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  • 稀有地球吗?

    With the discovery of exoplanets in the 1990s,  enthusiasm was  mounting   for  the search for life on other worlds . I n 2000, a popular book, "Rare Earth," was published by Ward and Brownlee suggesting that primitive life is very common  but technological life is rare.  Without a single other example of life ,  the book spurred a lot of discussion among scientists and the interested public . 
    ::20世纪90年代发现外行星后,人们开始热衷于在其它世界寻找生命。 2000年,沃德和布朗利出版了一本流行的书《稀有地球 》 ( “ 稀有地球 ” ) , 表明原始生活非常常见,但技术生活却很少见。 没有另一个生命范例,这本书激发了科学家和感兴趣的公众进行大量讨论。

    The premise of the book was that too many random, chance conditions were required for homosapiens  to evolve. In addition, once our technological species appeared, there were new threats to the survival of life (nuclear war, climate change, over-population). Some of the essential conditions and circumstances identified by Brownlee and Ward  are listed in the Table below. 
    ::该书的前提是,需要太多随机、机会条件才能使同性恋者发展,此外,一旦我们的技术物种出现,生命的生存就会面临新的威胁(核战争、气候变化、人口过剩),Brownlee和Ward指出的一些基本条件和情况列于下表。

    Essential Characteristic Reasoning
    Right mass planet Too low mass and the planet could not hang on to its atmosphere; too high mass and the high surface pressure would impede evolution of animals.
    Right mass star Too high mass and the star would have a very short lifetime and emit most of its energy at damaging ultraviolet wavelengths; too low mass and strong magnetic activity from the star would strip away the planet atmosphere.
    Habitable zone orbit Too close and the planet would experience runaway greenhouse warming, like Venus; too far and liquid water would be frozen and  biochemical reaction rates would be sluggish.
    Atmosphere An atmosphere is needed to protect life from ultraviolet radiation. 
    Oxygen The invention of photosynthesis was required to produce atmospheric oxygen for efficient aerobic metabolisms. Microbes could be successful organisms in anaerobic conditions, but not complex eukaryotes. 
    Liquid water Some surface water, but not  so much as to cover all of the land (hard for technology to develop under water).
    Plate tectonics To participate in a negative feedback loop and stabilize the climate.
    Global magnetic field Without a global magnetic field, the wind of charged particles from the surface of the host star will strip away the atmosphere. 
    Large moon To stabilize the tilt of the planet. 
    Mars-like world So prebiotic chemistry and life could get a head start.
    Gas Giant outer planet To act as a sink for incoming comets.
    Right location in the galaxy Too close to the galactic center and high energy radiation from massive stars would be a threat; too far out in the galactic disk and there would be less chemical enrichment.
    Mass extinctions Some mass extinctions to allow for biodiversity, but not too many.

     

    The counter point made by many scientists was that the argument was too anthropomorphic. Some characteristics make sense (right mass planet, atmosphere, water) but others seem less relevant, for a few different reasons :
    ::许多科学家提出的相反观点是,这一论点过于人类形态化。 有些特征(右质量地球、大气、水)是有道理的,但其他特征则似乎不太相关,原因如下:

    Not a bottleneck : The  circumstances that allowed  life on our planet might not be a requirement for life elsewhere. Some characteristics  do not present  a bottleneck. There are hundreds of billions of "right mass stars," and very few OB stars with short lives and peak energy emission of damaging ultraviolet radiation. It now appears that there are also billions of planets similar in mass to the Earth and orbiting in the habitable zone. Many of these stars and planets reside in intermediate regions in the galactic disk. 
    ::不是瓶颈:允许我们星球上生活的环境可能不是其它地方生活的必要条件,有些特征并不存在瓶颈。 目前有数千亿个“ 极右质量恒星 ” , 很少几个寿命短且能量排放高峰的离紫外线辐射的离子恒星。 现在似乎还有数十亿颗与地球质量相近的行星和在可居住区运行的行星。 许多这些恒星和行星都位于银盘的中间区域。

    Redundant : S ome of the characteristics are inter related.  I n the same way that there is redundancy in the requirement: " you have to have a tree and it has to have leaves and it has to have a trunk and branches," some of the characteristics follow "for free" from others. A large subset of Earth-like planets in the habitable zone will have liquid water, plate tectonics, and global magnetic fields. They will naturally outgas or accrete atmospheres.   
    ::冗余 : 有些特性是相互关联的。 同样, 要求中也存在冗余 : “ 您必须有一棵树, 树必须有树叶, 树必须有一个树干和树枝 ” , 有些特性与其它特性“ 免费” 相继。 在可居住区, 大量类似地球的行星将拥有液态水、 板块构造以及全球磁场。 它们将自然产生气体或吸收大气。 ”

    Uncertain impact : Other factors in Ward & Brownlee's list have an uncertain impact. Did we really need Mars to jump-start life on Earth?  If we didn't have Jupiter, would there be fewer objects trapped in the asteroid belt? Would a few more impacts have been a good thing for life on Earth? Does it matter if the obliquity of the Earth precesses at a faster or slower rate?  
    ::不确定的影响: Ward & Brownlee 列表中的其他因素具有不确定的影响。 我们真的需要火星来启动地球上的生命吗? 如果我们没有木星,那么小行星带中的物体会减少吗? 更多的撞击对地球上的生命是好事吗? 如果地球的模糊性以更快或较慢的速度发生,这是否重要?

    Mass extinctions fall on a spectrum of natural selection, which must be fundamental for life. It does seem likely that simple microbial life will be far more common than complex organisms. There is likely a pyramid scheme for life, with single cell organisms at the bottom, and complex organisms like whales, velociraptors, and humans near the top. This is exactly what astrobiologists are trying to learn. 
    ::大规模灭绝取决于一系列自然选择,而这些自然选择必须是生命的根本。 简单的微生物生命似乎很可能比复杂的生物更常见。 可能存在一种金字塔式的生命计划,单细胞生物在底部,而鲸鱼、巨兽和人类等复杂生物则在顶部附近。这正是天体生物学家正在尝试学习的。