- 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 by our Cool World donors.
In fact, I wanna thank two 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 you so much for your support.
If you too, wanna help us out,
you can click the link up above
and get access to special perks.
And until next time, see you around the galaxy.
(upbeat music)