[00:00.00] take...for granted be true of in theory detect
[00:05.09]认为…理所当然 对…适用 理论上 察觉
[00:10.19]in particular in connection with reliable for one thing
[00:14.90]尤其 有关 可靠的 首先一点
[00:19.62]stable orbit for instance be apt to(do)
[00:24.10]稳定的 轨道 例如 有…的倾向
[00:28.58]evolution in other words existence capable
[00:33.87]演变 换句话说 存在 有能力的
[00:39.16]endure dispose in the process(of) universe
[00:47.88]Is there life on other planets?Not on those surrounding our sun,it seems.But what of other stars?
[00:56.22]Do they have planets capable of supporting life?This article sets out to explore the possibilities.
[01:05.31]IS THERE LIFE ON PLANETS CIRCLING OTHER STARS? By Isaac Aslmov
[01:12.31]There is probably no life of our type in the solar aystem outside Earth itself.
[01:18.79]But is there life on planets circling other stars?
[01:24.04]2 Before we can really try to answer that,we have to ask if there are planets circling other stars.
[01:32.13]Over five hundred years ago,Nicholas of Cusa took it for granted that there were.Modern astronomers think
[01:41.80]he is likely to have been right,for if our solar system was formed from a cloud of dust
[01:48.75]and gas that automatically formed planets,
[01:53.53]that should be true of many other stars as well,and even,perhaps,of nearly all stars.
[02:00.50]3 But that is risky reasoning.It would be much better if one star,aside from our own sun,
[02:08.76]were actually found to have a planetary system.Unfortunately,even with our present-day instruments,
[02:16.88]we can't see any planets circling other stars.Such a planet would be 4.4 light-years away,
[02:25.68]even if it were circling the very nearest star,and it would be shining only by the reflected light of that star,
[02:34.57]so that it would not deliver enough light to be seen at that distance.There is an answer,however.
[02:42.77]Sirius B was discovered by Bessel because its gravitational pull was forcing Sirius A to move in a wavy line,
[02:53.06]not because it was seen through a telescope.Might a planet,or group of planets,do the same for the stars they circle?
[03:02.54]4 In theory,yes,though the effect would be extremely small.(1)The best chance for detecting a planet
[03:10.98]outside our solar system is to choose a star that is very close to us so that we can measure any deviation
[03:19.68]from its path most accurately.It should also be small,so that a planet could affect its motion sufficiently,
[03:29.79]and the planet itself would have to be very large to produce a sizable effect.
[03:36.11]5 The Dutch-American Peter Van de Kamp investigated nearby small stars for just that purpose.
[03:44.86]He felt that he had detected tiny irregularities in the motion of nearby stars such as 61 Cygni,Laland 21185,
[03:56.96]and,in particular,Barnard's Star.In addition to being very near us,Barnard's Star is quite small
[04:07.28]and Van de Kamp thought that from its motion he had detected a Jupiter-sized planet circling it.
[04:15.40]He found similar large planets in connection with the other stars he studied.
[04:21.56]But his work was at the very edge of what his instruments could detect,
[04:27.36]and later astronomers since have decided that his results were not reliable.
[04:34.42]6 On the other hand,in the last couple of years some bright stars have been found to be surrounded by bands of duat.
[04:43.30]It is hard to avoid thinking these might be asteroid belts,and where asteroids exist,larger planets ought to exist,too.
[04:53.41]Nevertheless,we still have not actually observed any planets circling other stars,
[05:00.12]and must be satisfied with reasoning they are very likely to exist just the same.
[05:06.94]7 If,however,there are planets circling most stars,what does that tell us about the possibility of life on those planets?
[05:15.56]8 Life certainly can't exist on any world that is part of another planetary system,
[05:22.32]just as it cannot exist on any world in our own planetary system.The planet has to be suitable for life.
