Oxygen is Not Definitive Sign for a Habitable World

Earth is covered with plants, which produce oxygen from carbon dioxide through photosynthesis and fill the atmosphere with it. This has led scientists to believe that oxygen must be a key element in a habitable world's atmosphere.

However, Norio Narita, a research assistant professor of the Astrobiology Center of National Institutes of Natural Sciences (NINS), and Shigeyuki Masaoka, an associate professor of the Institute of Molecular Science of NINS, hypothesized that could be possible for planets to have large quantities of non-biologically produced, or abiotic, oxygen, according to EurekAlert.

Prior to this study, scientists believed that for oxygen to be present on a extrasolar planet, there must be plants producing oxygen through photosynthesis. In turn, this caused scientists to look at oxygen as a definite biomarker for life. 

The study, published by Scientific Reports, has shown that abiotic oxygen, produced by the photocatalytic reaction of titanium oxide, an abundant gas found on the surfaces of terrestrial planets, meteorolites, and the Moon in the Solar System, may show Earth-like oxygen levels.

They found that even in the least efficient production case of a low-temperature star, it is possible that a habitable extrasolar planet could maintain an atmosphere with Earth-like oxygen, even without organisms to perform photosynthesis. The photocatalytic reaction of titanium oxide on just three percent of the surface could allow oxygen to reach those levels, according to Phys.

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

Non-biological chemical reactions affect atmospheric compositions of extrasolar planets, which means scientists must begin to look other places for signs of life, besides oxygen.