Even with a basic understanding of the universe, one quickly deduces that humanity isn’t
all that significant—in fact, when compared against the vast stretches of space between
stars and galaxies, our existence doesn’t seem all that important in the grand scheme
of things.
But as we’ll come to see today, it turns out that the emergence of life on our seemingly
insignificant pale blue dot and the development of a technologically advanced civilisation
such as ourselves is possibly one of the most improbable events to have ever happened in
the entire history of the universe.
Given the vastness of space and the great age of the universe, one can hardly doubt
the existence of other complex life besides ourselves.
Admittedly, we should be commenting on what is contained within all of that vastness rather
than the size of the universe itself.
Most of what we see is empty space but that’s not really what we’re interested in.
Rather, it is within this empty space that there are billions upon billions of potential
sites for life.
Our Galaxy alone contains upwards of 400 billion stars, each with the potential of housing
habitable planets that could very well contain what is the equivalent of humanity.
That raises the question: what is the likelihood of one of these star systems containing a
second Earth?
What is the probability of life-spawning planets in any given Galaxy such as the Milky Way?
We certainly know that it is possible.
We are living proof that life can emerge; that much is obvious.
And if it can happen once, surely it is bound to happen on other occasions.
Indeed, if we take on the Principle of Mediocrity and assume that there’s nothing particularly
special or unique about Earth; that it’s just a typical rocky planet orbiting a common
main sequence star in an average spiral arm of a galaxy, it follows that there should
be tens of thousands, if not, millions of sites possessing the conditions necessary
for life in our Galaxy alone.
What I’ve just described to you is perhaps the most common response to the age-old question:
“Are we alone in the Universe?”.
But despite its immense popularity and seemingly solid foundations, it doesn’t come without
several inherent flaws.
This argument, that life is bound to arise given enough time and places to do so, is
built upon the assumption that the sheer numbers involved of both the age of the universe and
the number of potential sites for life far outweighs the probability of life beginning
in such places, no matter how small that number may be.
Such that, even if the probability of the origin of life (which is better known as abiogenesis)
is infinitesimally small, the incomprehensible number of opportunities for life in the universe
brings back the scales, so to speak, and evens up the odds.
It thus follows that the Galaxy should be swarming with life everywhere we look given
the time other technologically advanced civilisations have had to emerge before our time and expand
out into the Galaxy.
The issue with that line of reasoning is that we cannot truthfully reach a consensus on
what the probabilities are for abiogenesis for any one planet.
It may be that the likelihood of a planet possessing the characteristics and suitable
environment needed for abiogenesis is so low that none of them do and that Earth is just
a single cosmic fluke, which certainly appears to be the case everywhere we look.
Our ongoing efforts in the Search for Extraterrestrial Intelligence have continuously come up dry
despite having spent a little over sixty years searching—not that technological civilisations
would be immediately obvious.
In fact, there could very well be alien probes in the asteroid belt or on the surface of
Pluto and we wouldn’t know any better.
Having said that, the argument still stands that surely given the clear immensity of the
universe, the Galaxy should be teeming with all sorts of technologically advanced civilisations
or, at the very least, we shouldn’t be alone.
Indeed, our apparent loneliness is rather odd.
As humanity prepare to take on the feat of interstellar travel within the next century
or so, we seem like an infant venturing out into the darkness of a forest that seems a
little too quiet.
This oddity is known as the Fermi paradox and we’ve discussed it in quite a few videos
on the channel.
It can be very easy to fall into the trap of the size argument.
It is, after all, a very attractive one.
However if we are to be entirely truthful and scientific, this argument is, for the
most part, invalid unless we know the exact probabilities or at least a ballpark figure
of the probabilities of two variables.
Firstly, how many habitable planets are likely to crop up within a given galaxy and second,
what number of those habitable planets will abiogenesis occur.
Now while there exists plenty of ambiguity and uncertainty about the probability of abiogenesis,
the rarity of habitable planets within our Galaxy is something that we can definitely
estimate to a far more accurate degree.
Make no mistake though, estimating even just one of two of these variables is a very challenging
endeavour.
After all, we only have a sample size of one to work with; Earth being the only success
story of life that we know of.
As far as we can tell, Earth is the only oasis—a mere pale blue dot—that is bobbing along
in the vast ocean of space.
But just how many of these Earth-like habitats are there dotted across the Galaxy or the
entire universe for that matter?
There is a standpoint that planets like Earth—planets that provide the optimum conditions for life
to arise as Earth has done for us—are incredibly rare.
This viewpoint on the existence of life in the universe is referred to as the Rare Earth
hypothesis, for obvious reasons.
The term was originally coined by Peter Ward and Donald E. Brownlee in their book “Rare
Earth” in 2000.
