There are two competing theories about the plenitude of intelligent life in the universe. The first says that Earth and the solar system appear to be prototypical of some pretty common planetary systems throughout the universe, so there must be other planets where intelligent life has evolved similar to Earth. We should be actively trying to find and communicate with them. This theory was espoused by Carl Sagan and Frank Drake in the early 60s, who famously predicted tens of thousands of communicating civilizations in the Milky Way galaxy and created the SETI program to find them.
A competing theory, espoused by Peter Ward and Donald Brownlee in their book “Rare Earth: Why Complex Life is Uncommon in the Universe” claims that the factors that enabled intelligent life on Earth are actually not common, so there will be few if any other communicating species on other planets in other star systems. They do not claim that life is rare, carbon-based life is probably very common, but the factors that enabled simple life forms to evolve into a species with brains and communications tools on Earth seem to be remarkable and unlikely to have occurred in other star systems.
Ward and Brownlee enumerate the following factors that were important, if not critical, to the evolution of intelligent life on Earth:
• The location of the sun in the galaxy – close enough to the center to benefit from nearby supernova events necessary to create the matter that seeds the planets, but not so close that constant bombardment sterilizes the planet frequently
• The size and brightness of the sun – smaller stars may not yield sufficient energy to provide an effective energy source for an evolving planet, but larger stars might burn out before an intelligent species could reach maturity
• An evolving planet with adequate size and composition (hydrogen, oxygen, carbon, iron, nitrogen, etc.) to provide the elements of life (whereas a smaller planet might enable life, a larger planet might generate too much gravitational attraction to permit species with calcium-based skeletal structures)
• An evolving planet in a circular orbit in a stable habitable zone where water exists in liquid form over billions of years – note that if the star is too small, the habitable zone may locate the planet in a tidally locked orbit, much like our moon or the planet Mercury are phase locked with their orbital centers, which would not permit day/night cycles, or seasons, which have clearly influenced the evolution of life here
• An evolving planet with a moon to stabilize its orbit over billions of years
• An evolving planet with another massive planet in the star system, similar to Jupiter, that captures through its gravitational field much of the debris that could sterilize the evolving planet over time
• An evolving planet with a system of plate tectonics that provides a long stable period of planetary evolution and enables biodiversity, which has clearly driven the evolution of intelligent life here
• An evolving planet with an internally generated magnetic field that repels cosmic radiation, which is harmful to cell function and likely a show-stopper for evolution
• A modest number of mass extinction events – enough to wipe the slate clean and restart whenever evolution results in dead ends, but not so many that intelligent life doesn’t have time to evolve
These factors don’t even consider the importance of the time window – in order to observe other intelligent species, they would have to be in relatively similar states of evolution coincident with us, and would have had to maintain their civilization long enough to remain coincident. (This begs the question of how long Earth can sustain an intelligent communicating species before war, famine, climate change, celestial debris, or some other extinguishing event terminates it.)
Another factor that Ward and Brownlee don’t mention is the time and frequency scale that we take for granted on Earth – units of seconds, hours, days, years, generations, have developed on Earth relative to specific physical constants, including rotational and orbital frequency of the Earth. Species on other planets may have evolved with generational cycles on the order of milliseconds or millennia, and we would likely not be able to communicate with those species on any reasonable basis.
Just consider the 24-hour rotational period of the Earth. This period provide a centripetal acceleration at the equator that is not present at the poles, and results in an effective decrease in a body’s weight at the equator by a fraction of a percent. But if the rotational period were 90 minutes instead of 24 hours – not that much different on a cosmic scale – then centripetal acceleration at the equator would exactly counter gravitational acceleration and would cause bodies to fly off into space. (Has anyone considered that that might be the source of the rings of debris around some of our outer planets?) I would argue that a planet where mass, including liquid water, is constantly being ejected is not a likely candidate planet for intelligent life to evolve.
These are some of the important factors that characterize Earth and the solar system, and that suggest that we may be unique in the galaxy, and possibly in the observable universe. Personally I find the Ward and Brownlee arguments persuasive, so I believe it is highly unlikely that we will ever find communicating alien civilizations trying to reach us. The search for new Earth-like planets and for evidence of life elsewhere in the extraterrestrial universe is exciting and is likely to bear scientific fruit, but is probably not going to result in any buddy planets with aliens for us to adopt as pen-pals.