Most definitions of life assume that, at a minimum, life is a physical form of matter distinct from its environment at a lower state of entropy than its surroundings, using energy from the environment for internal maintenance and activity, and capable of autonomous reproduction. These assumptions cover all of life as we know it, though more exotic entities can be envisioned, including organic forms with novel biochemistries, dynamic inorganic matter, and self-replicating machines. The probability that any particular form of life will be found on another planetary body depends on the nature and history of that alien world. So the biospheres would likely be very different on a rocky planet with an ice-covered global ocean, a barren planet devoid of surface liquid, a frigid world with abundant liquid hydrocarbons, on a rogue planet independent of a host star, on a tidally locked planet, on super-Earths, or in long-lived clouds in dense atmospheres. While life at least in microbial form is probably pervasive if rare throughout the Universe, and technologically advanced life is likely much rarer, the chance that an alternative form of life, though not intelligent life, could exist and be detected within our Solar System is a distinct possibility.

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外星世界的天体生物学：已知和未知的生命形式

大多数关于生命的定义都假设，生命至少是一种物质形式的物质，不同于环境，其熵处于比周围环境低的状态，它利用来自环境的能量进行内部维护和活动，并能够自主繁殖。这些假设涵盖了我们所知的全部生命，尽管可以设想到更多的外来实体，包括具有新型生物化学的有机形式，动态无机物和自我复制机器。在另一行星体上发现任何特定形式的生命的可能性取决于该外星世界的性质和历史。因此，在岩石覆盖，全球海洋被冰覆盖的岩石星球，没有地表液体的贫瘠星球，拥有丰富液态烃的寒冷世界中，生物圈可能会大不相同。在独立于宿主恒星的无赖行星上，在潮汐锁定的行星上，在超级地球上或在稠密大气层中的长寿命云中。尽管至少在微生物形式的生命可能会在整个宇宙中普遍存在，并且技术上先进的生命可能更罕见，但替代性生命形式（虽然不是智能生命）可能存在并在我们的太阳系中被发现的可能性很大。独特的可能性。