In its simplest form, the Rare Earth hypothesis argues that our existence is the culmination
of an endless series of highly improbable astrophysical and geological circumstances
and events that just happened to occur in the right place at the right time.
The hypothesis makes a case against the plausibility of complex, multicellular life but does not
argue against the possibility of microbial life being relatively abundant throughout
the Galaxy.
It argues that the environment and evolutionary triggers necessary for the jump from simple,
prokaryotic life to complex life is likely to be very rare—but microbial life in and
of itself may be somewhat common.
In this case, we are technically not alone but our only comfort is the presence of some
simple bacteria which isn’t much, if any, at all.
If this were found to be true, it would resolve the Fermi paradox in the sense that the reason
we aren’t seeing any galactic empires sprawled over every corner of the Milky Way is because
there are none.
The sheer number of stars and planets is no match for the unparalleled unlikelihood of
intelligence and of potential sites possessing the conditions necessary to develop such intelligence.
It wouldn’t necessarily mean that Earth is the only planet that has ever produced
an intelligent civilisation, it could be that such civilisations are so rare and sparse
that the interstellar distances separating them prevent them from ever discovering one
another.
The probability of abiogenesis may be as low as only allowing for one civilisation per
galaxy or perhaps as low as one per our observable universe—in which case, our existence is
a cosmic fluke—or anywhere in-between.
In fact, there could very well be as many as, say, a couple thousand civilisations distributed
evenly across our Galaxy and it would be unlikely that we would ever notice one unless they
sought to actively colonise the Milky Way or contact us at some point in their lifetime—which
leads me to believe that there aren’t any and have never been any since the Galaxy appears
to be about as empty and barren as Antarctica is.
Or perhaps any one technologically advanced civilisation destroys itself before it can
ever get close to colonising a significant chunk of the Galaxy.
The truth of the matter is that we just don’t know.
We just don’t know enough to give a definitive answer on how small the probability of life
spontaneously emerging is and on how many civilisations there may be in our Galaxy and
the greater universe.
We must remember that it is still early days in the world of astrobiology.
We lack any real evidence when it comes to the Search for Extraterrestrial Intelligence
and as I’ve mentioned, our dataset only consists of one instance where life has arose—and
that’s us here on Earth.
A sample size of one is not much to go on and naturally it induces a bias.
If we are to look for other potentially habitable worlds, we are naturally going to look for
ones that are like Earth in every way possible.
Our anthropocentric bias leads us to be narrow-minded in our search for potential sites for life
in the sense that there may be worlds out there, overrun with life, but are so unlike
Earth in every possible way that they go under the radar.
Science fiction has led us to be no stranger to the possibilities of alternative life and
biochemistries, introducing the ideas of silicon-based life-forms, life-forms that may use methane
or ammonia as a substitute for water, and so on.
However, today I would like to remain within the realms of science and not delve too much
into unknown territories such as hypothetical biochemistries as discussions within that
subject are often entirely speculative and opinion-based.
So let’s have a look at just how rare Earth-like planets may be.
Let’s assume for the meantime that life would only arise as it did here on Earth as
a rough guideline, but it need not be in the exact same manner.
So, what are the characteristics of our planet that seem critical for life that we should
be looking for in other potentially habitable worlds?
The first few are concerned with the relative position of the planet in space.
A habitable planet can only be so if it is within the Galactic habitable zone.
Unbeknownst to most, not all regions in a galaxy offer the same chances for life.
There are many factors that must be considered when deciding on a galactic habitable zone—from
the rate of major extinction events in each region to the metallicity of stars.
Naturally, some regions are going to be more active and therefore at more of a risk to
major catastrophic events such as nearby supernovae which could potentially sterilise its surrounding
region of all life.
In addition to this, life may be at odds with gamma-ray bursts and even gravitational disturbances
from nearby stars.
It must also be said that it is unlikely for there to exist a constant habitable zone in
a galaxy like the nature of a star’s habitable zone.
The Galaxy is not static; everything that we see in our Galaxy orbits the Galactic centre;
galaxies are constantly changing shape and evolving—albeit at a slow rate.
In this way, a habitable planet can only support life if its star system is continuously within
these safe regions such that it avoids any catastrophic events.
Within the star system itself, the planet must have a stable orbit at the right distance
from its parent star such that it essentially has no eccentricity and remains within the
habitable zone long enough to sustain liquid water and support life.
This is not as easy as it sounds as habitable zones vary with a star’s age.
For complex life, the star must also be of the right metallicity (which in astronomy,
just means all elements other than hydrogen and helium) and must be highly stable such
that it has little to no variance in its energy output as even a minor change could have a
significant impact on life on a planet within that star system.
