Read an article recently on the Great Filter, the notion that we may not come across any evidence of advanced civilizations beyond our own because something eventually rubs them out. We have been sending out electro-magnetic signals for over a hundred years and have been listening for almost as long. We have by now discovered almost 1800 exoplanets. An estimated 22% of sun-like stars in our galaxy may have earth-like planets orbiting in their habitable zones. That would mean 20 billion candidates for life such as ours. Four of such earth-like exoplanets planets have been identified within 50 light years of us, another two within 500 LYs.
There is no reason to assume that life would have to be similar to our carbon-based form or would require conditions similar to ours. Life on our planet sprung up quickly and the physics and chemistry of our universe seem to favor self-organizing processes. Life forms could be quite varied and perhaps universal.
Enrico Fermi suggested in 1950 that if any advanced civilization developed the ability to travel beyond its solar system, even at less than light speed, in ten million years it should be able to colonize the whole Milky Way (100,000 LYs in diameter). So why don't we see them? Why haven't we even heard anyone else? The Great Filter suggests various possibilities.
The first would be that advanced life is rare. The conditions for it to develop are quite special. While life on earth arose quickly, in just 400 million years after earth formed a solid crust, it took another almost two billion years for complex single cells to evolve. Add another billion years – about 550 million years ago – for multi-cellular creatures. Most of the history of life on earth is this long prelude to the development of us. Humans arose only in the last two million years of the earth's 4,500 million years. Along the way, life went through several mass extinction events. The last one, 65 million years ago, took out the dinosaurs leaving the ground clear for the development of mammals. The combination of events and circumstances that led to us may be so rare as to make us one of the very few – or only – lucky ones.
But with some probable 20 billion earth-like exoplanets and some 100 billion likely planets in all, chances are that however rare, odds would favor the development of a considerable number of advanced life forms in our galaxy. Some might have arose millions of years ago. Any signals they sent would have had plenty of time to reach us. Any earth-like planet with advanced life within 500 LYs would presumably have been heard by now. So far, the SETI project has found none.
Perhaps our listening capabilities are still not sensitive enough to pick up any signals. But clearly we are now able to tease out the existence of exoplanets themselves out some two thousand light years.
Maybe cosmic natural disasters – nearby super-novas, meteor strikes, etc – occur frequently enough to set back life and knock out civilizations before they can get very far? But we've gone 65 million years without one and there is no reason to expect any such for at least the next few hundred years.
Maybe someone is out there, able to hide themselves and/or tracking down and destroying any potential competitors before they get too far? This is a common science fiction trope. But it assumes that advanced civilizations would either be very modest – and thus hide themselves, perhaps quietly visiting and making crop circles or waiting for us to rise to the level where we could join their Federation – or especially vicious and aggressive. Based upon the only advanced civilization we know of – ourselves – one could not rule out the second possibility.
Finally, there is the possibility that there is something about advanced technologies that operates to cut short the civilization that develops them: industrial civilization leading to run-away climate change; biotechnology leading to – or failing to keep up with – disruptions in the present web of life; failure of critical management systems to handle increasingly complex and changing political, social, economic and ecological dynamics.
Bottom line, so far we have no evidence that we have company anywhere out there. We may be special. Question is, are we doomed to be filtered out and will we have ourselves to blame?