Why We Might Be Alone in the Universe (Script) (Patreon)

It’s so crazy that I just happen to be in one of the rare places in the universe where I don’t instantly asphyxiate or freeze or vaporize or dehydrate. ... Just lucky I guess. Actually, it turns out that our very privileged perspective on the universe from Earth’s comfortable biosphere tells us a lot. For example, it could explain why we don’t see aliens.

It shouldn’t be surprising that we live on a planet that can support our existence, in a universe that can produce such planets. The anthropic principle tells us that we shouldn’t expect to find ourselves in some random corner of the multiverse - there’s an observer bias. In upcoming episodes we’ll be exploring this principle and its two main versions - the strong and the weak anthropic principles. The strong anthropic principle tells us that than observed universe must be able to produce observers - and we’ll get to the implications of that soon - including the contentious idea that this predicts the existence of universes beyond our own. But today we’re going to focus on the weak anthropic principle, although it’s anything but weak. It says that we must find ourselves in a part of the universe capable of supporting us. For example, in a planetary biosphere rather than floating in the void between the galaxies. This may seems tautological, but accounting for this observer selection bias is important to understanding why the universe looks the way it does from our perspective. And the weak anthropic principle is much more useful than that. When combined with the apparent absence of alien civilizations, it may tell us that intelligent life is incredibly rare in our universe.

To get to this, let's think about what it means to be an intelligent observer. Your mental experience of thinking about these questions ... exists. It's happening right now. Your type of mental experience - your type of being - could be incredibly rare - even unique - and only be possible in very unusual environments. But you're having that experience - you ARE that experience. So no matter how rare these you-supporting environments really are - by definition you’re in one. The weak anthropic principle places no limit on how rare those you-supporting environments are. For example, if there’s only one life-bearing planet in the galaxy, or the universe, you’re going to be on it.

The rare earth hypothesis argues posits exactly this - that a range of factors made Earth exceptionally unusual and uniquely able to produce intelligent life. This hypothesis was inspired by some striking observations about our home planet, which I’ll get to - but also by one other piece of evidence. Or, rather, a lack thereof - the fact that we see no evidence for aliens. The Fermi Paradox notes the apparent contradiction between the massive abundance of potential opportunities for technical life to have emerged and spread through our galaxy and the apparent lack of galactic civilizations. We’ve talked about the Fermi Paradox before, and some potential solutions. But let’s consider the possibility that it is exactly how it seems - technological civilizations are exceedingly rare - and maybe that’s because Earth is exceedingly rare planet.

The solution to the Fermi paradox is often expressed in terms of one or more great filters - extremely difficult or unlikely steps in the development from barren planet to visible technological civilization. Such a filter might be after our own stage of development - climate change or nuclear obliteration or whatever - still waiting to wipe us out. But the Rare Earth hypothesis is a little more optimistic - it states that planets capable of spawning civilizations even at our own level are very rare. The idea was named and brought to popular attention in the book by Peter Ward and Robert Brownlee in 2000. It highlights a series of remarkable qualities of planet Earth that may have been needed for life and intelligence to arise here. Let’s take a look at them.

Actually, let’s start with something that is NOT rare about the Earth. Earth-like planets are common. By Earth-like I mean rocky planets about the size of Earth in orbit around stars very similar to the Sun, and at the right distance to sustain liquid water on their surface - in the so-called habitable or Goldilocks zone. The Kepler mission has revealed there should be 10 billion or so in our galaxy - 40 billion if we permit other star types. Billions of potential starting points for life in the Milky Way alone, even if we restrict ourselves to boring old carbon-based water-loving planet life. That’s billions of planets stewing for billions of years - if only one civilization had a tiny head start on us then it could have colonized the galaxy by now.

Unless Earth has special qualities that mean true Earth-like planets are much rarer. Let’s think about what Earth’s has got that seems critical for life and that could be unique. If we see that even one other planet has some life-critical quality, then we know that that quality could be relatively common. But if we’ve only ever seen that quality on Earth then it could be hugely uncommon - and the weak anthropic principle it’s still not surprising for us to find ourselves on one of the few planets with that quality.

We’ll start by comparing planets of our solar system, because our ability to probe extra-solar planets is still in its infancy. Broadly, Earth has two qualities not shared by other rocky planets in our solar system: 1) it has a very dynamic interior and 2) a very large moon.

