- What are the requirements
for life in the universe? Perhaps it is a fussy,
punctilious phenomenon. One that only occur in the rare cases where conditions are absolutely perfect. An event that demands
each and every molecule to be precisely aligned. Or perhaps life starts with relative ease, just throw together a few
random organic molecules and life inevitably spawns. Each of these possibilities
is broadly consistent with our current data of
life, which really is just us. All life on earth is genetically related, distant cousins harking back
to a single microbial ancestor known as LUCA, the Lowest
Universal Common Ancestor. So despite the biodiversity
that we see today, we're all just descendants
of this one success. Maybe there were others
that eventually fizzled out, but for now we have no
evidence for such a claim. Crucially, the fact that
there was at least one success doesn't actually teach us anything, because we already know that. Of course there had to be one success or else how could you and I
even ponder such questions? Life could be incredibly
rare in the universe. And if so, it's no coincidence
that we happen to live in one of the rare
places where it happened. Because for living
things such as ourselves, there could simply be no other way. This means that our mere existence, nor indeed that of the
entire tree of life, does not help us in trying to answer this grandest of questions. How often does life get started? If we were to one day
discover a second tree of life then that would prove that
life started at least twice. More importantly, that second start would have nothing to do with us. Our existence is not dependent upon whether it happened or not. Such a discovery would
break the ambiguity. It would likely be the most
important scientific discovery in human history, proving
that we are not alone. Of course there has
been many claims of life throughout the years,
living seas on the moon, engineered clouds on Mars,
fossil-like Martian meteorites, Venusian bio-signatures, and of course, claims of alien contact or sightings. Yet none of these past claims have crossed the scientific bar needed to call them a
definitive discovery. A reproducible set of observations with an unambiguous interpretation. The Martian canals is a
particularly interesting subject that we've discussed in
detail on this channel before, illustrating how easily
our human preconceptions can interfere with an objective analysis of data pertaining to alien life. Of course, an absence of evidence is not evidence of absence either. Life could be incredibly rare or very common or anything in between. So again, our existence in of itself doesn't provide any
useful information here. In fact, trying to use it in such a way actually forms a circular argument. Because of course, our
ability to ask these questions is predicated upon at least one success. A much better piece of
evidence is the timing of when life emerged on the earth, occurring cosmically quick, within just a few hundred million years. In one of our previous papers and videos I showed how this quick start is also still burdened with ambiguities. Yet overall, it suggests that if we had the ability to somehow rerun the clock on
earth another time over, then we'd give nine to one betting odds that life would indeed
reemerge quickly once again. If you're comfortable calling
nine to one a safe bet, then this single data point seemingly settles the debate, right? Re-run Earth's clock over
again and life would reemerge. Therefore life should be
common across the universe. Well, it's that second step where we need to tread very carefully. Because remember I'm
re-running Earth's clock here, so that means that a
planet with essentially identical conditions to the earth, the same orbit, the same mass, the same initial atmosphere,
even the same locations of each and every pebble
on the planetary surface. But if I move one of those
pebbles or one of those volcanoes or one of those continents,
does that screw things up? Like the butterfly effect,
perhaps even a small change might drastically change the fate of life to a potentially far more dire outcome. This is where the famous Rare
Earth Hypothesis comes in. First formulated by Professors Peter Ward and Robert Brownlee, the Rare Earth Hypothesis has now become a highly influential and
debated topic in astrobiology. As the name suggests, it
argues that the conditions of our planet are incredibly unusual. Finely balanced just right for life. If true, the idea that life starts quickly with Earth-like conditions
becomes almost irrelevant, because those conditions
themselves are so rare that life hardly ever gets
a chance to get going. Or a different take on the idea, and one that is more consistent
with original formulation, is that precise Earth-like
conditions are necessary for intelligent life in particular. Perhaps there's microbes on Mars, but if you want a civilization, you really need something
that's very similar to Earth's. So what exactly is it that
Ward and Brownlee suggest to be so special about our home? There's the fairly obvious
