[al:新概念英语（三）] [ar:MP3 同步字幕版（英音）] [ti:From the Earth: Greetings] [by:更多学习内容，请到chazidian.com搜索“新概念”] [00:01.45]Lesson 55 [00:04.07]From the earth: Greetings [00:13.62]Which life forms are most likely to develop on a distant planet? [00:20.55]Recent developments in astronomy have made it possible to detect planets in our own Milky Way and in other galaxies. [00:29.22]This is a major achievement because, in relative terms, planets are very small and do not emit light. [00:37.67]Finding planets is proving hard enough, but finding life on them will prove infinitely more difficult. [00:45.35]The first question to answer is whether a planet can actually support life. [00:50.96]In our own solar system, for example, Venus is far to hot and Mars is far too cold to support life. [01:00.55]Only the Earth provides ideal conditions, and even here it has taken more than four billion years for plant and animal life to evolve. [01:11.42]Whether a planet can support life depends on the size and brightness of its star, that is its 'sun'. [01:19.70]Imagine a star up to twenty times larger, brighter and hotter than our own sun. [01:26.81]A planet would have to be a very long way from it to be capable of supporting life. [01:32.84]Alternatively, if the star were small, [01:35.61]the life-supporting planet would have to have a close orbit round it and also provide the perfect conditions for life forms to develop. [01:44.37]But how would we find such a planet? [01:47.51]At present, there is no telescope in existence that is capable of detecting the presence of life. [01:54.10]The development of such a telescope will be one of the great astronomical projects of the 21st century. [02:01.78]It is impossible to look for life on another planet using earth-based telescopes. [02:07.07]Our own warm atmosphere and the heat generated by the telescope [02:11.57]would make it impossible to detect objects as small as planets. [02:16.44]Even a telescope in orbit round the earth, [02:19.21]like the very successful Hubble telescope, [02:22.46]would not be suitable because of the dust particles in our solar system. [02:27.65]A telescope would have to be as far away as the planet Jupiter to look for life in outer space, [02:34.59]because the dust becomes thinner the further we travel towards the outer edges of our own solar system. [02:41.42]Once we detected a planet, [02:43.43]we would have to find a way of blotting out the light from its star, [02:47.43]so that we would be able to 'see' the planet properly and analyse its atmosphere. [02:53.05]In the first instance, we would be looking for plant life, rather than 'little green men'. [02:59.83]The life forms most likely to develop on a planet would be bacteria. [03:05.23]It is bacteria that have generated the oxygen we breathe on earth. [03:10.16]For most of the earth's history they have been the only form of life on our planet. [03:15.72]As Earth-dwellers, we always cherish the hope that we will be visited by little green men [03:22.10]and that we will be able to communicate with them. [03:25.40]But this hope is always in the realms of science fiction. [03:29.57]If we were able to discover lowly forms of life like bacteria on another planet, [03:35.04]it would completely change our view of ourselves. [03:38.93]As Daniel Goldin of NASA observed, [03:42.50]'Finding life elsewhere would change everything. [03:46.32]No human endeavour or thought would be unchanged by it.'