55 · 来自地球的问候
From the Earth: Greetings
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Recent developments in astronomy have made it possible to detect planets in our own Milky Way and in other galaxies. This is a major achievement because, in relative terms, planets are very small and do not emit light. Finding planets is proving hard enough, but finding life on them will prove infinitely more difficult. The first question to answer is whether a planet can actually support life. In our own solar system, for example, Venus is far too hot and Mars is far too cold to support life. Only the Earth provides ideal conditions, and even here it has taken more than four billion years for plant and animal life to evolve.
Whether a planet can support life depends on the size and brightness of its star, that is its "sun". Imagine a star up to twenty times larger, brighter and hotter than our own sun. A planet would have to be a very long way from it to be capable of supporting life. Alternatively, if the star were small, the life-supporting planet would have to have a close orbit round it and also provide the perfect conditions for life forms to develop. But how would we find such a planet? At present, there is no telescope in existence that is capable of detecting the presence of life. The development of such a telescope will be one of the great astronomical projects of the twenty-first century.
It is impossible to look for life on another planet using earth-based telescopes. Our own warm atmosphere and the heat generated by the telescope would make it impossible to detect objects as small as planets. Even a telescope in orbit round the earth, like the very successful Hubble telescope, would not be suitable because of the dust particles in our solar system. A telescope would have to be as far away as the planet Jupiter to look for life in outer space, because the dust becomes thinner the further we travel towards the outer edges of our own solar system. Once we detected a planet, we would have to find a way of blotting out the light from its star, so that we would be able to "see" the planet properly and analyze its atmosphere. In the first instance, we would be looking for plant life, rather than "little green men". The life forms most likely to develop on a planet would be bacteria. It is bacteria that have generated the oxygen we breathe on earth. For most of the earth's history they have been the only form of life on our planet. As Earth-dwellers, we always cherish the hope that we will be visited by little green men and that we will be able to communicate with them. But this hope is always in the realms of science fiction. If we were able to discover lowly forms of life like bacteria on another planet, it would completely change our view of ourselves. As Daniel Goldin of NASA observed, "Finding life elsewhere would change everything. No human endeavour or thought would be unchanged by it."
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Recent developments in astronomy have made it possible to detect planets in our own Milky Way and in other galaxies. This is a major achievement because, in relative terms, planets are very small and do not emit light. Finding planets is proving hard enough, but finding life on them will prove infinitely more difficult. The first question to answer is whether a planet can actually support life. In our own solar system, for example, Venus is far too hot and Mars is far too cold to support life. Only the Earth provides ideal conditions, and even here it has taken more than four billion years for plant and animal life to evolve.
Whether a planet can support life depends on the size and brightness of its star, that is its "sun". Imagine a star up to twenty times larger, brighter and hotter than our own sun. A planet would have to be a very long way from it to be capable of supporting life. Alternatively, if the star were small, the life-supporting planet would have to have a close orbit round it and also provide the perfect conditions for life forms to develop. But how would we find such a planet? At present, there is no telescope in existence that is capable of detecting the presence of life. The development of such a telescope will be one of the great astronomical projects of the twenty-first century.
It is impossible to look for life on another planet using earth-based telescopes. Our own warm atmosphere and the heat generated by the telescope would make it impossible to detect objects as small as planets. Even a telescope in orbit round the earth, like the very successful Hubble telescope, would not be suitable because of the dust particles in our solar system. A telescope would have to be as far away as the planet Jupiter to look for life in outer space, because the dust becomes thinner the further we travel towards the outer edges of our own solar system. Once we detected a planet, we would have to find a way of blotting out the light from its star, so that we would be able to "see" the planet properly and analyze its atmosphere. In the first instance, we would be looking for plant life, rather than "little green men". The life forms most likely to develop on a planet would be bacteria. It is bacteria that have generated the oxygen we breathe on earth. For most of the earth's history they have been the only form of life on our planet. As Earth-dwellers, we always cherish the hope that we will be visited by little green men and that we will be able to communicate with them. But this hope is always in the realms of science fiction. If we were able to discover lowly forms of life like bacteria on another planet, it would completely change our view of ourselves. As Daniel Goldin of NASA observed, "Finding life elsewhere would change everything. No human endeavour or thought would be unchanged by it."
天文学近来的发展，使人们有可能探测到银河系乃至其他星系中的行星。这是一项重大成就，因为相对来说，行星体积很小，而且本身不发光。发现行星已经够难，要在行星上找到生命，就会困难得多。首先必须回答一个问题：这颗行星是否真的具备孕育生命的条件？比如，在我们的太阳系中，金星太热，火星又太冷，都无法维持生命。只有地球提供了理想条件；而即便在地球上，动植物生命的演化，也经历了四十多亿年。
一颗行星能否维持生命，取决于它所围绕的恒星，也就是它的“太阳”，有多大、多亮。设想一颗恒星，大小、亮度和热度都高达太阳的二十倍，那么一颗行星就必须离它非常远，才可能适合生命存在。相反，如果恒星很小，宜居行星就必须沿着贴近恒星的轨道运行，同时自身还要提供适合生命发展的理想条件。可我们怎样才能找到这样的行星呢？目前还没有任何一架望远镜，能够探测生命是否存在。研制这样的望远镜，将成为21世纪天文学的重大工程之一。
利用地面望远镜寻找其他行星上的生命，是不可能的。地球温暖的大气层，以及望远镜本身产生的热量，都会使我们无法探测行星这样小的天体。即便是像哈勃这样成就卓著、绕地运行的望远镜，也会受到太阳系尘埃的干扰，并不适用。若要搜寻外太空的生命，望远镜得远到木星所在的位置，因为越接近太阳系边缘，尘埃越稀薄。一旦发现行星，我们还得想办法遮去它所围绕的恒星的光，才能真正“看清”行星，并分析其大气。最初，我们寻找的会是植物生命，而不是什么“小绿人”。行星上最有可能出现的生命形式，是细菌。地球上供我们呼吸的氧气，正是细菌产生的；在地球历史的大部分时间里，细菌都是唯一的生命形式。作为地球居民，我们总怀着一种希望：有朝一日，小绿人会来访，我们还能与它们交流。但这种希望始终属于科幻的领域。假如我们能在另一颗行星上发现细菌这样低等的生命形式，就会彻底改变我们对自身的认识。正如美国航天局的丹尼尔·戈尔丁所说：“在别处发现生命，将改变一切。人类的一切努力和思想，都将因此而改变。”
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译文依据整理稿，由本站按中文表达重新校订；解析据吴应时、何景霖整理资料及首轮审校稿；词伙与练习提示为本站学习设计。原PDF第197—200页，2022年2月第3版。