大学英语综合教程 第三册 4textB

[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.