Which life forms are most likely to develop on a distant planet?
Recent developments in astronomy have made it possible to detect planets in our won Milky Way and in other galaxies. This is a major achievement because, in relative terms, planets are very small and old 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 t twenty times larger, brighter, 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 iron 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 endeavor or thought would be unchanged by it."
【參考譯文】
天文學方面最新發(fā)展使得我們能夠在銀河系和其他星系發(fā)現(xiàn)行星。這是一個重要的成就，因為相對來說，行星很小，而且也不發(fā)光。尋找行星證明相當困難，但是要在行星上發(fā)現(xiàn)生命會變得無比艱難。第一個需要解答的問題是一顆行星是否有能夠維持生命的條件。舉例來說，在我們的太陽系里，對于生命來說，金星的溫度太高，而火星的溫度則太低。只有地球提供理想的條件，而即使在這里，植物和動物的進化也用了40億年的時間。
一顆行星是否能夠維持生命取決于它的恒星——即它的“太陽”——的大小和亮度。設想一下，一顆恒星比我們的太陽還要大，還要亮，還要熱20倍，那么一顆行星為了維持生命就要離開的它的恒星非常遠。反之，如果恒星很小，維持生命的行星就要在離恒星很近的軌道上運行，而且要有極好的條件才能使生命得以發(fā)展，但是，我們如何才能找到這樣一顆行星呢？現(xiàn)在，沒有一臺現(xiàn)存的望遠鏡可以發(fā)現(xiàn)生命的存在。而開發(fā)這樣一臺望遠鏡將會是21世紀天文學的一個重要的研究課題。
使用放置在地球上的望遠鏡是無法觀察到其他行星的生命的。地球周圍溫暖的大氣層和望遠鏡散出的熱量使得我們根本不可能找到比行星更小的物體。即使是一臺放置在圍繞地球的軌道上的望遠鏡——如非常成功的哈勃望遠鏡——也因為太陽系中的塵埃微粒而無法勝任。望遠鏡要放置在木星那樣遙遠的行星上才有可能在外層空間搜尋生命。因為我們越是接近太陽系的邊緣，塵埃就越稀薄。一旦我們找到這樣一顆行星，我們就要想辦法將它的恒星射過來的光線遮暗，這樣我們就能徹底“看見”這顆行星，并分析它的大氣層。首先我們要尋找植物，而不是那種“小綠人”。行星上最容易生存下來的是細菌。正是細菌生產(chǎn)出我們在地球上呼吸的氧氣。在地球上發(fā)展的大部分進程中，細菌是地球上唯一的生命形式。作為地球上的居民，我們總存有這樣的希望：小綠人來拜訪我們，而我們可以和他們交流。但是，這種希望總是只在科幻小說中存在。如果我們能夠在另一顆行星上找到諸如細菌的那種低等生命，那么這個發(fā)現(xiàn)將徹底改變我們對我們自己的看法。正如美國國家航空和宇宙航空局的丹尼爾.戈爾丁指出的“在其他地方發(fā)現(xiàn)生命會改變一切。任何人類的努力和想法都會發(fā)生變化。”
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