Are there extraterrestrials out there sharing the galaxy with us, and have they visited our planet? Are they perhaps visiting it now? Who knows? I know I don't. But here are some interesting facts to consider when pondering such questions.
The galaxy is probably not brimming with life, at least not planets to which the life is indigenous. Unfortunately we don't have a large sample of life-producing planets from which to extrapolate. Earth is the only one we know of with any degree of certainty, but it has a number of features that make it very unique, which would indicate that it is unlikely that ecosystems comprised of complex multicellular life forms can arise on most planets.
First of all, our solar system is located far out toward the edge of a spiral arm of the galaxy. By being so far out of the way, we are exposed to a lot less clutter than the vast majority of other star systems. This means that we get fewer strikes from asteroids and comets, which are notorious for causing mass extinctions. An unlucky collision from a large enough body could conceivably wipe out all life on the planet.
Furthermore, earth is the only planet in the solar system to rotate around the sun in an almost perfect circular path. The other eight planets all have eliptical orbits, which play a huge rôle in determining their weather patterns. As the planet gets farther from the sun, it gets colder; as it gets nearer, it warms up. On earth, climate change tends to be much less severe and is determined by the earth having a tilted axis. The less drastic the seasonal changes, the less stress there is to the ecosystem. Circular orbits may be very uncommon for planets; they certainly are not the rule.
Add to this the fact that our moon is also essential for regulating our climate. Without its stabilizing gravitational influence, earth's axis would not only be far more tilted than it is; it would wobble all over the place. There would be occasions when Antarctica would lie on the equator and be a tropical continent. Hawaii could just as easily find itself at one of the poles. The moon's gravity also helps to deflect the paths of approaching asteroids away from the earth. We are very fortunate to have the moon, but the prevailing theory on its origin says that for the earth to have a moon of that size, something very unlikely had to have occurred. During the formation of the planets through regular collisions of material around the sun, our earth and moon were formed when two planet-sized bodies collided, merged, and then split apart. If the collision had been any weaker, they would have merged, forming one large planet. If the collision had been stronger, the moon would have been expelled with too much force to be caught by the earth's gravity and would have gone on its merry way.
If the emergence of complex life were a common occurrence throughout the galaxy, it is highly unlikely that we would find ourselves living on a planet with so many unlikely advantages. That's a depressing notion for people like myself who keeping hoping that any day now the aliens will pop in for a visit. However, there are some reasons to believe that they may be out there.
First of all, even if the emergence of simple lone-celled life forms is rare, over millions of years it is probably common for microbes to travel from one planet to another. All that is needed is for a large enough asteroid impact to throw rock and dirt and ice up into space. Occasionally some of the larger chunks may find their way into the atmosphere of another planet. If just one or two organisms thaw out and survive and can find enough nutrients to keep themselves going, eventually they will probably adapt to the new environment and spread all over the place. Some people have suggested that the fossilized Martian microbes (assuming that's what they are) may be descended from earth microbes or even that the reverse may be true -- life may have begun on mars and traveled to earth. I don't find it at all unlikely that when we eventually take samples from the gas giants, we will find their atmospheres full of bacteria. And over billions of years, there is no reason life wouldn't be able to make it to planets in other star systems.
Of course none of that makes much of a difference if there aren't any planets out there that can nurture the microbes sufficiently to allow them to go multicellular. But we really only need a few good planets out of the billions in the galaxy where this can occur. Of course this would probably put the ETs thousands of light years away from us. It would be hard for contact to occur in that case. But I will always remember something that Carl Sagan said in his Cosmos television series. According to him, based on what we know about the galaxy and physics, if a spacefaring race of beings were to set out from its home planet to colonize a world in another nearby star system at conventional sublight speed, and if each settled world eventually sent out more ships, there would be a geometric progression of settlement and exploration. In just a quarter of a million years, the aliens would have visited, and possibly settled every starsystem in the galaxy. A quarter of a million years is nothing! The earth is billions of years old. The dinosaurs went extinct 65 million years ago -- that is 260 quarter million year periods.
So if such civilizations do occasionally arise, then it is almost a certainty that the earth has been visited multiple times. Considering how rare the earth is in terms of the features listed above, one would think that the aliens would take a huge interest in it. After all, among billions of worlds in the galaxy, ours may be one of only a handful on which a complex ecosystem arose. One would think that our planet is under constant observation. Even if the aliens have no interest in making contact at this time (probably for our own good), it's highly likely that they are out there and that we are well known to them.
I find the reports of UFO encounters over nuclear weapons facilities to be intriguing, given that the aliens' most serious concern would be that humans might wipe out all life on the planet. It is comforting to think that they may be out there, unwilling to make themselves known but ready to intervene to save us (or at least our environment) from ourselves.
