OK, This one keeps coming up and most investigators have to deal with it. So here is my take, and a few reasons why I feel this way about it.
The first thing we need to do is define Alien Life. If we use our own world as an example it becomes obvious that life is abundant. However with the exception of humans, not one form of life, plant or animal, has an ability to reach out and ask that very question. So is apparent intelligent life represents only a small portion of all life. And when we look back into our history we find that life has existed for eons, yet intelligent life is simply a blip on the screen. Extending that to other planets it can be assumed that even of the planets that contain any life, the chances of it being able to make itself known across the universe is slim.
The probability of finding alien life was best put forward in 1961 by Frank Drake who put it in the form of a probability using the Drake Equation:
Drake's Original estimates were as follows:
- R* = 1/year 1 star formed per year, on the average over the life of the galaxy
- fp = 0.2-0.5 (one fifth to one half of all stars formed will have planets)
- ne = 1-5 (stars with planets will have between 1 and 5 planets capable of developing life)
- fl = 1 (100% of these planets will develop life)
- fi = 1 (100% of which will develop intelligent life)
- fc = 0.1-0.2 (10-20% of which will be able to communicate)
- L = 1000-100,000,000 years (which will last somewhere between 1000 and 100,000,000 years)
While some of the factors have been validated, many are still just estimates and speculation. Thus we cannot use this equation as any kind of definitive marker. But recent discoveries have shed some light on his original estimates. First, the number of stars forming is actually a bit higher, more like 5 or 6 per year. Second, planets around distant stars may be quite common.
However other factors seem quite high. Many of the planets are quite large and very near their parent stars. Thus we can assume they would be extremely hot and not conducive to life as we know it. Thus factor Ne is very low.
Another consideration is that Fl is also very low. Using our own earth as an example intelligent civilization is a very recent occurrence. For most of earth's history, even after life arose, the only life were single cell protozoa and plant life. Also consider there is no evidence that our neighbor planets, especially Mars ever had life on its surface. Consequently I would put the probability of any life developing as less than the 100% Drake claims. Perhaps 20 - 30% would be a good guess although based on current data from our Mars rovers it would appear that even that is high.
And based on the number of years Earth was uncivilized, primitive plant and animal life it would seem that many planets that harbor life simply fail to develop civilization. Thus, unlike Drake's estimate of 100% of life developing civilization, that figure is actually much lower on the order of 2 - 5 %
The final figures regarding the length of such civilizations seem extraordinarily high. Consider our own planet, how long can a civilization such as ours burn through resources until it collapses in on itself? What percentage of our own have we consumed in just the last 200 years versus what we used in the preceding 2,000? Can we keep this up for another 800 years? If we did that would only represent Drake's minimum value of 1,000 years. And that's only if we don't blow ourselves up in a war or something.
Another factor is desire to reach out to the stars. We assume that aliens are visiting us, learning or studying us, or maybe trying to control us. But that bases their desire on our standards. Even if alien civilizations exist, what makes us assume they share out goals? Maybe they have reached a high level of agrarian culture and could care less about the stars in their sky. Or what about the costs? Even if a civilization had the technology, what financial cost might they encounter to travel between star systems? And would the rewards be worth it to them?
Of course all this is speculation. We can't say until they land and identify themselves if aliens are flying craft through our skies. It is also possible their technology has circumvented the distance and cost issues. Just as it is possible the aliens aren't aliens at all, rather they may be us visiting ourselves through time. Or maybe aliens not from out there but from a different dimension. Or what if they are simply another manifestation of what some see as ghosts or spirits? Just a few ideas to consider when one asks if alien life exists.....