ones, like mass and size. Too small and the planet's
gravity weakens so much that it can't hold onto
a substantial atmosphere. Too heavy and the planet's higher gravity risks runaway growth into a giant planet. The next obvious factor is
being the right distance, from your star that liquid
water is stable on the surface. When the book was published in 2000, well, we really didn't know much about either of those factors. But of course in the years since, exo planet discoveries have amassed and now we finally do have some insights. For example, we now
know that rocky planets are common around typical stars. However, we still don't yet know how common are rocky planets on Earth-like orbits
around sun-like stars. You can check out our
previous video to learn why. Rare earth goes much further than just mass and orbit though. To gain some insight, I'm gonna highlight four key additional factors that are often invoked
by rare earth proponents. Factor one, a habitable
planet needs to have a magnetosphere to protect it
from high energy radiation. This effect is coming into recent focus as we consider sending astronauts to the moon or Mars on a long term basis. The Earth's magnetic field protects us from ionizing radiation,
deflecting charge particles along it's field lines. Since the moon and Mars don't have an intrinsic global magnetic field, this ironizing radiation poses
a health risk to astronauts leading to cancerous cells
and radiation sickness. Of course, one solution is for astronauts just to hunker down under some regolith, either in caves or throwing
sandbags on top of their base. In fact, the earth
essentially does this for us by virtue of its much thicker atmosphere. Indeed, renowned
Astrobiologist, Jim Kasting has argued that Earth's thick atmosphere is in fact sufficient to protect us even without a magnetic field. An idea which has support from the fact that life clearly persisted during Earth's frequent
magnetic pole reversals. So perhaps the field isn't
so crucial after all. Moving on, factor two
is that plate tectonics are necessary for life
and may also be rare. The main thrust here is
that bio geochemical cycles of rock, nutrients and chemicals are enabled by the subduction of crust. When an oceanic plate
subducts under a continent, we essentially recycle chemicals locked up in rocks, the fossils, and oil. The elements within them are liberated under the intense heat and eventually re emerge
through volcanic activity. It is thought that this recycling is an advantage for biology, maintaining a fresh supply
of life's key ingredients. However, here too, there's been pushback. Some arguing that earth mass objects inevitably form plate tectonics, and that even without it, you
can still have active geology. For factor three, we turn to our moon. Our moon is freakishly large compared to the planet
that it goes around. Whereas most moons in the solar system are no more than 1/10000th
the mass of their host planet. Our moon is 100 times greater, at over 1%. A consequence of this is that it has an oversized
gravitational influence on us, most noticeable through the tides. But going deeper, it's also thought to stabilize the axial tilt of our planet, which is locked in at an angle
of 23 and a half degrees. It was Jacques Laskar
who advanced this idea in a seminal paper in 1993. But since then, astronomer
Jack Lissauer has pushed back, suggesting that stable obliquities can occur without a big moon too. A large moon could help in other ways, too such as generating huge
constant covering tides shortly after its formation. And perhaps even kick
starting plate tectonics via the giant impact that formed it. But in both cases, it's deeply unclear how crucial these are for life. The final factor I'll
discuss is death from above. Impacts from comets and asteroids that could extinguish life on the earth. Clearly, too many of
these is bad for life, although it's also been suggested that not enough is a problem too. Perhaps sometimes evolution
gets stuck in a rut and needs a clean slate, a
reset to make progress again. In this context, Jupiter
has been suggested to play an important role, perhaps protecting us from giant impacts. For example, in 1994, we
watched from the earth as Jupiter took one for the team. As the 1.8 kilometers Shoemaker-Levy
comet smashed into it. However, simulation work doesn't provide such a clear picture. In fact, astronomers Horn and Jones found that Jupiter actually led to more meteorite impacts
on the earth, not less. So as you can see, there has
been considerable pushback about the validity of
these individual factors. Now, rather than doing a
deep dive on each of these, I think it's more useful just to ask how can we combine these together? Can we somehow use them to calculate just how rare earth is? It was John Kramer who suggested
that factors like these who be combined together to build a so-called Rare Earth Equation. It computes the number of
Earth-like planets in the galaxy, NE, as a function of the various inputs. So as an example, we could
write something like this. Where FP is the fraction
