We review the latest findings on extra-solar planets and their potential to support Earth-like life. Focusing on planets orbiting Red Dwarf (RD) stars, the most abundant stellar type, we show that including RDs as potential host stars could increase the probability of finding biotic planets by a factor of up to a thousand, and reduce the estimate of the distance to our nearest biotic neighbor by up to 10. We argue that binary and multiple star systems need to be taken into account when discussing exoplanet habitability. Early considerations indicated that conditions on RD planets would be inimical to life, as their Habitable Zones (where liquid water could exist) would be so close as to make planets tidally locked to their star. This was thought to cause an erratic climate and expose life forms to flares of ionizing radiation. Recent calculations show that these negative factors are less severe than originally thought. It has been argued that the lesser photon energy of the radiation of the relatively cool RDs would not suffice for Oxygenic Photosynthesis (OP) and other related energy expending reactions. Numerous authors suggest that OP on RD planets may evolve to utilize photons in the infrared. We however argue, by analogy to the evolution of OP and the environmental physiology and distribution of land-based vegetation on Earth, that the evolutionary pressure to utilize infrared radiation would be small. This is because vegetation on RD planets could enjoy continuous illumination of moderate intensity, containing a significant component of photosynthetic 400-700 nm radiation. We conclude that conditions for OP could exist on RD planets and consequently the evolution of complex life might be possible. Furthermore, the huge number and the long lifetime of RDs make it more likely to find planets with photosynthesis and life around RDs than around solar type stars.
Photosynthetic exergy I. Thermodynamic limits for habitable-zone planets
Università di Napoli Federico II（那不勒斯费德里科二世大学）；INFN, Sezione di Napoli（意大利国家核物理研究所那不勒斯分部）；INAF – Osservatorio Astronomico di Capodimonte（意大利国家天体物理研究所卡波迪蒙特天文台）；Università di Roma Tor Vergata（罗马托维加塔大学）
Super-Earths, M Dwarfs, and Photosynthetic Organisms: Habitability in the Lab
几年后，太空望远镜将调查我们的银河系，主要通过观察岩石行星来发现生命的证据。在过去的十年中，对系外行星大气的观测和对生物特征气体的理论研究经历了相当大的加速。系外行星领域最吸引人的特征是40%的M矮星拥有超级地球，其最小质量在1到30个地球质量之间，轨道周期短于50天，半径在地球和海王星之间（1- 3.8 R $_\oplus$）。此外，最近发现蓝细菌能够利用远红光（FR）进行光合作用，这是由于叶绿素$d$和$f$的合成，将体内光吸收扩展到750\ nm，这表明在M矮星周围的行星中可能存在外来光合作用。使用创新的实验室仪器，我们将不同的蓝藻暴露于M矮星模拟照射，比较它们对太阳和FR模拟光的反应。正如预期的那样，在FR光下，只有能够合成叶绿素$d$和$f$的蓝藻可以生长。令人惊讶的是，所有菌株，无论是能够或不能使用FR光，生长和光合作用下的M矮产生的光谱以类似的方式太阳光和更有效地比FR下的。我们的研究结果强调了模拟M矮光谱的可见光和FR光分量的重要性，以正确评估暴露在这种异国情调的光~条件下的产氧生物的光合性能。
Ultraviolet-Driven Atmospheric Degeneracies Challenge Conventional Biosignature Frameworks for Terrestrial Planets with Ultracool M Dwarf Hosts: An Archean-Analog TRAPPIST-1 e Case Study
University of California, Riverside（加州大学河滨分校）；Blue Marble Space Institute of Science（蓝大理石太空科学研究所）；University of Maryland, Baltimore County（马里兰大学巴尔的摩县分校）；NASA Goddard Space Flight Center（NASA戈达德太空飞行中心）；SETI Institute（搜寻地外文明研究所）；NASA Ames Research Center（NASA艾姆斯研究中心）
White Paper on Phototrophic Biosignatures: Research Priorities for the Search for Life on Other Worlds
NASA Goddard Institute for Space Studies（NASA 戈达德太空研究所）；University of Wisconsin, Madison（威斯康星大学麦迪逊分校）；Arizona State University（亚利桑那州立大学）；Washington University in St. Louis（圣路易斯华盛顿大学）；NASA Ames Research Center（NASA 艾姆斯研究中心）；University of Alabama（阿拉巴马大学）；Technion⎯Israel Institute of Technology（以色列理工学院）；Georgia Institute of Technology（佐治亚理工学院）；California Institute of Technology（加州理工学院）；University of California, Riverside（加州大学河滨分校）；North Carolina State University（北卡罗来纳州立大学）；National Astronomical Observatory of Japan（日本国立天文台）；Loyola University Chicago（芝加哥洛约拉大学）；University of Chicago（芝加哥大学）；The University of Tokyo（东京大学）；Ibaraki University（茨城大学）；Purdue University（普渡大学）；University of Padova（帕多瓦大学）
Global Abiotic Sulfur Cycling on Earth-like Terrestrial Planets
Instituto de Astrofísica e Ciências do Espaço, Observatório Astronómico de Lisboa（葡萄牙里斯本太空天体物理与空间科学研究所，里斯本天文台）；Blue Marble Space Institute of Science（美国蓝大理石空间科学研究所）；Earth and Environmental Science, Rensselaer Polytechnic Institute（美国伦斯勒理工学院地球与环境科学系）；Planetary Habitability Laboratory, University of Puerto Rico at Arecibo（波多黎各阿雷西博大学行星宜居性实验室）；Earth-Life Science Institute, Tokyo Institute of Technology（日本东京理工大学地球生命科学研究院）；Department of Chemistry, Howard University（美国霍华德大学化学系）
