对于宜居系外行星，氧气不是生命存在的重要证据

中国科技网9月14日报道（张微 编译）地球的大气层中含有氧气，是因为植物通过光合作用不断地产生氧。这种充足的氧气供应使得像动物这样的生命形态能够生生不息。因此，氧气也一直被认为是系外行星上存在生命的一个重要生物标记。但是现在，国家自然科学研究所天体生物学中心(成立于2015年4月)的研究助理教授Norio Narita，和国家自然科学研究所分子科学中心的研究助理教授Shigeyuki Masaoka共同提出了一个新的假设，行星上可能存在大量的非生物氧(由非生物产生的)。这项研究是一个跨学科研究的范例，将不同科学领域的知识相结合，促进天体生物学对于系外行星生命的搜索和研究。这项研究的成果发表在2015年9月10日出版的《科学报告》上。

直到现在，人们一直认为，如果一颗行星上有氧气，那就意味着是某种形态的植物通过光合作用产生的。因此，当寻找宜居系外行星上是否有生命存在时，大气层中氧气的出现往往被视为重要的生物标记。现在由Narita博士领导的研究团队发现，在类地行星表面、陨石和太阳系中的月亮上含量丰富的、由二氧化钛的光催化反应产生的非生物氧是不能被忽视的存在。

对于一个类太阳系的行星系统，约0.05%的行星表面上存在钛氧化物的连续光催化反应，可以产生与目前地球大气层中等量的氧。此外，该团队还估算了围绕其他寄主恒星运转的宜居行星在不同质量和温度下可能产生的氧气量。他们发现，即使在一个低温恒星产量最低效的情况下，3%的行星表面的钛氧化物光催化反应也能够通过非生物过程，将大气中的氧气维持在这个水平。换句话说，即使没有生物体进行光合作用，宜居系外行星上也可能保持一个类似地球的氧气大气层。

Narita博士说：“通过天文观测寻找系外行星上的生命，我们需要结合各种学科领域的知识，提升天体生物学研究实力来建立生命存在的确定性标准。虽然氧气仍然是一个可能的生物标记物，但是根据目前的研究结果来看，我们有必要寻找新的生物标记物。”

Oxygen is not definitive evidence of life on habitable extrasolar planets

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 terrestrial planets, 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 titanium oxide 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 photosynthesis.

Dr. Narita said, "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. Although oxygen is still one of possible biomarkers, it becomes necessary to look for new biomarkers besides oxygen from the present result."

http://www.wokeji.com/guojipindao/dujiabianyi/201509/t20150914_1678620.shtml

中国科技网9月14日报道（张微 编译）地球的大气层中含有氧气，是因为植物通过光合作用不断地产生氧。这种充足的氧气供应使得像动物这样的生命形态能够生生不息。因此，氧气也一直被认为是系外行星上存在生命的一个重要生物标记。但是现在，国家自然科学研究所天体生物学中心(成立于2015年4月)的研究助理教授Norio Narita，和国家自然科学研究所分子科学中心的研究助理教授Shigeyuki Masaoka共同提出了一个新的假设，行星上可能存在大量的非生物氧(由非生物产生的)。这项研究是一个跨学科研究的范例，将不同科学领域的知识相结合，促进天体生物学对于系外行星生命的搜索和研究。这项研究的成果发表在2015年9月10日出版的《科学报告》上。

直到现在，人们一直认为，如果一颗行星上有氧气，那就意味着是某种形态的植物通过光合作用产生的。因此，当寻找宜居系外行星上是否有生命存在时，大气层中氧气的出现往往被视为重要的生物标记。现在由Narita博士领导的研究团队发现，在类地行星表面、陨石和太阳系中的月亮上含量丰富的、由二氧化钛的光催化反应产生的非生物氧是不能被忽视的存在。

对于一个类太阳系的行星系统，约0.05%的行星表面上存在钛氧化物的连续光催化反应，可以产生与目前地球大气层中等量的氧。此外，该团队还估算了围绕其他寄主恒星运转的宜居行星在不同质量和温度下可能产生的氧气量。他们发现，即使在一个低温恒星产量最低效的情况下，3%的行星表面的钛氧化物光催化反应也能够通过非生物过程，将大气中的氧气维持在这个水平。换句话说，即使没有生物体进行光合作用，宜居系外行星上也可能保持一个类似地球的氧气大气层。

Narita博士说：“通过天文观测寻找系外行星上的生命，我们需要结合各种学科领域的知识，提升天体生物学研究实力来建立生命存在的确定性标准。虽然氧气仍然是一个可能的生物标记物，但是根据目前的研究结果来看，我们有必要寻找新的生物标记物。”

Oxygen is not definitive evidence of life on habitable extrasolar planets

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 terrestrial planets, 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 titanium oxide 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 photosynthesis.

Dr. Narita said, "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. Although oxygen is still one of possible biomarkers, it becomes necessary to look for new biomarkers besides oxygen from the present result."

http://www.wokeji.com/guojipindao/dujiabianyi/201509/t20150914_1678620.shtml