Ever since Copernicus showed us we’re not
the center of the Universe, we have tried to emphasize that Earth is probably not that
special or unique. As it turns out, that might not be case after
all. So today we return to the Great Filters series
to continue our look at the Fermi Paradox. As a reminder, one of the more popular solutions
to the Fermi Paradox, the strange-seeming absence of other intelligent life in our huge
and ancient Universe, is that Occam’s Razor applies in this case and that the reason for
the seeming absence of other civilizations is exactly that, they don’t exist. While we can consider the possibility that
they existed in the past, but all died out after getting technology, the alternative
explanation is that they just don’t occur much. Since there is no obvious reason why they
wouldn’t, we tend to assume that there are, instead, a lot of little steps and conditions
involved in a planet becoming life-bearing and staying that way long enough for life
to get intelligent. Each of those steps and conditions is called
a filter, basically a hurdle you would need to get past that filters out potential civilizations
arising on a planet. Last time we defined a number of different
types of filters, from lesser to greater, the former being ones that most will pass
through, coin flip odds or better, while the latter is more comparable to lottery odds
or less. I also mentioned before that you can often
group related smaller filters into one Great Filter, though you can’t always do the reverse
and break up a great filter into a collection of smaller component ones. This series focuses on possible Great Filters
and while we discussed three last time, we are only looking at one for this episode and
will generally stick to that. Our focus for today is the famous “Rare
Earth” Hypothesis, and more specifically the conditions about Earth and our solar system,
which might make it nearly uniquely suited among planets to host intelligent life and
technological species. I want to emphasize “intelligent and technologically
advanced” right up front, because for the Fermi Paradox we don’t care about some barren
twilight planet hosting simple lichen, not unless that lichen has developed a complex
and enlightened civilization building spaceships and radio transmitters. So while we can look at a place like Pluto
and say “Yes there’s a chance life might develop there in some deep subsurface pocket
of muddy water warmed by nuclear decay or tidal heating”, we can also rule it out
as a place to look for alien homeworlds. There’s just not enough energy flux there
to allow a complex and fast ecosystem that would permit intelligence to arise. Brains are very expensive in terms of energy
and organs demanding resources and have to be constantly of value at every step of an
evolutionary progression. An Oak Tree doesn’t have a brain, for instance,
because it would offer no advantage to its survival, not when it has no means to implement
any decisions that brain makes. So outside of the specific case of a Boltzmann
Brain or something parallel, where the brain just sort of blunders into existence fully
formed, we are justified in assuming only a high-energy environment could offer enough
life to generate such a predator-prey mental arms race and do so quickly enough for it
to have already happened. Evolution takes time, but more importantly,
it takes generations; and more complex critters tend to have longer generational cycles, bacteria
can reproduce hourly, we do not. There have been an estimated 100 billion humans
who have ever lived, or 10^11. We can estimate that maybe a billion times
that many mammals have ever lived, or 10^20, most on an annual or longer generational cycle. Throw in insects who are far more numerous
and have been around much longer, and you can go up about a billion times more, 10^29,
and if we include bacteria, perhaps a trillion times that many, 10^42, have ever lived. Each of those organisms represents an evolutionary
event, mostly a failed or meaningless one, but that gives you an idea how many of these
events need to take place to generate some folks sitting around a campfire sharpening
stone axes and wondering what the pretty points of light in the sky mean. Most of those events had nothing to do with
your genetic makeup, a better designed oak leaf is unrelated to your own DNA, but growing
intelligence is a reaction to the entire ecosystem evolving so it does matter. It’s not hard for an organism to learn to
eat grass properly, they don’t need a ton of brains or sensory gear for that, they need
it for responding to other animals who either want to eat the same grass or eat them. We’ll talk more about the brain race in
the future, but for today I mention it primarily to emphasize how many overall generations
and individual evolutionary events were needed to get here. If that’s fairly normal, then we have to
consider that any planet that can support less life, or requires it to live slower,
is less likely to have gotten technology by now because of fewer of those evolutionary
events. Make the year 10% longer and critters that
breed annually around the food supply of seasons evolve 10% slower, though that is probably
optimistic since they would need bigger food stores to survive the longer winter and therefore
support fewer of those critters to do the evolving. We’ll talk about this more in the future,
but for now it’s important to remember that we’re not discussing where life could exist,
