There is probably no life of our type in the solar system outside Earth itself. But is there life on planets circling other stars? 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. Modem 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.
    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.
    If, however, there are planets circling most stars, what does that tell us about the possibility of life on those planets?
    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.
    For one thing, a planet would have to have a reasonably stable orbit. 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.
    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.
    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. In other words, we need stars about the size of our sun.
    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.
    However, what are the chances that on one of these planets intelligent life has developed, capable of developing a technology like ours?
    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.
    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?
    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.
    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.
    Questions:

问答题   What factors are mentioned as necessary for a planet to be suitable for life?

问答题   Why are large stars unlikely to have life on surrounding planets?

问答题   Why are small stars unlikely to have life on surrounding planets?

问答题   What can we infer from Paragraph 12?

问答题   What is the topic of the passage? What is the author's attitude towards it?