It's certain not everyone agrees with Alan Bass, but this fits the trend of the past few years (emphases mine):
Earths Common as Dirt: Scientific American Podcast
“We’re on the verge of finding out how frequently habitable planets occur in the universe.” That was astronomer Alan Boss at the AAAS meeting on February 14th. “And I think we’re going to find out that that number is very close to one.” Meaning that each solar-type star is probably orbited by, on average, one Earth-type planet. So how many habitable planets might be out there?
“10 to the 11th in our galaxy and then there are something like 10 to the 11th galaxies. We’re up to about 10 to the 22 Earths, plus or minus a few.
“You don’t have to just believe that this speculation is going to be correct or not. NASA will be launching the Kepler space mission, and Kepler’s entire purpose is to count how many Earths there are around a population of stars in the constellation Cygnus.” Kepler launches on March 5th.
“Then about three or four years from now, there’ll be a press conference at NASA headquarters, and Bill Borucki, the Kepler PI will stand up and tell us just how frequently Earths occur. And once we know that we’ll know how to take the next steps in the search for living planets, and some of that work will involve not only telling if the planets are habitable, but actually searching for signatures in their atmospheres if they could be inhabited, as well.”
Presumably including signatures in the atmospheres that might also indicate widespread use of hydrocarbons.
Incidentally, 10*22 can be written as 10,000,000,000,000,000,000,000 planet Earth's throughout the observable universe, and 10*11 means 10,000,000,000 in our galaxy alone.
Which brings one to the Drake Equation. In 2007 I reviewed a "Damn Interesting" summary of this simple equation ...
Damn Interesting has a very nice Drake Equation/SETI review today. Allan Bellow's even touches lightly on the Fermi Paradox, though he doesn't get into the various paradox resolutions.
The highlight of the article is an interactive Drake Equation calculator. Users start with various presets, including the 'rare earth' and the 'Drake 2004' options, then add their own biases. Two of the "terms" of the Drake Equation are now relatively accepted, below I show them and 3 variations on the rest: Drake 2004, rare earth, and me...
Here was my guess then using the interactive Drake Equation calculator
Average number of life-compatible satellites = 0.10
Percentage of planets where life does appear = 87.50%
Percentage where intelligent life evolves = 20.00%
Percentage of civilizations which send signals into space = 90.00%
Average years that civilizations will send signals = 200.00
Average civilizations in our galaxy = 9.5
If I boost the first number I get (emphases mine) ...
New Milky Way stars per year = 6.00
Proportion of stars which have planets = 95.00%
Average number of life-compatible satellites = 1.00
Percentage of planets where life does appear = 74.00%
Percentage where intelligent life evolves = 20.00%
Percentage of civilizations which send signals into space = 100.00%
Average years that civilizations will send signals = 200.00
Average civilizations in our galaxy = 168.7
Actually, there's a defect in the calculator. You can't make the percentage of life-bearing planets where intelligent life evolves > 20%.
That may be an underestimate. From The Economist (2/8/2009, emphases mine):
Charles Darwin's revolution is unfinished | Unfinished business | The Economist
.. Gould’s view was thus that the evolution of human intelligence while not exactly an accident, since it was a response to a long series of circumstances, was certainly not a foregone conclusion. If that series of circumstances had been even slightly different, there would have been no egg-headed Homo sapiens...
That view is being questioned. For example, in a study published last year in the Proceedings of the National Academy of Sciences a group of researchers looked at crustaceans (crabs, shrimps, woodlice and so on) over the past 550m years and found far more examples of groups of species evolving towards complexity than in the other direction. Matthew Wills of the University of Bath, in England, commented at the time that it was the “nearest thing to a pervasive evolutionary rule that’s been found.” In this study, the only crustaceans that became simpler were either parasites or those living in remote habitats, such as isolated marine caves.
Simon Conway-Morris, a palaeontologist at Cambridge University, in England, is the champion of a new interpretation of evolution—one that challenges the view that it is largely governed by the accident of circumstances. Unlike Gould, he thinks that if evolution were replayed from the beginning, a lot of things would turn out the same.
Dr Conway-Morris has arrived at this view from a detailed study of what is known as convergent evolution. Darwin himself was intrigued by this phenomenon, in which different groups of organisms independently evolve similar solutions to similar problems, whether these solutions are teeth, eyes, brains, ecosystems or societies...
.. His argument is that, given the nature of physics and chemistry, there may be only a limited number of ways in which things can work. Evolution will be channelled into these successful paths, and thus does have trends. Two of these, he thinks, are towards complexity and intelligence. He adds that things “don’t just happen in chemistry”. They happen because of pre-existing causes. Whether it is the molecules of crystallin that are used to build an eye or the haemoglobin that makes blood carry oxygen, the nature of molecules themselves means that evolution is more likely to follow a path determined by their basic structure. Evolution is a mechanism, and it works within rules...
With far less evidence, I wrote about similar suspicions about six years back. Back then the oddity of expanding brains was also noted in Matt Ridley's book Genome ...
If information processing (IP) is an adaptive advantage in systems where natural selection applies (eg. all systems - see below), then it will increase over time...
.. Natural selection applies to systems where there is competition for scarce resources, inherited variability, and where some variations enhance competitive advantage. These systems may be biological or a pure information system -- such as an economy. Natural selection then applies to the human brain, human information processing tools (writing, calculating) and computing systems -- and perhaps to the cosmos itself.
One may imagine "intelligence", or information processing, as a parasitic process which begins on simple chemical systems and migrates across various hosts. On our planet the primary host is humans, but computational devices are secondary hosts as are, to some extent, books. The host may change, but the process is self-perpetuating.
(This is not an entirely untestable hypothesis; I suspect it has been tested in simulated evolutionary models. There is one odd historical example, though it seems so odd as to be more likely coincidental. In Matt Ridley's book Genome he writes "the brains of the brainiest animals were bigger and bigger in each successive [geologic] age: the biggest brains in the Paleozoic were smaller than the biggest in the Mesozoic, which were smaller than the biggest in the Cenozoic, which were smaller than the biggest present now" (p. 27). Unfortunately Mr. Ridley did not cite a source for this statement, but a quick Google search found this possibly relevant reference...
If we redo the simple Drake equation with a higher probability of intelligence evolving whenever it can, say about 60% of the time, we get about 540 likely coexisting civilizations in our galaxy alone -- and that assumes they only broadcast about 200 years before going silent (we've been sending since about 1910 or so). If they broadcast for even 1000 years than that there would be about 3,500 such civilizations in play right now.
Ahh, but then one comes to one of the most tantalizing questions in science. Where are they? If the numbers were so large, one would think that at least a few hundred would have broadcast in a way we can receive, or sent self-replicating probes throughout the galaxy.
Alas, the best response to this Fermi Paradox is that the number is not that great -- because technological civilizations are short lived. They either self-destruct or change to something that's not interested in communication with us.
Such a fascinating question ...
