary to look for new biomarkers besides from the present result."

Explore further: Distant planet's interior chemistry may differ from our own

More information: Titania may produce abiotic oxygen atmospheres on habitable exoplanets, Scientific Reports, 2015 Sep 10. www.nature.com/articles/srep13977

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5 / 5 (6) Sep 10, 2015
It is good to know that atmospheric modeling has shown that oxygen by itself can constitute tens of percent in some planetary atmospheres from atmospheric processes alone. So it was already known (from last year, I believe) that oxygen by itself is not a definitive biomarker.

Astrobiologists hence propose to minimize false positives by looking at combinations of gases. (Say, observable amounts of O2 and CH4 are only present in an atmosphere of an inhabited planet.)

The proposed process is, as opposed to the known abiotic sources, self-limiting. (Such quantities of oxygen leads to an ozone sheild, that cuts down surface UV.) I would therefore suspect that atmospheric modeling should show smaller, not comparable, amounts of free oxygen. Mostly though, if this was a major pathway to oxygen on planets, where is the evidence of early oxygen on Earth? There is none.
4.3 / 5 (3) Sep 10, 2015
Good comments Torbjorn.
It was Professor Lovelock who proposed that oxygen was a biomarker when asked to comment on the probability of life on Mars before we sent probes there.
His argument had been misrepresented in this article. He argued that because oxygen is so reactive it would have to be continuously replenished.
Further, of it was held at a constant value over long periods of time, that would imply that there was some sort of feedback mechanism as an oxygen atmosphere would be Naturally unstable.
vlaaing peerd
4 / 5 (1) Sep 10, 2015
I'm not very aware of the photocatalytic reaction of titanium oxide, so I must look that up.

However, if that reaction is not a continuous process, would on a dead planet the oxygen not easily react with any material until all the oxygen is gone into random material-oxides?
not rated yet Sep 10, 2015
TiO2's catalytic activity is well known. I didn't trace back the references to see how active meteoric tio2 is. I have doubts. I also doubt that its presence in oceans would result in much catalysis, I suspect it would be rapidly and permanently poisoned. The authors arguments seem to be a reasonable 'napkin calculation' that abiotic O2 accumulation needs to be modeled. I'd be willing to bet dollars to donuts that it (near surface NUV processes) will not be significant contributor to atmospheric O2, but we won't know without further study.
not rated yet Sep 10, 2015
Since we have a weak/poor understanding of how planets are "put together", this question isn't likely to be definitively answered anytime soon (not in my lifetime, sigh) Especially since Earth seems not to be a representative example of "Earth-type" planets. Arguably.

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