Day 094 - Rare Earth

Submitted by Sam on 23 August, 2011 - 00:11

Most arguments for the existence of extraterrestrial life rest on the 'principle of mediocrity', which states that the properties and evolution of our solar system, including the processes that led to life on Earth, are not unusual in any important way and could be common throughout the Universe. The principle of mediocrity's counter argument is the 'rare earth hypothesis', which concludes that all of the conditions which conspire to permit complex life on Earth are exceptionally rare. The rare earth hypothesis has been presented as a plausible solution to the Fermi paradox, and an application of it concludes Stephen Webb's book Where is Everybody?.

In order to support life as we know it, a star system needs to be located in a very specific segment of the galaxy. It must be close enough to the galactic centre that its star contains a certain proportion of heavy elements, which are crucial for the formation of rocky planets containing the molecular components for life's building blocks. Additionally, the planetary system needs to be located far enough away from the dangerous centre of the galaxy, away from its high levels of radiation and its supermassive black hole, so that carbon-based life can develop. This is known as the galactic habitable zone, and it may only encompass around 20% of the stars in our galaxy; the other 80% are exposed to conditions that make the evolution of complex lifeforms unviable.

Of the stars in the galactic habitable zone, perhaps only about 5% are like our sun – stable, not too bright and not too dull, and so we need only consider planets orbiting sun-like stars in our search for complex life in the form that we know already. And then of these planets, we need only consider those with circular orbits which allow them to remain in their star's continuously habitable zone for billions of years at a time – that is, those planets without erratic orbits and which are close enough to their star to maintain liquid water on their surface for large, continuous periods of time. As a star's habitable zone moves outward as it ages, as the star grows hotter and brighter with time, a planet needs to be in a very particular orbit to avoid either permanently icing over or burning up over the course of the billions of years required for complex life to evolve. Perhaps, as Webb suggests, as few as 0.1% of the remaining planets orbit this continuously habitable zone.

Now, with the conditions on these remaining planets set to support the kind of carbon-based life that we know here on Earth, we have to contend with the probability that life will evolve. In itself, this could be an exceptionally rare event, or it could be a probable occurrence whenever conditions permit. We don't know, but if we say that the chance of life evolving now that everything is prepared is 0.05, then there are half a million planets in our galaxy supporting life.

Whittling the chance that these planets will evolve intelligent life will be the disasters and mass-extinctions that life cannot recover from. Asteroid impacts, global glaciation and supervolcanoes are potential culprits, and may account for up to 20% of all life-bearing planets permanently losing their inhabitants, or at least preventing the formation of complex multi-cellular life. In fact, the evolution of eukaryotic cells (those containing complex structures like mitochondria or chloroplasts) from prokaryotic cells (those without a nucleus or other complex structures) took many millions of years on earth, and is by no means necessarily inevitable. It may not happen on many of these worlds at all, and they may never see multicellular life. Webb estimates that one in forty life-viable planets might have the conditions that conspire to permit multicellular life to evolve from single-celled ancestors, but he cautions that this (like all of the other variables) is merely a guess.

Finally, in order for these planets to bear the kind of advanced, abstractedly intelligent civilizations that could have developed the technologies to contact or visit us, Webb believes that the remaining planets need to have the conditions necessary for complex language to develop – the crucial enabling step which allowed our species to truly master technology. This final criterion is perhaps sufficiently unlikely to evolve that it has only ever happened once, here on Earth. In this scheme, there are still planets in our galaxy where life is common, but it is frequently only unicellular, rarely multicellular, and uniquely rarely intelligent. This model of the universe leaves us with many fabulously diverse extraterrestrial ecosystems to explore, but ultimately alone as an advanced intelligence. It neatly explains why we have never heard nor seen an alien.

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