新概念英語(yǔ)第三冊(cè)（美音版）From the earth: Greetings

[ti:未知] [by:https://www.lrc.com.cn] [00:02.79]Lesson 55 [00:05.50] From the earth: Greetings [00:13.81]Which life forms are most likely to develop on a distant planet? [00:21.33]Recent developments in astronomy [00:23.60]have made it possible to detect planets in our own Milky Way and in other galaxies. [00:30.35]This is a major achievement because,in relative terms, [00:34.31]planets are very small and do not emit light. [00:39.02]Finding planets is proving hard enough, [00:42.02]but finding life on them will prove infinitely more difficult. [00:47.06]The first question to answer is whether a planet can actually support life. [00:53.15]In our own solar system,for example, [00:55.65]Venus is far to hot and Mars is far too cold to support life. [01:03.35]Only the Earth provides ideal conditions,and even here [01:07.74]it has taken more than four billion years for plant and animal life to evolve. [01:14.94]Whether a planet can support life [01:17.15]depends on the size and brightness of its star,that is its 'sun' [01:23.57]Imagine a star up to twenty times larger,brighter and hotter than our own sun. [01:31.04]A planet would have to be a very long way from it to be capable of supporting life. [01:37.60]Alternatively,if the star were small, [01:40.42]the life-supporting planet would have to have a close orbit round it [01:44.67]and also provide the perfect conditions for life forms to develop. [01:50.08]But how would we find such a planet? [01:53.33]At present,there is no telescope in existence [01:56.47]that is capable of detecting the presence of life. [02:00.40]The development of such a telescope [02:02.28]will be one of the great astronomical projects of the 21st century. [02:08.73]It is impossible to look for life on another planet using earth-based telescopes [02:14.46]Our own warm atmosphere and the heat generated by the telescope [02:19.05]would make it impossible to detect objects as small as planets. [02:23.95]Even a telescope in orbit round the earth, [02:26.74]like the very successful Hubble telescope, [02:29.98]would not be suitable because of the dust particles in our solar system. [02:35.57]A telescope would have to be as far away as the planet Jupiter [02:40.12]to look for life in outer space, [02:42.58]because the dust becomes thinner [02:44.48]the further we travel towards the outer edges of our own solar system. [02:50.06]Once we detected a planet, [02:51.87]we would have to find a way of blotting out the light from its star, [02:56.12]so that we would be able to 'see' the planet properly and analyse its atmosphere [03:02.37]In the first instance,we would be looking for plant life,rather than 'little green men [03:09.34]The life forms most likely to develop on a planet would be bacteria. [03:15.12]It is bacteria that have generated the oxygen we breathe on earth. [03:20.16]For most of the earth's history [03:21.91]they have been the only form of life on our planet. [03:25.86]As Earth-dwellers, [03:27.20]we always cherish the hope that we will be visited by little green men [03:32.21]and that we will be able to communicate with them. [03:35.45]But this hope is always in the realms of science fiction. [03:39.84]If we were able to discover lowly forms of life like bacteria on another planet, [03:45.39]it would completely change our view of ourselves. [03:49.86]As Daniel Goldin of NASA observed, [03:53.55]'Finding life elsewhere would change everything. [03:57.34]No human endeavour or thought would be unchanged by it.'

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