Stephen Hawking recently stirred up some controversy by stating that human contact with an extraterrestrial species might not be a good thing—for humans. Some have responded to his assertion on the likelihood of life forms existing in other planetary systems by citing some sort of religious authority to “prove” that humans were created by God in his own image and therefore, as his special creatures, we are the only intelligent life form in the universe. That is superstitious nonsense and betrays a remarkable misunderstanding of the size and complexity of the universe.
We live on a planet that orbits a fairly ordinary star in the outer reaches of a fairly ordinary galaxy. From examination of other stars and other galaxies it is clear that neither our sun nor its neighborhood are unusual. Many planets have been discovered in other systems, and current theories about how planets are formed indicate that planetary systems are quite common. So our environment, in an astronomical sense, is a quite common and ordinary one.
As to the origin of life, it is clear that for life as we know it, meaning carbon-based with amino acids for building blocks, under the right conditions and given enough time, self-replicating molecules (life forms) will emerge.
When we talk about the probabilities of life emerging elsewhere, we need to understand the immense numbers involved. Our Milky Way galaxy has upwards of 200 billion stars. Estimates of the total number of galaxies range from 200 billion to 500 billion. Taking the lowest estimate for the number of stars in a galaxy and the lowest estimate for the number of galaxies, we arrive at an astonishing number of stars: 40,000,000,000,000,000,000,000. Because our sun is a typical star and stars like it are quite common, there are more than a few planetary systems out there where life forms could arise. Because of the nature of living systems, especially their ability to adapt to whatever conditions may exist, it is likely that there is a vast number of planets on which life has arisen. At least some of these have undoubtedly evolved into intelligent tool users. Tool-using is critical, because even though dolphins are undoubtedly quite intelligent, they could never build a spaceship.
So there is a high probability that there are intelligent, technologically advanced cultures out there somewhere. They might be gentle and benevolent, but there is no reason to assume they have any higher regard for other species than humans do. There is no reason to assume they are not belligerent and aggressive. A close encounter of the third kind could well turn out to be as devastating for us as a species as Columbus’ voyage was for the indigenous population of North America.