or even where technology might eventually develop, but where it is likely to have already
developed. After all, the Universe is fairly young still. We’ll start there. Once the universe explodes into existence
it takes a long while to develop a structure where Earth-like planets can be common, and
today we are going to try to assume that it almost has to be an Earth-like planet to offer
a good chance for civilizations to emerge. So what do we know about Earth that matters? The first step is to ask about its basic position
in time and space. Earth is not an old planet at all, the Universe
was about 9 billion years old already when it formed. If we compare the entirety of time to one
24-hour day starting at midnight, Earth formed at about 4 o’clock in the afternoon, two-thirds
of the way through the day. Life didn’t crawl up on land till an hour
before midnight. Now you already know that it took a long while
for enough heavy elements to form in dying stars to produce rocky planets like Earth,
but it did not take that long. Earth’s composition relates to the Metallicity
of our own Sun, that being all the stuff in a star besides hydrogen and helium, the only
two elements that existed in any real quantity prior to stars making them. We often also look at this as the ratio of
Iron to Hydrogen in a star, iron incidentally being the most abundant element on Earth,
just beating out oxygen, which is the third most common element in the Universe. The relation of those two is pretty important
as well, since our oxygen-rich atmosphere only came after the iron in the upper crust
of the planet became saturated with more oxygen than it could sequester even with tectonic
activity turning over new iron. Oxygen is obviously very important to high-energy
mobile lifeforms, as is the oxygenic photosynthesis process that supplies all of our food and
releases that oxygen. Oxygen that also consequently poisoned off
the life that lived before then and used anoxygenic photosynthesis or other far weaker energy
sources that don’t rely on sunlight. Likewise, oxygen is an excellent fuel, or
oxidizer, and its sheer abundance everywhere in the Universe should tend to make it the
preferred substance for critters to consume, we each use nearly a kilogram of it every
day. Our sun’s metallicity, Z, is 0.0196, or
meaning just under 2% of the Sun’s mass is stuff other than hydrogen and helium. As I said, we are also often interested in
the iron-to-hydrogen ratio, partially because it matters for planet formation, but mostly
because it’s a lot easier to determine both iron and hydrogen content of stars than most
of the other stuff in them. From this we can make a good guess at the
rest of elements. Metallicity is a bit more intuitive though,
and is what I will use to discuss this. However, if you go hunting for more data you
will see as many graphs with Fe/H on an axis as Z for Metallicity, and it helps to avoid
confusion. So the sun is 2% metals, and so essentially
were all of the early planets forming out of the nebula that became our solar system. That got quite hot and when the sun ignited
it started adding radiation and solar wind to the mix. Planets lost all their helium and neon, the
second and fourth most abundant elements in the modern Universe. They are quite light and easily blown away,
and being Noble gases they’re so aristocratic and snobby they won’t even hang out with
each other, let alone the peasant elements. Hydrogen, oxygen, nitrogen and carbon are
all pretty light too, but they can form heavy molecules that stuck around. However the over-abundance of hydrogen meant
that most couldn’t find any dance partners and so they also blew away. Oxygen is likewise very common and will dance
with almost anyone, so most of the hydrogen that didn't blow away paired up with them
as water. So the warmer planets of the inner solar system
are rocky, not so much because they started out with more of those elements than the Sun
or outer gas giants, but rather because those are the main things that stuck around under
the heat. Their absence from the thousands of exoplanets
we’ve found so far doesn’t indicate rarity, it is just that it’s far easier to see big
planets and those close to their sun, so we see a lot of very big and hot planets. It would seem a safe bet though, that you
need a metallicity decently close to our own Sun’s or higher to get many large rocky
planets. The metallicity of stars is always higher
the younger they are, not because they lose that as they age, but because as the Universe
ages the amount of metals increases as more and more stars go supernova so the younger
newer ones tend to be higher. However, there are plenty of stars younger
than ours with lower metallicity and somewhat older ones with higher metallicity. Most aren’t however, we are pretty much
in the thick of the bell-curve in that regard. You can find a fair number which are a billion
or more years older than ours with higher metallicity, but they start getting very scant
at more than 8 billion years of age. We can, however, safely conclude that systems
with lots of rocky planets with the same basic composition as our own, have been quite common
since at least a billion years before our own came into existence, and perhaps a few
more. We generally divide stars into populations
1 and 2, and really old ones are labeled population 3, based on how much metal they have. That’s a very broad and arbitrary categorization,
so it might be that not all population 1 metal-rich stars like our sun have enough metal or that
