We are alone in the universe (part 2)

You are probably wondering what evidence one could possibly be brought to the table that would suggest that we are alone. If you are wondering this, you are in the right place. Get ready to rumble.

From part 1 we learned that the Principle of Mediocrity suggests that the frequency of complex earth-like life is an indicator for the frequency of intelligent life regardless of how exotic extraterrestrial life ends up being. This is because humans are much more likely to be an easy way to evolve intelligent life rather than a difficult way. We are not special like a snowflake; we are mediocre. But, that’s OK. Actually, that’s a good thing for this analysis.

The central question now is: what do we think is the frequency of complex earth-like life? The general feeling of space enthusiasts is that our universe teeming with microbes and with intelligent civilizations popping up a few times per galaxy or so. Feelings and guesses are fine and dandy, but there is a more reasoned approach that has concluded that earth-like complex life is incredibly rare. This is the Rare Earth Hypothesis. The analysis goes like this: we can see to produce complex life and intelligent life on Earth, several factors were important including:

  1. Galactic habitable zone – not too close to the central black holes which emit gamma radiation, not too dense region of stars which poses danger of supernova and gravitational perturbations
  2. Favorable star – must have adequate lifespan for evolution
  3. Planet in Goldilocks zone – allows for liquid water
  4. Good Jupiter – protects from asteroid impacts (Bad Jupiter refers to a gas giant in a closer orbit to the sun than the earth and would cause detrimental gravitation perturbations)
  5. Stable orbit – for climate stability
  6. Planet composition – need solid surface in addition to oceans of water
  7. Plate tectonics – for carbon cycling and greenhouse effect
  8. Magnetosphere – protects from harmful radiation
  9. Billions of years of stable climate – don’t freeze or have runaway greenhouse effect, just look at Mars and Venus to see what could have happened to Earth
  10. Abiogenesis (or panspermia?)
  11. Abiogenesis occurs early in planetary life
  12. Not too many mass extinctions
  13. Other factors

Any individual factor is not likely rare in itself (except the factors which must stay true over long time periods). For example, we know extrasolar planets are not rare. Statistically every star has at least one planet. Also, planets in the Goldilocks zone are not rare. According to Kepler Space Telescope data around 20% of stars have rocky planets in the Goldilocks zone.

The lesson we learn here is that it’s not that individual factors are rare, it’s that the combination of factors is rare. It’s like rolling a cosmic dice over and over and having to get the right combination of numbers by chance. Chance and coincidence are at work here. There are 12 factors listed above, but how many factors are there really? There could be far more, but we don’t know for sure. Furthermore, we don’t know the frequency of each of these factors yet.

Let’s perform some calculations to get a feel for how chance will affect the frequency of complex earth-like life. Looking out at the observable universe there are about 100 billion galaxies each with about 100 billion stars. That means there are 10^22 stars in the observable universe! Now, let’s make some assumptions for the sake of analysis. Let’s assume there is one planet per star. Let’s further assume there are 22 individual planetary factors (like the 12 listed above) necessary for complex life and each factor has a 10% chance of occurring. How many planets will harbor complex earth-like life? With these assumptions, only one single planet in the whole observable universe will! What if we increase the average frequencies of the factors to 20%? There will be just over 4 million planets with complex earth-like life which is far less than one per galaxy. What if we increase the number of planetary factors to 400, what would the average frequency need to be for just 2 planets with intelligent life? About 88%. Doing these calculations is constrained by our starting assumptions, but this exercise is helpful because it shows us how the universal lottery may require substantial luck just for a few planets in the observable universe with complex earth-like life.

The thing that pushed me over the edge in this discussion is the factors which must remain true over very long time periods. Complex life is very fragile and that is evidenced by the extinction of so many species. How many dinosaurs have you seen today? If you go to the Creation Museum then Adam and Eve walked alongside dinosaurs, but the fossil record completely fails to support this. The dinosaurs were wiped out during a mass extinction event around 65 million years ago partly caused by a 10 km diameter asteroid slamming into the Earth causing severe climate change. About 50,000 years ago there occurred a similar event called the Toba catastrophe theory which nearly wiped out all of humanity. It is thought that the human population was reduced to around 6,000-10,000 individuals! How lucky are we to have persisted? These kinds of extinction events are common in the fossil record, and even more interesting may help accelerate evolution by opening up niches. Evolving complex life may require a delicate balance of extinction and speciation. But, how often does a delicate balance happen by chance in the universe?

Astrobiologist, David Waltham thinks that the most lucky feature of our planet is its 4 billion years of climate stability. Think about our neighboring planets who probably started out with compositions similar to that of Earth. Due to the sun’s gradual increasing solar output and a runaway greenhouse effect, the surface of Venus is more than 400 C, far too hot for earthly life of any sort. Even extremophiles would find this to be hell. And, Mars once had oceans of water and possibly life, but now is a freezing desert and bombarded with lethal doses of radiation. It may have pockets of microbial life, but certainly nothing complex like on Earth. We are lucky to have enjoyed such climactic stability.

How often does abiogenesis occur? How often do earth-like planets fail to produce complex life from simply life? How often on earth-like planets does extinction events set back evolutionary progress? How often does a planet enjoy 4 billion years of climate stability? If your answer is, “Not very often” then you might be a proponent of the Rare Earth Hypothesis.

We are alone in the universe (part 1)

Suppose there are many different ways for the universe or multiverse to evolve intelligent life. There will be easy ways to evolve intelligence and difficult ways, and these ways will fall on a spectrum as such. In fact, it is reasonable to suppose that this spectrum can be plotted as a frequency distribution and will be a bell-shaped curve. Where would humans fall on this curve?

Before answering this let’s pay homage to the debate of the Anthropic Principle. This principle states that the universe is geared towards producing us. It is derided by modern scientists because it seems like cosmic hubris. Since the time of Copernicus, we have been moving away from this thinking starting with the heliocentric model of the solar system. In keeping with this trend the latest proposal is the multiverse which solves the problem of fine-tuning of physical constants. This change in thinking is called the Copernican Principle, or Principle of Mediocrity, and would suggest that we are most likely an average way to make intelligent life. We are not found at the tail ends of the bell curve, rather smack in the middle. Earth-like biology is probably a rather easy way to make intelligent life in this universe/multiverse. Applying the Principle of Mediocrity, the frequency of earth-like complex life, is a surrogate marker for the frequency of intelligent life in the universe. That is very important because we can actually say something about the possibility of earth-like life out there. What does science say? How difficult is it to make earth-like life?

If you think we are in an infinite multiverse where all possibilities become actualities, then this question might be of less importance to you. Because even if it’s one in a zillion zillion, there ought to be an infinite number of earths out there in the multiverse. This is theoretical physicist Brian Green’s take on the matter. There is another Naïve Thinker out there but who is actually the President of Mars, but this doppelganger must be almost infinitely far away. If you are going to be this generous with reality, you will run into a problem. If absolutely everything possible is actualized, then God must exist. And, an all-powerful being would also be God of the whole multiverse. Also, the Flying Spagetti Monster would exist, but God would eat it for lunch. Alright, alright come back down to reality now! This escapade proves the point that we should not be too generous. Such bizarre notions of the possible do not respect the elegant universe we can actually observe, and it’s not a multiverse. . . yet. And, if we eventually find we are in a multiverse, it will not necessarily be infinite. How could we even prove that it is infinite?

Barring an infinite multiverse which I think is reasonable, how difficult is it to make earth-like complex life? We will address this in more detail in part 2.

The Gaia Hypothesis and Extraterrestrial Life

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.