Of course, the vast majority of the extra-solar worlds discovered to date are quite unlike our own: many are gas giants, and some are boiling hot while others endure everlasting chills. Just a handful are close in size to our planet, and only a few of those may be rocky like the Earth, rather than gaseous like Jupiter or icy like Neptune.

But within the next few years, astronomers expect to find dozens of alien earths that are roughly the size of our planet. Some of them will likely be in the so-called habitable zone, where the temperatures are just right for liquid water. The discovery of “Earth twins,” with conditions similar to what we find here, will inevitably bring questions about alien life to the forefront.

Detecting signs of life elsewhere will not be easy, but it may well occur in my lifetime, if not during the next decade. Given the daunting distances between the stars, the real-life version will almost certainly be a lot less sensational than the movies depicting alien invasions or crash-landing spaceships.

The evidence may be circumstantial at first — say, spectral bar codes of interesting molecules like oxygen, ozone, methane and water — and leave room for alternative interpretations. It may take years of additional data-gathering, and perhaps the construction of new telescopes, to satisfy our doubts. Besides, we won’t know whether such “biosignatures” are an indication of slime or civilization. Most people will likely move on to other, more immediate concerns of life here on Earth while scientists get down to work.

If, on the other hand, an alien radio signal were to be detected, that would constitute a more clear-cut and exciting moment. Even if the contents of the message remained elusive for decades, we would know that there was someone “intelligent” at the other end. The search for extraterrestrial intelligence with radio telescopes has come of age recently, 50 years after the first feeble attempt. The construction of the Allen Telescope Array on an arid plateau in northern California greatly expands the number of star systems from which astronomers could detect signals.

However it arrives, the first definitive evidence of life elsewhere will mark a turning point in our intellectual history, perhaps only rivaled by Copernicus’s heliocentric theory or Darwin’s theory of evolution. If life can spring up on two planets independently, why not on a thousand or even a billion others? The ramifications of finding out for sure that ours isn’t the only inhabited world are likely to be felt, over time, in many areas of human thought and endeavor — from biology and philosophy to religion and art.

Some people worry that discovering life elsewhere, especially if it turns out to be in possession of incredible technology, will make us feel small and insignificant. They seem concerned that it will constitute a horrific blow to our collective ego.

I happen to be an optimist. It may take decades after the initial indications of alien life for scientists to gather enough evidence to be certain or to decipher a signal of artificial origin. The full ramifications of the discovery may not be felt for generations, giving us plenty of time to get used to the presence of our galactic neighbors. Besides, knowing that we are not alone just might be the kick in the pants we need to grow up as a species.