some population 2 stars do. Indeed we’ve found planets around fairly
low metallicity stars too. What this establishes however, is that we
can’t consider the metallicity of our own sun and its age to be more than a lesser or
minor filter itself in terms of setting boundaries, though those lesser filters can stack up as
we’ve discussed before. Ditto the fact that we often discard the inner
galaxy as a place for life because of all the radiation and higher star density sterilizing
planets or perturbing their orbits, but the spiral arms of our galaxy are quite heavy
in the metal-rich population 1 stars so this isn’t much of a filter either. It should be noted, though, that the further
you get from a galactic core in galaxies like our own, the less metals you tend to have. So we have a galactic Goldilocks zone, too
close, radiation and perturbation, too far and there are not enough metals, but realistically
it is hard to argue this is much more than a lesser filter. So, that gives us two lesser filters so far,
we’ll treat both as 50/50 at the end when we total everything up, though I suspect we’re
being generous for both and it’s more likely less than 50/50. Of course everything I just said applies only
to stable spiral arms galaxies; when you look at other types, the conditions tend to be
less favorable and as I’ve mentioned on this topic before, you can’t just write-off
other galaxies from the Fermi Paradox. However, for today we will focus on our own
galaxy, so we can use its total stellar population as our comparison number at the end and also
bypass further complicating stuff like quasars, galactic mergers, and so on. Though it is also worth remembering that our
own galaxy is a serious cannibal and has eaten a ton of lesser galaxies, though by and large
this sort of process shouldn’t tinker too much with how habitable various individual
solar systems are. This and other factors have to be considered,
for example, a star’s orbit around a galaxy being stable and not passing through the hazardous
core or too close to big dense pockets of stars. I will add that as another lesser filter and
we’ll call that the “Safe Galactic Orbit” filter. A lot of stars are also binaries or in packs
so dense that they would tend to destabilize stable planetary orbits, and we will make
that our fourth lesser filter, again that’s probably being generous. Now another one that gets suggested a lot
is being hit by a supernova shock wave or a dreaded gamma ray burst, but that needs
some quick context. You need to be very close to one to get a
planet sterilizing event that can kill off even deep ocean life and that won’t be too
common. Nevertheless, if it kills off all large land
life, which would take a lot less impact, you do get a substantial reset. Yes lots of bacteria will survive, probably
even lots of insects, and certainly stuff deep in the ocean unless we got really whammied,
but they will recover fairly quickly and let evolution fill up the missing niches again. Extinction events are hard to classify for
the Fermi Paradox though, that’s because it’s hard to predict how common major or
near-total ones were even in Earth’s history, let alone that of other solar systems in general. Coupled to that, such events can often be
beneficial, clearing and leveling the Darwinian battleground as it were. Dinosaurs were probably already on their way
out, but if that asteroid hadn’t hit, we might not be here today. So I am going to ball up all the various extraterrestrial
extinction causes – supernovae, gamma-ray-bursts, asteroid bombardment, etc into one rather
nebulous filter and that one I will make our first minor filter. As a reminder, lesser filters were the kind
we thought lowered the odds to no worse than a coin flip, while minor was anything that
most didn’t pass through but wasn’t terrible odds, no lower than 1%, and for our final
calculation today I’ll treat it as 10%, again we are using the favorable values. Now not all Suns are the same, and while we
call our sun a yellow dwarf, it is actually one of the most massive stars out there. It got that misnomer because in early cataloging
we could only see the biggest and brightest stars, just as now we can only see the biggest
and hottest exoplanets. Interestingly, the Sun is about as massive
as a star can be and still support life. Stellar lifetime shortens exponentially with
a rise in mass, as its cube, so a star twice our mass lives one-half-cubed, or one-eighth
as long. So if our Sun was even 30% more massive it
would be just about ready to die by now. To make things worse, since a star gets hotter
as time goes by, it’s habitable zone will probably have shifted much more noticeably
during that time rendering some planets uninhabitable that used to be habitable. That is expected to happen here on Earth in
the next billion or so years, even if the Sun’s main-sequence normal lifetime will
still have a few billions years left on the clock. This is not a good filter though, because
only a small percentage of stars are actually massive enough to live so short a time. It matters only if we are assuming life is
limited to other G-type stars. As a G2 star, our sun is fairly large even
for other G-types stars, that scale goes from 0 to 9, with 0 being largest and 9 smallest. Of the seven classic Spectral Types, O, B,
A, F, G, K, and M, only K and M are smaller than G but each of those groups has a higher
population than the other five combined. The habitability of all those stars is therefore
