where:
N is the number of cvilizations in our galaxy with which communication might be possible; and
R* is the average rate of star formation in our galaxy
fp is the fraction of those stars that have planets
ne is the average number of planets that can potentially support life per star that has planets
fℓ is the fraction of the above that actually go on to develop life at some point
fi is the fraction of the above that actually go on to develop intelligent life
fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L is the length of time such civilizations release detectable signals into space.
The total number of stars in the milky way is around 400 billion.
The fraction of stars that have stars that have planets is lossely estimated to be a quarter.
Therefore, the number of planetary systems is around 100 billion.
If each system has 10 planets, that makes 1 trillion planets.
If around 1/5 of those can support life, that makes 200 billion planets capable of supporting life.
Of course no one knows as we have nothing to compare but if you take the fraction of those planets on which life does arrise as a half, that makes 100 billion planets with some form of life at some point in the planet's life.
The reat gets more difficult.
If you take
fi and
fc to each be a tenth, that makes 1 billion planets on which civilizations have orizon at least once.
However, if you were gaining hope that that seems a large number, there's one other factor. Consider that in the lifetime of our star, we have only had advanced civilization for a few decades. Also with advanced technology comes the idea that we could all destroy each other tomorrow. Taking earth as an example, as a proportion of the life of a planet, the window of time where civilizations might actually arise is probably only of the order of 1/100million. Therefore, the number of planets with civilized life that we may communicate with at any moment is around 10.
Ten planets in a galaxy the size of the milky wayis s near as make no difference, nothing.
However, you could also take the view that some civilizations might learn to live with advanced technology without destroying themselves and continue to co-exist and evolve for a time comparable to that of a planet. If instead of 1/100million, if 1% of civilizations do this, the equation then becomes of the order of millions.
The sky could very well be humming with messages sent out by civilizations much older and wiser than ourselves.
[ame="http://www.youtube.com/watch?v=p86BPM1GV8M&feature=related"]YouTube - Carl Sagan - Pale Blue Dot[/ame]
The pale blue dot in question was from a photo taken by Voyager 1 after it had completed it's mission was going to leave radio contact just past Saturn. It turned round to take one last picture of Earth and the astronomers were startled at first because they couldn't find it. Then they looked a bit closer and where Earth should have been was a tiny blue pixel.
that is really amazing and interesting, i love things like that. Geez we are small!
