[00:01.040]--- lesson 55 From the earth: Greetings
[00:07.200]--- Listen to the tape then answer the question below.
[00:13.200]--- Which life forms are most likely to develop on a distant planet?
[00:20.120]Recent developments in astronomy have made it possible to detect planets in our own Milky Way and in other galaxies.
[00:28.840]This is a major achievement because, in relative terms, planets are very small and do not emit light.
[00:37.240]Finding planets is proving hard enough, but finding life on them will prove infinitely more difficult.
[00:44.960]The first question to answer is whether a planet can actually support life.
[00:50.560]In our own solar system, for example, Venus is far too hot and Mars is far too cold to support life.
[01:00.080]Only the Earth provides ideal conditions, and even here it has taken more than four billion years for plant and animal life to evolve.
[01:10.960]Whether a planet can support life depends on the size and brightness of its star, that is its 'sun'.
[01:19.320]Imagine a star up to twenty times larger, brighter and hotter than our own sun.
[01:26.440]A planet would have to be a very long way from it to be capable of supporting life.
[01:32.360]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.
[01:43.960]But how would we find such a planet?
[01:47.120]At present, there is no telescope in existence that is capable of detecting the presence of life.
[01:53.680]The development of such a telescope will be one of the great astronomical projects of the twenty-first century.
[02:01.360]It is impossible to look for life on another planet using earth-based telescopes.
[02:06.640]Our own warm atmosphere and the heat generated by the telescope would make it impossible to detect objects as small as planets.
[02:16.000]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.
[02:27.200]A telescope would have to be as far away as the planet Jupiter to look for life in outer space,
[02:34.280]because the dust becomes thinner the further we travel towards the outer edges of our own solar system.
[02:41.080]Once we detected a planet, we would have to find a way of blotting out the light from its star,
[02:47.240]so that we would be able to 'see' the planet properly and analyse its atmosphere.
[02:52.680]In the first instance, we would be looking for plant life, rather than 'little green men'.
[02:59.480]The life forms most likely to develop on a planet would be bacteria.
[03:04.840]It is bacteria that have generated the oxygen we breathe on earth.
[03:09.800]For most of the earth's history they have been the only form of life on our planet.
[03:15.280]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.
[03:25.000]But this hope is always in the realms of science fiction.
[03:29.120]If we were able to discover lowly forms of life like bacteria on another planet, it would completely change our view of ourselves.
[03:38.520]As Daniel Goldin of NASA observed, 'Finding life elsewhere would change everything.
[03:45.880]No human endeavor or thought would be unchanged by it.'