Yesterday, the environmentalists-Leninists were celebrating Vladimir Lenin's birthday once again – or the Earth Day, as these watermelons (green on the surface, red inside) like to call it in order to superficially look hip and modern. We would hear that there is no Planet B again, and so on.
Well, just a few days earlier, NASA's Kepler spacecraft announced the discovery of Kepler-186f which is pretty much nothing else than such Planet Earth B, the most Earth-like habitable-zone planet discovered so far. The alternative home is located 492 light years from us. At the current speed, Voyager spacecrafts would get there in (20,000 times 500) nine [thanks, Gene] million years (which is still short relatively to the lifetime of a star or a planet) but let's hope that in the future, we will have speedier spaceships (and ETs should have faster ones, too).
Their mother star, Kepler-186, is a main-sequence M1-type dwarf star whose radius as well as mass is 1/2 of the Sun's values but the luminosity is 1/25 of that of the Sun. The surface temperature, below 4,000 kelvins, is cooler than the Sun's temperature and all these numbers combine to the fact that at the orbital radius of 0.5 AU (4 light minutes), Kepler-186f is in the habitable zone. The temperature should allow liquid water which seems "enough" for those who believe that ETs are almost everywhere. The size of the planet exceeds the Earth just by 11% – so it's almost a twin sister.
Hontas Farmer thinks that the probability that an industrial civilization is thriving over there exceeds 50%. Not bad (he or she even claims to see some intelligent pattern in some electromagnetic noise). I suppose that the reasoning is that life has to exist there sometime in the future or the past and we might be more advanced or less advanced than them, with the chances for both options at 50%. Count me as a skeptic, my guess would be well below 1%.
We have clearly entered the era in which planets with physical characteristics extremely close to Earth's may be discovered pretty often. I have personally no doubt that the Earth's temperature plus minus 10% or so is just fine for life; the size of the Earth plus minus dozens of percent (and perhaps plus minus an order of magnitude) is OK for life, too. And I have no doubt that there are billions of planets just in the Milky Way that have these OK enough characteristics.
So why don't I believe that the smart ETs "probably" live on such a random planet?
My main reason is the simplest empirical one, one that Enrico Fermi pointed out. We haven't seen too many ETs or their signals here on Earth. If civilizations on habitable-zone planets routinely get to the point we are enjoying here and now, many of them should have sent at least some spacecrafts to all good enough planets that are at most 1,000 light years from them. Note that it's enough to reach the required industrial power – to send these thousands of spacecrafts – once. I don't need to assume that the civilizations survive for millions of years.
But we haven't been contacted by the "ambassador spacecraft" sent from Kepler-186f. We haven't been contacted by any "ambassador spacecraft" from the hundreds of other hypothetical planets where life should be much more advanced than ours and that are about as close as Kepler-186f. It seems bizarre to me. I think that a universe – or the Earth – drowning in the cosmic sea of hi-tech planets simply looks very different from ours.
The absence of non-bogus observations of ETs on Earth as of today seems to be the most important approximate measurement of the concentration of ETs that we seem to have, I think. It's a "negative" measurement. The most important "positive" piece of evidence indicating that ET life should be common is the fact that life began to evolve relatively quickly here on Earth. If some unlikely processes that may be fixed by repetition were needed during the creation of life, it would be far more likely that we find ourselves living on a planet whose star is going to die soon – a star that is as old as you can get. It doesn't seem to be the case.
However, the birth of life may have depended on some unlikely conditions that cannot be improved by waiting. The composition of chemicals – heavier elements, perhaps some special compounds, and possibly even some simpler or more complex organic molecules (or panspermia-like seeds distibuted by comets, asteroids, and dust, but only in some regions of the galaxy) – we have here on Earth may be rare. Some debris from several different types of dying stars could have met at a single point (on the Earth) and this just doesn't happen too often – and it will be happening even less frequently as the Universe keeps on expanding and diluting and stars keep on dying.
I have used this analogy before. The RNA/DNA/protein-like complex molecules that are needed for life may be as rare in the Milky Way as good string theory groups are rare on Earth. To establish a meaningful string theory group, you need to concentrate and accumulate the intellectual resources. In a similar way, certain – many – economic activities require some concentration of the capital. Because of the required concentration, these things just won't occur and can't occur everywhere.
The number of precise enough measurements of the number of ET civilizations – or the amount of robust enough evidence of any kind, including very indirect theoretical arguments – is extremely low which means that our prior beliefs – also known as prejudices – are obviously playing a very important role for our opinions about the number of advanced ET civilizations. Needless to say, I will instantly turn the coat once we start to watch our third alien TV channel if not earlier than that. ;-) But so far, the idea that we're very special – and perhaps the only ones in the Milky Way or perhaps the visible Universe (and there may even be some reasons why it is so), seems to admit a rather natural model that is compatible with all the evidence I am aware of.
Many people seem to be convinced about the idea that the "ETs are almost everywhere" because they think that they can disprove the idea that life is very rare. The main difference between them and myself is that I think that their disproof is just invalid. They believe that the "fast start of life" here on Earth immediately implies the "fast start on every good enough planet". But it's just not necessarily so – there may be differences in the "genetic setup" of the Earth, some very special detailed properties that planets either possess or (usually) not possess from the very beginning and the Earth's "genetic predispositions" simply could be extremely special and rare.
On the other hand, I feel that the advocates of "ET life everywhere" often hold beliefs that are in conflict with our observations of zero ET signals and ambassadors accessible to all of our tools of detection by 2014 AD. I think that their belief that an O(1) fraction of the hospitable-zone planets inevitably turns into the home of superadvanced civilizations predicts certain observations of ETs that our actual observations have refuted.