We know now that our galaxy is filled with rocky planets, a percentage of which will almost certainly fall within the category of ‘earth-like’. (100 billion stars mean even a 1:1000000 chance allows for 100 000 earth-like planets). If we ever do find evidence of microscopic life on Mars the odds will go way up as well – meaning that life is much more likely to exist elsewhere. And yet, we see no evidence of extraterrestrial life in our observations of the galaxy, a fact known as the ‘Great Silence’.
There could be many explanations for this phenomenon. A short and not at all comprehensive list includes:
- We are leading the development curve. Other life has not yet reached or surpassed our current development level, and those who have are so far away the evidence still has not arrived (1000 light years is really far away). This theory makes the assumption that humans are outliers, which is always possible but statistically less likely.
- We are well behind the development curve, and would not recognize evidence of extraterrestrials because we haven’t developed far enough yet. Just as an Amazon tribesman might not grasp the significance of a Predator drone at 20,000 feet, we might not recognize or understand what we are looking at. For a long time we assumed that radio transmissions would be evidence of life, but would a 1970s SETI researcher have recognized a typical WiFi or cell phone transmission as evidence of sentience, or merely dismissed it as noise? Even if an ET civilization did have recognizable transmissions at some point, it wouldn’t have been obvious to anyone on Earth until the middle of the last century. It is possible that ETs developed beyond us while we were bashing away at each other with clubs or muskets. The thought that irrefutable evidence of ETs might have been readily available in the form of recognizable radio transmissions during the Spanish Inquisition makes me chuckle for some reason.
- Life is harder than we think. Just because it happened here doesn’t mean that it is easy. Perhaps the places where it might happen number in the dozens. Add in some random elements like planet killing asteroids, and we have a quiet galaxy.
- The Great Filter. The notion that some point in the development of a civilization is very hard to overcome, and that most species do not actually succeed. If this filter is in the past, something like the development of language or writing, then we may be outliers leading the pack. If it is in the future, like developing into a star-faring civilization before we destroy our own planet, then we might be in trouble. We certainly aren’t doing well at planet management so far, it isn’t hard to imagine humans entering a downward spiral long before leaving the solar system.
Whatever the situation, our current rush of exoplanet discoveries and the dawn of private space travel make me hopeful that humans will be able to explore at least our own solar system, with an eye to exploring further in some capacity (biological or not). Meanwhile we should really be trying to find ways to avert civilization killing catastrophes like war and climate collapse.