2 Before we can really try to answer that, we have to ask if there are planets circling other stars. Over five hundred years ago, Nicholas of Cusa took it for granted that there were. Modern astronomers think he is likely to have been right, for if our solar system was formed from a cloud of dust and gas that automatically formed planets, that should be true of many other stars as well, and even, perhaps, of nearly all stars.
3 But that is risky reasoning. It would be much better if one star, aside from our own sun, were actually found to have a planetary system. Unfortunately, even with our present-day instruments, we can't see any planets circling other stars. Such a planet would be 4.4 light-years away, even if it were circling the very nearest star, and it would be shining only by the reflected light of that star, so that it would not deliver enough light to be seen at that distance. There is an answer, however. Sirius B was discovered by Bessel because its gravitational pull was forcing Sirius A to move in a wavy line, not because it was seen through a telescope. Might a planet, or group of planets, do the same for the stars they circle?
4 In theory, yes, though the effect would be extremely small. (1) The best chance for detecting a planet outside our solar system is to choose a star that is very close to us so that we can measure any deviation from its path most accurately. It should also be small, so that a planet could affect its motion sufficiently, and the planet itself would have to be very large to produce a sizable effect. 这在理论上是成立的，尽管其作用将是极其微小的。探测太阳系外行星最有可能的机会是选择一颗离我们相当近的恒星，这样我们就能非常精确地测量其运行轨道的任何偏离。这颗恒星要小，这样行星就能明显地影响其运行，而那颗行星一定要相当之大，足以对其产生相当的影响。
5 The Dutch-American astronomer Peter Van de Kamp investigated nearby small stars for just that purpose. He felt that he had detected tiny irregularities in the motion of nearby stars such as 61 Cygni, Lalande 21185, and, in particular, Barnard's Star. In addition to being very near us, Barnard's Star is quite small and Van de Kamp thought that from its motion he had detected a Jupiter-sized planet circling it. He found similar large planets in connection with the other stars he
studied. But his work was at the very edge of what his instruments could detect, and later astronomers since have decided that his results were not reliable. 荷兰裔美国天文学家彼得·范德肯普为此观测了附近的小恒星。他认为自己观测到了附近恒星运行的细微的异常之处，如天鹅座61，拉兰德21185，尤其是巴纳德恒星。巴纳德恒星不仅与地球距离相当接近，而且比较小。彼得·范德肯普认为，他在该恒星的运行过程中发现有一颗与木星一样大小的行星环绕其运行。他发现同样大小的行星与他所研究的其他恒星也有这种联系。但他的研究超出了他的器材所能观测的范围，后来的天文学家认定，他的研究结果并不可靠。
6 On the other hand, in the last couple of years some bright stars have been found to be surrounded by bands of dust. It is hard to avoid thinking these might be asteroid belts, and where asteroids exist, larger planets ought to exist, too. Nevertheless, we still have not actually observed any planets circling other stars, and must be satisfied with reasoning they are very likely to exist just the same.
7 If, however, there are planets circling most stars, what does that tell us about the possibility of life on those planets?
8 Life certainly can't exist on any world that is part of another planetary system, just as it cannot exist on any world in our own planetary system. The planet has to be suitable for life. 生命当然不会在别的行星系的任何一个星球上存在，正如生命并不存在于我们的行星系中的任何一颗星球上一样。有生命存在的行星必须拥有适合生命存在的条件。
9 For one thing, a planet would have to have a reasonably stable orbit. (2) If it had an erratic orbit, there might be times when its temperature would rise above the boiling point of water or, at other times, drop below Antarctic temperatures, and there would not be much chance of finding life as we know it. What's more, a planet would have to be massive enough to hold on to an atmosphere and an ocean, but not so massive that it collected hydrogen and helium.
10 (3) But even assuming that a planet is the right size and has the proper chemical composition and a stable orbit neither too far from its star nor too close, so that its temperature is at all times in the range of liquid water (as is true of Earth except for the polar regions), a great deal would still depend on the kind of star it was revolving about. Stars that are much more massive than the sun, for instance, would not be very apt to have such planets; their lives on the
main sequence are too short. After all, here on Earth, organisms as advanced as primitive shellfish did not appear until life had existed on the planet for 3 billion years. If that is the normal rate of evolution, then a planet circling a star such as Sirius could never have life advanced beyond the simplest form of bacterial life, for after a mere half-billion years, Sirius would become a red giant and destroy the planet.
11 Furthermore, if a star is very small and dim, a planet must be very close to it to get enough light and heat to support life as we know it. But at that close distance, tidal effects would cause the planet to face only one side to the sun, so that half the planet would be too hot and half too cold. 再者，如果一颗恒星又小又暗，行星要获得足够的光和热以维持我们所熟悉的生命，就必须与该恒星靠得相当近。但距离过近，潮汐作用就会导致其一面朝向恒星，这样该行星的一半球体会过于炎热，另一半则太冷。
12 In other words, we need stars about the size of our sun.
13 Then again, such stars cannot be part of close binaries or in other regions where there would be too much energetic radiation from surrounding stars. Suppose we decide that only one out of three hundred stars has a chance of possessing a planet that would be hospitable to our kind of life, and only one out of three hundred of such stars has a planet of the right size, chemical composition, and temperature to actually support life. That might still mean the existence of millions of life-bearing planets scattered among the stars.
14 However, what are the chances that on one of these planets intelligent life has developed, capable of developing a technology like ours?
15 There are no optimistic answers to that question. After all, Earth had to exist for 4.6 billion
years before a life form appeared that was capable of developing technology.
16 Even if the chances of its happening are small, it might still be that thousands of technologies have developed among the stars, but then there's a still more difficult question: How long would such technologies endure?
17 Intelligent beings, as they learn to dispose of great sources of energy, might use them for self-destructive purposes. Certainly, now that mankind has developed advanced technologies, we have begun to use them in ruinous wars and are in the process of destroying our environment with them. If this is typical, then the universe might be full of life-bearing planets that have not yet achieved a technology, and equally full of others that have already achieved an advanced technology and have destroyed themselves. There would be only a very, very few besides ourselves who had achieved the technology and had not yet had time to destroy themselves.
18 In about 1950, the Italian-American physicist Enrico Fermi asked the question: Where are they? What he meant was, if the stars are rich in technologies, why hasn't some alien life form reached us? (4) (We can't count wild tales of flying saucers and ancient astronauts, because the evidence in their favor is extremely weak.)
19 Perhaps aliens have not appeared because the distances between the stars is too great to cross, or they have reached us and decided to let us develop in peace, or have failed to appear for any number of other reasons. We can't be sure that simply because no alien is here, there are no aliens somewhere out there.