What are the chances that we will encounter some alien form. of life, as we explore the ga

laxy. If the argument about the time scale for the appearance of life on Earth is correct, there ought to be many other stars, whose planets have life on them. Some of these stellar systems could have formed 5 billion years before the Earth. So why is the galaxy not crawling with self designing mechanical or biological life forms？ Why hasn't the Earth been visited, and even colonized. I discount suggestions that UFOs contain beings from outer space. I think any visits by aliens would be much more obvious, and probably also, much more unpleasant.

What is the explanation of why we have not been visited？

One possibility is that the argument about the appearance of life on Earth is wrong. Maybe the probability of life spontaneously appearing is so low that Earth is the only planet in the galaxy, or in the observable universe, in which it happened.

Another possibility is that there was a reasonable probability of forming self-reproducing system, like cells, but most of these forms of life did not evolve intelligence.

A third possibility is that there is a reasonable probability for life to form, and to evolve to intelligent beings, in the external transmission phase. But at that point, the system becomes unstable, and the intelligent life destroys itself. This would be a very pessimistic conclusion. I very much hope it isn't true.

I prefer a fourth possibility: there are other forms of intelligent life out there, but we have been overlooked. There used to be a project called SETI, the search for extraterrestrial intelligence. It involved scanning the radio frequencies, to see if we could pick up signals from alien civilizations. I thought this project was worth supporting, though it was cancelled due to a lack of funds. But we should have been wary of answering back, until we have developed a bit further. Meeting a more advanced civilization, at our present stage, might be a bit like the original inhabitants of America meeting Columbus. I don't think they were better off for it.

The author thinks that ______.

A．it is impossible that we encounter some alien form. of life

B．life only exists on Earth

C．it is likely that we'll encounter some alien form. of life

D．beings from outer space will never visit the earth