55 From the Earth Greetings

名称：55 From the Earth Greetings
内容简介：
新概念英语第三册（英音）
[al:新概念英语（三）]
[ar:MP3 同步字幕版（英音）]
[ti:From the Earth: Greetings]

[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.'