- It's so crazy that I just happen to be in one of the rare places in our 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 may tell us a lot about our reality, and perhaps resolves the Fermi paradox.

(upbeat electronic music) 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 an 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 gonna 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.

Now this might seem 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 your 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 in the universe you're gonna be on it.

The Rare Earth hypothesis, 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 strang observations about our home planet.

Which I'll get to, but also by one other piece of evidence, or rather a lack there of.

The fact that we see no evidence for aliens.

The Fermi paradox notes the apparent contradiction between the massive abundance of potential opportunities for technological life to have emerged and spread throughout our galaxy and the apparent lack of galactic civilizations.

Now 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 an 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, nuclear obliteration, whatever.

Still waiting out there 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 by something that is not rare about Earth.

Earth like planets are common, and by Earth like I mean rocky planets about the size of the Earth, in orbit around stars very similar to the sun, at the right distance to sustain liquid water on its surface.

In the so called habitable or Goldilock zone.

The Kepler mission has revealed that there should be ten billion or so in our galaxy, 40 billion if we commit other star types.

So 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 of 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 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 says it's still not surprising for us to find ourselves on one of the very few planets with that quality.

We'll start by comparing planets of our solar system because our ability to probe extrasolar planets is still in its infancy.

Broadly Earth has two qualities not shared by the other rocky planets in our solar system.

One, it has a very dynamic interior.

And two, 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 Earths core is solid mantle which still flows due to its heat.

This drives plate tectonics on the surface, plates of earths crust float around and are periodically drawn back into the mantle or subducted.

This results in shifting connections between ecosystems and that 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 from the biosphere long ago.

So Earths 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.

Earths moon is ridiculously gigantic, no other rocky planet in our system has anything like it.

Its size and also its composition in orbit suggest that it formed when Marsish size planet collided with the Earth right after its formation.

The debris thrown up during this collision became our moon, now 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 could be critical if seasons are important driver of evolution.

Too large a tilt and seasons become too extreme for life to thrive.

Earths tilt seems to be just right, perhaps even rare.

That impact may even have kick started Earths extreme tectonic activity by fragmenting Earths early crust into moving plates, and the moons 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 may have 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 tides are half the size so fewer tidal pools.

More recently alternative hypotheses for the location of abiogenesis have gained favor, particularly geothermal events on the ocean floor.

Okay so Earth is weirdly dynamic and has a weirdly giant moon, but there's more, our entire planetary system is pretty weird.

We've only figured this out as the Kepler mission wrapped up its sensors of other planetary systems.

Our solar system has a huge range of planet properties from the tiny rocky Mercury, to the giant 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.

That 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 that would otherwise have hit 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 Rare Earth factors, is we may have an unusually hospitable atmosphere, and water content, and may have been lucky avoiding various cosmic catastrophes like gamma-ray bursts.

But 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 common.

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 matched simple cell types fused one absorbing the other perhaps as a failed attempt at dinner.

The absorbed cell became mitochondria, an energy powerhouse that allowed the new chimerical cell to massively increase its complexity ultimately leading to the first multicellular organisms.

There are many factors that shaped Earths formation and development, what if the Cambrian explosion had 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 Rare Earth hypothesis is just how luck were we.

Many of Earths life critical qualities or development steps, have not been seen elsewhere.

Nor do they seem to have happened more than once, at least 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 space time.

(electronic music)