Kepler's haul = 3588 in sampled region of space
In Goldielocks zones = 1966
of these 51 are earthlike (mass) and extrapolating for undetectable (by telescope) 22500 earths in that region of space
==> galaxy wide 15-20 billion earths.
Of cause we now know that large moons orbiting large gas giants can also be likely homes for life even if they are well outside the Goldielocks zone of the parent star.
Anyone's guess how many have evolved life and likely not many will have "smart" inhabitants , even fewer will have inhabitants at least as well developed technologically as us or have been colonised by nonindigenous spacefaring beings .
And the Goldilocks Zone is such only if you assume carbon based life forms somewhat like us and our close (biochemistry) cousins, the amoeba and the yucca plant. Non-carbon-based life is not just possible, but probable. Most of these theories and models fail to account for that when projecting the probability of life elsewhere in the Universe.
But our current understanding of physics helps explain why it is, in a Universe that is suspected to be teaming with planets suitable for carbon based life, we aren't pen pals with any of our suspected neighbors. The fastest mode of communication we have at our disposal is light, which tops out at the speed of light. The fastest mode of physical transport, with or without a human aboard, tops out at about 38,000 mph (current velocity of the Voyager probes).
Even if we could project a coherent light beam capable of being targeted on the nearest non-Goldilocks exoplanet orbiting Alpha Centauri (dubbed Alpha Centauri Bb), it would take the beam 4.23 years from emission to arrive at the target. Traveling to Alpha Centauri (robotically or otherwise) at 38,000mph would take approximately 78 thousand years, for reference. What intelligent technological life was on Earth 78 thousand years ago? Over the next 78 thousand year span, what is the likelihood of mass extinction events eliminating most life forms on Earth? The nearest Goldilocks planet orbits Tau Certi which is more than 11 light years away. You do the math.
The distances involved are so vast, that without some form of FTL travel and/or communication, the chances of one intelligent, technologically capable civilization coexisting simultaneously with us and being in close enough proximity to us to be able to technologically communicate with us, are exceedingly remote. Your odds of being struck dead my a meteor whilst being attacked by a Great White shark on the very day you won a half billion Dollar jackpot in the Powerball Lottery are hundreds of millions of times better.
Evolution is slow. How long did it take for Homo Habilis to evolve into new species leading to Homo Sapiens? For Homo Sapiens, how long has it taken us to evolve from early examples of the species to current stature (greater average height, weight, physical and intellectual capabilities, etc.)? What are the odds of any two proximate (in a galactic location sense) intelligent technological civilizations each achieving a level of evolution and technology allowing mutual communication over these vast distances at the same point in time? Astronomical. But eventually evolution will give us the cranial capacity necessary for our entire species to abandon ignorance and primitive superstitions. As a species we've already made great strides along this path, but the path is long and we have many miles to go before we rest.
Pending credible technological advancements or physics discoveries for overcoming the great governor, the speed of light, I will acknowledge that while it is almost 100% certain that other intelligent technological life exists in the Universe right now, it is also almost 100% certain that no person living today will ever know it. We are, for all practical inter-planetary civilization communication, combat or commerce intents and purposes, currently alone.
Given such, let's not **** up the Big Blue Marble too badly, eh?
Edited by jrbarnett, 20 February 2015 - 03:29 PM.