of stars with planets. FR is the fraction of those
planets that are rocky. FH is the fraction of those planets that are in the habitable zone. FM is the fraction of
those with a large moon. And FJ is the fraction of those with a Jupiter-like companion. Five factors and yes, I'm ignoring the geology factors in this example. Notice that by construction, each one is a subset of the previous. So FH for example, is not the fraction of all planets in the habitable zone, but rather it's just for the rocky ones. This is known as conditional probability and it's gonna be important to remember. Now this sequence of multiplicative, conditional probabilities
very much resembles the famous Drake Equation, which calculates how many
civilizations are in the galaxy. In fact, the relationship is even deeper because the Drake Equation
has an explicit parameter for the fraction of
planets that are earths. So that means that we could
replace this one Drake term with the latter four factors from our Rare Earth
Equation just like this. In effect, then, the Rare Earth Hypothesis can thus be seen to kind of
expand that single parameter. Either earth at the Drake Equation into a myriad of finer grain parameters that are necessary for true earth analogs. I think presented like this, it ostensibly seems
like an upgrade, right? We're improving the Drake Equation we are posing a refined
mathematical version of it. But is really true? Certainly one advantage
of this mathematical form is that it's very flexible. Look, you might disagree that a large moon is necessary for life, but instead think that a
sun-like star is needed. Okay, fine. You can easily just swap out
that factor in the equation which would still leave you
with four rarer factors. That would give us this equation, which we'll call scenario A. Alternatively, perhaps you think that the earth is a far more rarefied and finely-tuned concept
than these four factors alone can allow for. Perhaps you add on a parameter for the solar system's galactic location and then another for the
plant's initial atmosphere and then another for
the land to ocean ratio, et cetera, et cetera. Eventually, maybe you end up with a list of say 80 factors, 80 distinct properties, that are all simultaneously necessary for a plant to be truly habitable. Let's call this scenario B. So what is the effect
of having four factors in scenario A, versus 80 in scenario B? Well, for simplicity, let's imagine that each of these factors has
a 50% chance of being true. So for example, the first factor is the probability of a planet
being rocky, which is 50%. Which coincidentally, is
about the right answer. In scenario A, we have
four rare earth factors. So the final probability
of getting an earth is one half multiplied
by itself four times, which we can write is one
half to the power of four. Working that out, that gives us a final probability of 1/16th or 6.25%. Okay, so unusual, but not extremely rare. Now let's try scenario B. With 80 factors, we have to do one half, but now to the power of 80, which equals one in a trillion trillion, or 10 to the power of -24. That's a number so ridiculously small that even considering every single star in the observable universe, you still wouldn't get
a single other earth. Of course, the choice of
a one half probability in that example was entirely arbitrary. The real point of that little exercise was to highlight just how important the number of factors we
include in the equation are. I think often we get hung
up arguing about the values of those individual parameters, Should they be 1% or 10%? But in actuality, it's
often far more important to argue about how many parameters should go into that
equation in the first place. Now, of course, we don't really know how many factors to include. Remember that these factors are intended to describe the different
conditions necessary to make a planet capable
of supporting life. If we had a proven, universal
theory for the origins of life then we could figure out
the necessary parameters on the blackboard ab initio. But we don't. We know hardly anything
about how life began. In the absence of that, one might try looking at our own planet as a single data point and trying to extrapolate,
reverse engineer, what the conditions for life
in the universe truly are. Of course, trying to extrapolate off a single data point
is incredibly precarious. And indeed, this is what
many critics have suggested that Ward and Brownlee are really doing. And whilst they have reasonable arguments for the factors they
consider to be relevant, we've seen how there's considerable debate about them as well. Furthermore, there's no
way to test these claims until we start to detecting
life elsewhere in the universe. And I think this is where my
own concerns come into focus. The whole philosophy of
reverse engineering ourselves as an idealized example
of life in the universe is inherently, inherently,
anthropocentric. It tacitly endorses a narrow perspective. One that assumes that the
way things happened here is the only way which