of great interest to us, as they are way more numerous, and we did one of earliest episodes
on that topic, BUT, we actually don’t care about them for today's topic because of what
I mentioned earlier about total evolutionary events. For a civilization to emerge it needs a decently
high number of those total events. It is a regrettably clumsy and ham-fisted
way to look at the issue and we’ll spend more time on it later in the series, but for
now we have to consider that if a planet is tidally locked to a small red dwarf star,
as we have decent reason to think would tend to be the case, that planet has a far smaller
habitable region to support life. Less space, less life, therefore fewer evolutionary
events. Again, we don’t care if they’ve got life,
we care if they’ve evolved into civilizations. We also need to keep in mind that photosynthesis
on Earth does not use all of the sun’s spectrum, and smaller stars emit even less light in
that photosynthetic range. Less useful available energy, less life, more
of the material in that life devoted to basic fuel gathering rather than other survival
tasks, and so on. It doesn’t mean all those stars are uninhabited,
there’s a lot of question marks about tidal locking, atmosphere slowing locking or being
torched away, solar activity being more erratic, and so on, but they aren’t good candidates
to produce civilizations and they make up 90% of the stars. So I will give this one Minor Filter Status
too, though we will also attach to this category things that make stars erratic in their output
as well, our sun is pretty stable compared to most. So we are at 4 lesser and 2 minor filters
so far. How about planetary position? Each star has a habitable zone, we roughly
classify that as any place where water is a liquid, not too hot, not too cold. That’s very debatable since you could have
moons around gas giants that were warm enough by tidal heating for instance, but it is a
decently solid filter. Not because such zones are rare, every star
has one, and we think most stars have planets these days, so one being there isn’t too
improbable. However we have to consider three important
factors on this. First, what are the odds that planet is about
as massive as Earth? Doesn’t matter if some tiny planet like
Mars is there, it could never hold an atmosphere that close to the Sun for billions of years. Doesn’t matter if a big planet like Jupiter
is there, no life is evolving on a gas giant, not to technological civilizations anyway,
although maybe a large moon around one might. That last wouldn’t seem too probable though
since no moon in our solar system is even close to being as massive as Earth. Ganymede is the biggest, actually larger than
Mercury though less massive, and has a surface gravity that’s only 15% of Earth’s. So we have to consider that it needs to be
a planet decently close to Earth’s own size, though how close is hard to say. I said three things though and the second
would be that it needs to be staying inside that habitable zone, most planets don’t
have circular orbits, Earth has one of the lowest orbital eccentricities, Venus lower
still, while Mercury has the highest. Odds are pretty good that the official habitable
zones are already too generous and those near the edges won’t be hospitable for life,
if you add eccentricity into that it gets worse. And of course even if a system has a habitable
zone with planets near it, there might not be one actually in it, so we’ll make that
Lesser Filter 5, no planet of any sort happens to occupy the habitable zone, when the system
forms. For #6, we’ll include the chance that the
planet gets itself knocked out of orbit or captured or ejected out of the system or into
the Sun. Orbits aren’t nearly as stable as we often
think; odds are better than good that not all of the planets we started out with in
our solar system are still here and probably aren’t all quite where they began. Indeed, it seems likely that at least one
time Earth got smacked by one, and it’s also a pretty popular theory that one nailed
Venus causing its peculiarly slow spin, thus having a day longer than its year. Systems with more planets would probably be
even more vulnerable to such things, especially if they had big planets closer in. The mass of a planet in the habitable zone,
if we have one, matters a lot. We do not have a good inventory of planets
yet to say what percentage of them are of any certain size, but we have started finding
a lot of Super-Earths out there in habitable zones or closer so we know there’s no special
barrier against their existing. In between things like Mercury and Jupiter
is a mass difference of about 1000... and you could go bigger or smaller and still have
that habitable zone dominated by that planet so no others could be there. We can’t assume planets are evenly distributed
by mass, that there’s as many planets twice the size of Earth as there are half the size
or ten times the size. Indeed, what we do know suggests otherwise,
but if we did assume that for the moment, then we have a Major Filter right there, because
of possible planets -- from dwarf planet’s just big enough to dominate their orbit to
super-Jovians nearly big enough to count as a star - only a tiny range of those could
plausibly support the kind of life we’re interested in even if we are stretching plausibility. We’ll give this one Minor Filter Status,
our third, though it would not be hard to argue it was a Major Filter all on it own...