Earth’s solid iron inner core spins suspended in a molten metal outer core, and this motion generates a powerful magnetic field that protects Earth from dangerous space radiation and solar storms. Above Earth’s core is a solid mantle which still flows due to its heat. This drives plate tectonics on the surface - plates of Earth’s crust float around and are periodic drawn back into the mantle, or subducted. This results in shifting connections between ecosystems. This may have been a critical driver of evolution, promoting biodiversity. The periodic subduction of tectonic plates recycles nutrients from the crust into the mantle and then back into the atmosphere through volcanic activity. Without this biogeochemical cycle, many life-critical elements may have been lost to the biosphere long ago.

So Earth’s dynamic interior seems to be life-critical in multiple ways. By comparison, Mars is tectonically dead and Venus is at best tectonically weak - certainly neither have protective geomagnetic fields. We don’t know whether tectonic activity is rare in exoplanets, but it may be. Which brings us to the moon.

Earth’s moon is ridiculously gigantic - no other rocky planet in our system has anything like it. Its size and also its composition and orbit suggest that it formed when a Mars-ish-sized planet collided with Earth right after its formation. The debris thrown up during this collision became our moon. This could be an incredibly rare scenario, even galaxy-wide. It may also be that our moon and the event that formed it was critical to the development of life.

That impact likely gave Earth its rapid rotation rate - with short nights essential for photosynthesis, and also its axial tilt. A moderate tilt would be critical if seasons are an important driver of evolution. Too large a tilt and seasons become too extreme for life to thrive. Earth’s tilt seems just right - perhaps even rare. The impact may even have kickstarted Earth’s extreme tectonic activity by fragmenting Earth’s early crust into moving plates. And the moon’s later tidal influence may also be an important factor in enhancing ongoing tectonic activity.

And a final possible result of our weirdly large moon is that it enabled the first appearance of life. It enabled abiogenesis. One hypothesis for the first formation of life is that it evolved in tidal pools, with complex chemicals and eventually proto-cells emerging as a primordial soup sloshed in these pools, baking in the sun. Without a large moon you don’t get significant tides, and so no tidal pools. More recently, alternative hypotheses for the location of abiogenesis have gained favor - particularly geothermal vents on the ocean floor.

OK, so earth is weirdly dynamic and has a weirdly giant moon. There’s more. Our entire planetary system is pretty weird. We’ve only figured this out as the Kepler mission wrapped up its census of other planetary systems. Our solar system has a huge range of planet properties - from the tiny rocky Mercury to the gigantic gaseous Jupiter and Saturn. In contrast, the planets of most other systems tend to be all around the same size as each other, and planets as large as Jupiter and Saturn are pretty rare - only around 10% of systems. And yet Jupiter in particular was probably pretty important for the development of life. The planet acts like a gigantic gravitational vacuum cleaner, absorbing a lot of the debris left over from the formation of the solar system. It no doubt sucked up many comets and asteroids would otherwise have hit the Earth. If we’d had a significantly higher rate of mass-extinction-level impacts, perhaps evolution would not have progressed so far. Perhaps life would have been wiped out entirely.

There are a few other possible factors - Earth may have an unusually hospitable atmosphere and water content and have been lucky in avoiding various cosmic catastrophes like gamma ray bursts.

The final thing that may make Earth a cosmic rarity is the path taken by evolution. Perhaps life is extremely common - or at least extremely simple life is. Perhaps the great filter is one or more extremely improbable steps that happened in the evolutionary transition from single-cellular life to complex life, or to intelligence. Just one example of this: the evolution of the eukaryote cell. This seems to have been a freak evolutionary incident, in which two much simpler cell types fused - one absorbing the other, perhaps in a failed attempt at dinner. The absorbed cell became mitochondria, an energy power house that allowed the new chimerical cell to massively increase its complexity, ultimately leading to the first multicellular organisms.

There are many factors that shaped Earth’s formation and development - what if the Cambrian explosion never happened, or the asteroid never wiped out the dinosaurs, or an extra asteroid wiped out our ancestors? There are lots of ways that it seems Earth got lucky. The question raised by the rare earth hypothesis is just how lucky were we? Many of Earth’s life-critical qualities or development steps have not been seen elsewhere, or to have happened more than once - yet. The weak anthropic principle allows that these singular events were phenomenally unlikely - we simply can’t assign them larger probabilities until we get more evidence - which ideally means seeing them happen more than once. It’s very possible that a combination of extremely unlikely factors means it’s extremely rare for planets to spawn intelligence. The Fermi paradox surely has a solution, and that solution may be that the galaxy is as empty as it looks. We find ourselves in the only place we could be: gazing out from our rare earth into the untamed, unpopulated reaches of spacetime.