大學(xué)英語綜合教程第三冊 8

[00:00.00] take...for granted be true of in theory detect [00:05.09]認(rèn)為…理所當(dāng)然 對…適用 理論上 察覺 [00:10.19]in particular in connection with reliable for one thing [00:14.90]尤其 有關(guān) 可靠的 首先一點(diǎn) [00:19.62]stable orbit for instance be apt to(do) [00:24.10]穩(wěn)定的 軌道 例如 有…的傾向 [00:28.58]evolution in other words existence capable [00:33.87]演變 換句話說 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. [10:52.46]持久 處理 在…的過程中 宇宙

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