We are now in the Golden Age of exoplanet science. The Kepler Space Telescope has tantalized us with data showing a rich diversity of exoplanets including the discovery of Kepler-186f which orbits in the habitable zone of its sun and has a radius comparable to the Earth. Kepler-186f is the best Earth-analog we have discovered as of September 2014, and probably just the tip of the iceberg of what is out there. Does life exist outside of our solar system? This blog post examines the possibility of intelligent life existing outside of our solar system focusing on the factor of climate stability. It was first inspired when I read the book Lucky Planet by David Waltham which I would recommend for anyone interested in this subject.
Defining the problem
According to the Solar Standard Model, the sun has dramatically evolved over the course of its lifetime. If we could measure the solar output 4 billion years ago, we would find the output to be about 30% less than today. Around this time liquid water emerged on the surface of the Earth. We have evidence of liquid water on Earth as early as 3.8 billion years ago and hints of life date to as early as 3.5 billion years ago. The question is: how could a 30% dimmer sun coincide with an ocean on the Earth? This question was first asked by astronomers Carl Sagan and George Mullen in 1972 and has been dubbed the “faint young sun paradox”. Before addressing this let’s dig a bit deeper.
Since life began billions of years ago we know that liquid water had to exist on Earth. In fact, the Earth has enjoyed a remarkably stable climate to have continuous liquid water and complex life. The geological temperature record shows the global temperature has remained at 15 plus or minus 10 degrees C over the past 500 million years and with an overall cooling trend. The general explanation given by scientists is that the Earth started with a stronger greenhouse effect which compensated for a dimmer sun. Over time the sun became brighter (following the main sequence for stars) and geological and/or biological processes decreased the greenhouse effect and this cancellation led to a relatively stable climate. But, why did it happen this way?
When thinking about an explanation of the Earth’s remarkable climate stability, there seems to be only two games in town: the Gaia hypothesis or blind luck. We will try to differentiate between these possibilities. The Gaia hypothesis was first proposed by James Lovelock and has been debated by the scientific community ever since. Heavyweights like Richard Dawkins and Stephen J Gould have argued against Gaia. We are left with a nuanced discussion. For our purposes the Gaia hypothesis will be stated as: the biosphere interacts with the environment in such a way to promote a stable climate. This is accomplished unconsciously by climate sensing and feedback systems that exist within the biosphere. There are many criticisms of Gaia, but I want to focus on one that I find particularly convincing.
The Great Oxygenation Event is evidence against Gaia
Several billion years ago the Earth’s atmosphere was largely composed of nitrogen, carbon dioxide, and methane. Then, life evolved photosynthesis which introduced oxygen into the environment. It is thought that oxygen initially reacted with minerals which prevented it from building up significantly in the atmosphere for several million years. Eventually oxygen levels built up in the atmosphere which is called the Great Oxygenation Event. What was the consequence of this atmospheric change? Oxygen chemically reacts with methane and eliminates it. Methane’s greenhouse effect is 30X as potent as carbon dioxide. Since photosynthesis both removed carbon dioxide and eliminated methane, it greatly reduced the greenhouse effect and led to global cooling. Eventually ice at the poles advanced toward the equator and a period of global glatiation began, the Huronian glatiation. We have evidence that the entire Earth was frozen in what is called the Snowball Earth! Almost all life became extinct, but some pockets of life survived. Photosynthetic organisms could have survived under several meters of transparent ice at the equator. Also, ecosystems relying on volcanic vents in the ocean could have survived. What seems to have rescued the Earth from the snowball conditions is volcanism and possibly asteroid impacts which reintroduced greenhouse gases into the atmosphere eventually warming the Earth and melting the snowball.
Now, if we look at the advent of photosynthesis that led to a Snowball Earth episode that wiped out almost all life, we can infer that there was certainly no biological foresight. But, more importantly there seems to be no evidence of a biological climate sensor. If there was a biological climate sensor, it was totally powerless to provide adequate feedback to prevent the cooling. From this we can also infer that there were not significant Gaian mechanisms at work at this point in evolution. Life was lucky that nonbiological forces happened to rescue the Earth from this deep freeze. There are two criticisms to anticipate. First, perhaps the biosphere had simply not yet evolved strong Gaian mechanisms. The problem with this idea is that the biosphere itself cannot be considered life because it does not self-replicate. This is not a matter of semantics, self-replication is required for evolution to occur through natural selection, therefore Gaia will not emerge from evolution. Second, perhaps Gaian mechanisms became stronger by chance after this Snowball Earth episode. The problem with this idea is that it does not help Gaia triumph over blind luck. Lucky-Gaia is just blind luck.
Conclusion: how this alters the likelihood of finding complex or intelligent life
To recap, the Earth has enjoyed billions of years of climate stability, of course with a few blips like the Snowball Earth episode. The sun’s initially lower but steadily increasing output was counterbalanced by a decreasing greenhouse effect, and this was not an inevitable consequence of the biosphere. It seems to be just blind luck. How does this alter the likelihood of finding complex or intelligent life? The probability of complex or intelligent extraterrestrial life is severely diminished in this way because it increases reliance on the universal lottery. Of course, that doesn’t mean it doesn’t exist or that we shouldn’t look for it.