intelligent life can emerge. That our experience, our
history, is the sole channel through which sentiants can blossom. And look, we're all often guilty
of this in day to day life. We base much of our philosophy of life upon our own direct experiences, ignoring the very disparate,
often painful experiences that our fellow human
being is voyaging through. A full understanding of
life, both individually, and in terms of biology, requires a larger view,
a wider perspective. That might sound like I'm waxing a little over philosophically here. And sometimes I'm indulgent of that. But let me just illustrate a specific mathematical
consequence of this when it comes to the origins of life. Look, I can buy the argument
that the origins of humanity are indeed the product of a series of highly improbable events. First, you need X, then you
need Y, and then you need Z. And as we add more and more terms, yes, I agree that probabilities
become vanishingly small. But crucially, that recipe may not be the only path to intelligence. There may be entirely
separate, disparate avenues to arrive at a civilization. So for example, even
though a hypothetical world might never have had a magnetosphere, it was a moon orbiting a giant planet. And thus, protected by the
giant's magnetic field instead. A completely alien setup,
yet one which nevertheless, could eventually lead to intelligent life. Equations like the Drake Equation and the Rare Earth Equation assume an inherently
monolithic, singular pathway. Because remember each
term in that equation is conditional upon the previous one, forming an ever narrowing funnel towards something which
increasingly looks like us. But really, the search for
intelligent life in the universe, isn't just a search for our doppelgangers, it's far broader than that. And so perhaps we should imagine adding some parallel
paths to those equations. For example, let's imagine that the planet had failed to form a large moon. That doesn't mean that the doors to intelligent life are closed. In fact, that closed door
might open up another one. Another pathway towards intelligence that is distinct and
alien to that of our own. In this way, we would have
to add parallel versions of the equation to itself. We no longer have a series
of multiplying factors. No, now we have a sum of a
series of multiplying actors. Perhaps there's a few
paths, maybe just one, or perhaps there's a countless number. We simply don't know. And so, because of this, I
personally don't subscribe to the Rare Earth Hypothesis. Now you might jump to the conclusion that I therefore believe there are lots of earths out there, right? I mean, if I don't believe in X, then surely that means I believe in the opposite of X, right? Sadly that's often become
the kind of dichotomous logic that the modern media often
tries to drum into us. But it's a false dichotomy. In science, it's okay to remain
agnostic about a question until evidence is presented either way. And right now, we have no evidence. So I indeed withhold my belief. I'm not a skeptic, I'm not an advocate. I just await the result
with keen interest. As I've spoken about
on this channel before, I think this philosophy
is deeply important. Consider that you tell yourself and others that you believe in say,
the Rare Earth Hypothesis, despite the fact we have really no compelling evidence for it. Okay, fine. But now you face a
possible day of reckoning when the data comes in, then maybe you see that you were wrong. And so often, so often there is nothing that we find harder in life than to admit that we were wrong. Especially if we express
that view publicly. Instead, there is a tragic tendency to manipulate the information. To devise ever more elaborate narrative-fitting interpretations and to seek out any snippet,
any sound bite we can find to agree with our initial preconception, even sometimes at a subconscious level. Because admitting that
you were wrong feels bad. But believing that you
were right feels good. And look in a way, I don't
really need to tell you this because hey, we all
live in this modern era. But the real point is
that even in astronomy, when it comes to questions
such as how often life begins, or the abundance of
civilizations in the universe, we have essentially no data. And so it's okay to withhold your belief until you see evidence either way. You don't have to plant a
flag on issues with no data. We'll get to the data on this one day, but it may be a very
long road ahead of us. And yet, when that day comes let's make sure that we leave
our anthropocentric bias, our preconceptions,
our faith, at the door. And just listen into what the universe is saying to us with
open ears and clear eyes. So until next time, stay
thoughtful and stay curious. (galactic music) Hey, thank you so much for
watching this video, everybody. This video is supported
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of our latest supporters, that's Joseph Alexander
and a special thank you to one of our new Illuminati-level donors, that is Mike Hedlund, thank
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