[05:31.02]9 For one thing,a planet would have to have a reasonably stable orbit.(2)If it had an erratic orbit,
[05:39.75]there might be times when its temperature would rise above the boiling point of water or,at other times,
[05:47.90]drop below Antarctic temmperatures,and there would not be much chance of finding life as we know it.
[05:55.55]What's more,a planet would have to be massive enough ot hold on to an atmosphere and an ocean,
[06:02.55]but not so massive that it collected hydrogen and helium.
[06:07.99]10 (3)But even assuming that a planet is the right size and has the proper chemical composition
[06:14.99]and a stable orbit neither too far from its star nor too close,so that its temperature is at all times
[06:23.56]in the range of liquid water(as is true of Earth except for the polar regions),
[06:29.83]a great deal would still depend on the kind of star it was revolving about.
[06:35.86]Stars that are much more massive than the sun,for instance,would not be very apt to have such planets;
[06:43.70]their lives on the main sequence are too short.After all,here on Earth,
[06:50.20]organisms as advanced as primitive shellfish did not appear until life had existed on the planet for 3 billion years.
[07:00.34]If that is the normal rate of evolution,
[07:03.94]then a planet circling a star such as Sirius could never have life advanced beyond the simplest form of bacterial life,
[07:13.32]for after a mere half-billion years,Sirius would become a red giant and destroy the planet.
[07:21.34]11 Furthermore,if a star is very small and dim,
[07:25.62]a planet must be very close to it to get enough light and heat to support life as we know it.
[07:32.26]But at that close distance,tidal effects would cause the planet to face only one side to the sun,
[07:40.36]so that half the planet would be too hot and half too cold.
[07:45.35]12 In other words,we need stars about the size of our sun.
[07:50.20]13 Then again,such stars cannot be part of close binaries or in other regions
[07:57.02]where there would be too much energetic radiation from surrounding stars.
[08:02.72]Suppose we decide that only one out of three hundred stars has a chance of possessing a planet
[08:10.16]that would be hospitable to our kind of life,and only one out of three hundred of such stars
[08:18.42]has a planet of the right size,chemical composition,and temperature to actually support life.
[08:26.10]That might still mean the existence of millions of life-bearing planets scattered among the stars.
[08:33.65]14 However,what are the chances that on one of these planets intelligent life has developed,
[08:40.86]capable of developing a technology like ours?
[08:45.48]15 There are no optimistic answers to that question.After all,
[08:51.28]Earth had to exist for 4.6 billion years before a life form appeared that was capable of developing technology.
[09:00.76]16Even if the chances of its happening are small,it might still be that thousands of technologies have developed among the stars,
[09:09.46]but then there's a still more difficult question:How long would such technologies endure?
[09:16.64]17 Intelligent beings,as they learn to dispose of great sources of energy,might use them for self-destructive purposes.
[09:25.11]Certainly,now that mankind has developed advanced technologies,we have begun to use them in ruinous wars
[09:33.26]and are in the process of destroying our environment with them.If this is typical,
[09:39.79]then the universe might be full of life-bearing planets that have not yet achieved a technology,
[09:46.84]and equally full of others that have already achieved an advanced technology and have destroyed themselves.
[09:55.04]There would be only a very,very few besides ourselves
[09:59.75]who had achieved the technology and had not yet had time to destroy themselves.
[10:05.83]18 In about 1950,the Italian-American physicist Enrico Fermi asked the question:Where are they?What he meant was,
[10:17.43]if the stars are rich in technologies,why hasn't some alien life form reached us?
[10:23.88](4)(We can't count wild tales of flying saucers and ancient astronauts,because the evidence in their favor is extremely weak.)
[10:33.26]19 Perhaps aliens have not appeared because the distances between the stars is too great to cross,
[10:40.03]or they have reached us and decided to let us develop in peace,or have failed to appear for any number of other reasons.
[10:48.57]We can't be sure that simply because no alien is here,there are no aliens somewhere out there.
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