The Rare Earth Hypothesis

Enrico Fermi was a genius, and his mind worked lightning fast. When his co-workers were audibly speculating on how many alien civilisations might live in our galaxy, he simply looked up and asked: “Where are they?”

What he saw, so quickly, was that with about 200 billion star systems in our galaxy, and with about 20 billion estimated to be at least 3 billion years older than our own, if alien civilisations were common, some of them would already have colonised the entire kit and kaboodle to the point of overpopulation. So, where were they?

This discrepancy, between speculated large numbers of civilisations, and the lack of any sign of any of them, has since been called the Fermi Paradox. Where are they?

Let the weirdness begin! A wide range of amazing and unbelievable explanations have launched themselves into the literature, to explain why we have seen no sign of aliens, in spite of the almost statistically certain conclusion that any alien presence in our galaxy should have resulted in them being everywhere a long time ago. For example, would you believe the hypothesis that they were all stay at home philosophers who spent millions of years contemplating the whichness of the why? Or that all civilisations are doomed to extinction after a very short time?

I do not believe a word of it. What I suspect is overlooked is variability between species. How much? We do not know. We are limited in our studies to the sample size of one. But what do we see on our single sample? We see ‘intelligence' in so many forms. Squid. Octopus. Various parrots. Various members of the crow family. Elephants, Cetaceans. Various carnivores. Great apes. The variation between different reasonably intelligent species is incredible. How much more will alien species differ from each other? To suggest they must ALL die out in a short time is just silly.

Nor will this vast range of potential variants all want to stay at home. Here on Earth, evolution has equipped living things with a big collection of different mechanisms to drive dispersal of populations. With intelligent species, it is logical to suspect that most, if not all, will have behavioural adaptations to ensure dispersal. Among humans, we see a wanderlust and curiosity about other places that is so very common. To suggest that all alien species will not want to colonise the wider galaxy seems an unlikely conclusion.

So why no aliens? This is where the Rare Earth Hypothesis arises, which I personally think is the most likely explanation. Rather than suggest that lots of alien civilisations all mysteriously fail, is it not more rational to suggest that those civilisations simply do not exist? This is the Rare Earth Hypothesis.

The name comes from a book written by scientists Peter Ward and David Brownlee of the University of Washington. The full title is “Rare Earth, Why Complex Life is Uncommon in the Universe.” Their thesis is that the conditions required to develop advanced life are special and found on very few planets. So advanced and intelligent life will be very, very rare.

What conditions? Well, looking at our sample of one, it appears that a suitable environment for abiogenesis must be coupled with time to evolve, four billion years of reasonable stability with an environment suitable for life, but with enough driving influence to cause life to evolve over that time. Abiogenesis. Four billion years of stability. Factors driving evolution.

The “Goldilocks Zone”

Any good hypothesis will generate testable predictions. What can we predict from this one? The obvious prediction is that the star system and the planet we live on will be special, different to other ones in our galaxy. Our Earth must be rare. Fortunately, we are now in a position to gather data to support or refute that prediction. Studies of other star systems and exoplanets are now becoming sophisticated enough to draw at least a few conclusions.

Let's take a look at the factors we may need for life to develop. I have drawn up 20 such ideas.

1. The young sun must spin slowly to reduce solar flares that might otherwise blast volatile materials off the young Earth.
2. The Earth must have a reasonably stable and consistent rate of geothermal activity, from plate tectonics, over 4 billion years, yet fail to produce massive or common supervolcanoes.
3. The system must have orbits close to circular, to avoid excessive temperature swing from season to season. Results to date show that such orbits are rare.
4. The parent star must be reasonably stable over 4 billion years. No major outpourings of radiation, unlike the closest next star (Proxima Centauri) which emits substantial levels of ultraviolet and X-rays from time to time. Our sun is unusual in having such stability.
5. The world must have a large moon to stabilise its spin. Earth is unusual in this, being almost a double world.
6. A large planet like Jupiter is needed, positioned well out to mop up bodies of rocks that might otherwise be a threat. This appears to be rare, with gas giants often close to the parent star in other systems.
7. A position somewhat distant from the galactic core to avoid intense radiation and regular bolide collision, but not so far out as to reduce metallicity.
8. No tidal lock, such as we would expect from a planet close to a red dwarf.
9. An orbit within the Goldilocks zone, where water is liquid.
10. The planet must be rocky, of the right size and chemistry.
11. No large planets near the life bearer, which would perturb its orbit.
12. The parent star must be long lived to permit 4 billion years of evolution. Blue stars, for example, have relatively brief lives.
13. The planet needs a good magnetic field to divert damaging radiation.
14. Sufficient environmental variation to stimulate evolution, but not so much as to be a threat.
15. No planetary collisions during the time of evolution. Such collisions may not be uncommon. For example, Earth appears to have been struck before life began, by a body now called Theia.
16. The parent star must never pass close to another star. Over 4 billion years, this may be difficult. Our sun and solar system orbits through a denser spiral arm about every 100 million years.
17. The parent planet must have reasonably rapid rotation to evenly distribute temperatures.
18. The parent galaxy must also be stable. This is true for the Milky Way, with few collisions with other galaxies.
19. The central black hole must not have too much or too little activity.
20. The parent star must be solitary, not binary.

Much is unknown. This list is going to be incomplete, and some of the conditions listed are probably not truly required. More data is required. The following argument may not be valid but allow me to pluck some numbers from the ether to illustrate the point.

If there are just 12 vital conditions required for life to develop and evolve over 4 billion years, and each condition is found in 10% of cases, then advanced life will be found on just one planet in 50 Milky Way sized galaxies. Thus, anything Earth-like will be very very rare.

It is worth reiterating that this remains “just” a hypothesis. It may not be correct. But it is still the very best explanation I have come across for the Fermi Paradox.