we just don’t know and where there is uncertainty, we will err on the side of generosity today. So 3 minor and 6 lesser. I’m going to bypass our day length and axial
tilt because those are very dependent on our moon, arguably so is our crustal composition
since the Moon formed when we got whacked by some other planet, or rather when two planets
smacked each other to form our current one and the moon. For a long time we thought the Moon itself
such an anomaly that it might be a great filter all by itself, but newer models say that such
giant moons might not be so uncommon after all. Uncommon or ultra-rare, our big moon is a
huge factor in all sorts of things that make our planet stable and livable, yet at the
same time many of these factors just seem to better the odds for life not really exclude
it for Earth-like planets without a moon, so while we should probably still list the
Moon as a major Filter overall, I will keep it at Minor filter status. We used to be sure the Moon was a key factor,
as I said, but one of the examples for that is that life began in tidal pools, and the
Moon dominates those, but the sun alone causes decent tides and we also tend to tilt more
to expecting life to have emerged around underwater thermal vents, so I feel we should hedge our
bets and say Minor. Speaking of those thermal vents, our planet’s
tectonic activity and molten core are hugely important both towards providing us with a
protective magnetosphere, as well as ensuring plate tectonics that both give us thermal
vents and help create a geological cycle that replenishes elements on the surface and ensures
there is surface land that hasn’t been eroded away, making the planet all one big sea. Yet while such activity is vitally important,
and probably less common on smaller planets for instance, we’ve already excluded those
and we don’t have too much reason to think there’s a very narrow zone of these permissible
that most Earth-sized planets would lack. These are both very big factors though. A planet’s magnetosphere controls whether
or not it can keep an atmosphere in the long term, and the spin rate of planet effects
that, while at the same time, the faster a planet spins, the stronger the erosive winds
and storms trying to erode away the land. Too much tectonic activity could be devastating,
indeed, considering how many cities were ruined by Earthquakes in the past, if those were
stronger and more common folks might never settle down to build cities… and yet too
little of such activity and you won’t have land masses or the geological pump that brings
up new minerals and helps sequester oxygen early on. Too many uncertainties, so again we’ll be
cautious and call each just a lesser filter, placing us at 8 lesser and 4 minor. There are so many others we could look at,
and many we’ve shoved together into broader filters. There are many more still to come too, we’ve
just looked at those which are characteristic of our planet’s composition and position,
the core Rare Earth filters, but that was our focus for today so let’s total them
up. Remember that we classified a Great Filter
as something virtually none passed, one in a million at best and often worse. We also said that for our totaling today we
would treat a lesser filter as 50/50 and a minor one as 1 in 10. Many of them we looked at today were probably
way worse odds than that, but let’s see how we come out with 8 lesser filters and
4 minor ones. Cumulative odds can just be multiplied together,
the odds of flipping a coin heads is 1 in 2, the odds of doing so twice in row, one
half times one half, or 1 in 4, three times 1 in 8. So our 8 lesser filters are 1 in 2^8, or 1
in 256. Not bad, our 4 minor filters, 1 in 10, are
worse, 1 in 10^4, or 1 in 10,000. Combined together they equal 1 in 2,560,000. Less than 1 in a million, so we establish
a good case for the conditions on Earth to be a Great Filter. Incidentally if we treated those 4 minor filters
as just 1%, our lower end value for minor filters, it would be 1 in 2,560,000,000, not
a million. But, our goal was just to show that it qualified
as a Great Filter, not the worst case scenario. Even that value though would still leave a
hundred planets in our galaxy with those conditions, and our more moderate value would allow for
a hundred thousand. But it does seriously lower the odds for civilizations
to emerge and flourish. Some folks stop here, indeed many of the filters
we used could easily be boosted to Minor Filter status, not Lesser, and in doing so would
smash the odds down to a lot less than one such planet a galaxy, but for my part I tend
to think we are in about the right zone with our figures, and that it’s the filters that
come after this in developing intelligence and technology that help sweep the odds further
down. There’s so much uncertainty to all these
values, and the reasoning behind them, that we just can’t say much for sure. But this is the basis of the Rare Earth Solution
to the Fermi Paradox, and as you can see, it makes a pretty good case for Earth-like
planets that can foster technological civilizations to be quite uncommon and possibly so rare
that you won’t find another one even in the entire galaxy. Next time in this series we will look at the
biological side of things in more detail and see how all those factors can stack up to
form yet another Great Filter, but before that we will be discussing some other topics,
and in next week’s episode we’ll examine the popular science fiction concept of force
fields and see if there’s any room for that to become science reality, not science fiction,
and also discuss some of the fun things you could do with them if you had them. For alerts when those episodes come out, make
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have a great week!
Not one of his best episodes
But he's such a fantastic commentator, and his videos are so interesting