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:
- 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
- Favorable star – must have adequate lifespan for evolution
- Planet in Goldilocks zone – allows for liquid water
- 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)
- Stable orbit – for climate stability
- Planet composition – need solid surface in addition to oceans of water
- Plate tectonics – for carbon cycling and greenhouse effect
- Magnetosphere – protects from harmful radiation
- 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
- Abiogenesis (or panspermia?)
- Abiogenesis occurs early in planetary life
- Not too many mass extinctions
- 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.