The star system must be modelled more or less as the Solar System is, with rocky inner planets
and outer gas giants.
A Jupiter-like gas giant further out seems to be another necessity for life.
Within our solar system, Jupiter acts a giant vacuum cleaner, essentially ‘sucking up’
huge amounts of debris, some of which may have been on a collision course with Earth
and eradicated all life before it got a chance to evolve into us.
With that being said, plenty of debris gets past our Jovian vacuum cleaner and in some
cases, Jupiter can even send asteroids and the like our way—in this way, a star system’s
vacuum cleaner can very much be a double-edged sword.
As for the planet itself, it must be a terrestrial planet of suitable size (roughly between one
and two Earth diameters), capable of holding an atmosphere.
In addition to this, the planet must also have a large moon.
As far as we can tell, the Earth-Moon system is incredibly unique.
Given the dozens of planetary satellites that we observe in the Solar System, how is this
so?
Well, it comes down to the fact that the Moon is strange in that it is relatively large
compared to its parent body.
It certainly is not the largest—there are four moons larger than our moon in the solar
system—but in comparison to their parent bodies, none of them come close to replicating
the Earth-Moon system.
The Moon is hypothesised to have been absolutely essential in the development of life here
on Earth.
If life is only viable with a system similar to the Earth-Moon system, it follows that
Earth-like planets must be incredibly rare.
The Moon is thought to have been the result of a collision between an early Earth and
a Mars-sized body, which was named Theia.
This giant impact and the subsequent formation of the Moon gave Earth its axial tilt and
the perfect rotation rate, especially for photosynthesis.
Without the Moon, the ocean tides would be solely produced by the Sun’s gravity and
would only be half of that of the lunar tides.
The combined presence of both the Moon and Sun allows for tides strong enough to form
tidal pools, which is theorised to have played a very important role in the development of
life.
Plate tectonics also played an essential role as well as there being a magnetic field present.
As you can see, there is a seemingly never-ending list of hurdles on the road that leads to
the emergence of life and potentially intelligence.
The list of characteristics of our place in the universe that made life viable here (that
I just quickly overviewed) is in no way comprehensive and there still is a lot of uncertainty about
these characteristics.
It can often be difficult to make the distinction between what is merely coincidental and unrelated
to our existence and circumstances and events that, in fact, played a huge role in the road
to intelligence.
For instance, with a sample size of only one, we can’t tell if a Jupiter-like planet a
tad bit further out from the planet in question is essential to life emerging in absolutely
every case.
I doubt it is.
There is likely to be a vast number of different circumstances in which life could arise.
I can imagine life around binary stars; life emerging within the confines of a tidally
locked planet; even some form of sentient marine life in subsurface oceans.
I doubt there would be much in common, if anything at all, between us and their circumstances.
In searching for these characteristics that only Earth possesses (characteristics that
we are unsure about, mind you), we may be limiting our search and not allowing ourselves
to see the bigger picture of all the places in our Galaxy where life may arise.
Now that we’ve had a fairly in-depth look at what criteria must be met in generating
an Earth-like planet, let’s take a step back and have a little fun in estimating the
probability of technologically advanced civilisations emerging in our Galaxy more generally without
our anthropocentric bias clouding our view.
For this, we can make use of the famous Drake equation.
Astrophysicist and astronomer Frank Drake constructed this equation to allow us to calculate
a rough estimate on the number of active, communicating civilisations that we could
be hearing from now.
In his equation, he included a series of fractions of different astrophysical, biological and
sociological factors.
As you multiply more and more of these filters for life, the probability gets smaller and
smaller until you’re left with how many communicating civilisations may be out there
with technology betraying the secret of their existence—whether intentionally or not.
I want to emphasise though that there is a lot of uncertainty in some of these factors,
particularly the biological factors, and so this equation is really only meant to provide
an approximation rather than any precise figure.
I’ve altered the equation slightly to make our calculation easier.
The factors are as follows: the number of stars in the Galaxy, the fraction of suitable
stars with a planet in the habitable zone, the fraction of those worlds where life emerges,
the fraction of life-bearing worlds that eventually develop intelligent life, the fraction of
intelligent life that develop the necessary technology to communicate and finally the
average lifespan of civilisations.
First of, the number of stars in the Milky Way.
That’s an easy one.
We know it to be about 300 billion.
Next, the fraction of stars that are suitable for life and have a planet in their respective
habitable zones.
We must eliminate stars with unwanted characteristics such as having too short a lifetime, too little
luminosity or too much variance in their luminosity.
We know stars like our sun—G-type stars—offer great chances of life because, well, we’re
around one.
K-type stars fulfil our needs pretty nicely as well.
G and K stars account for about 20% of all stars in the galaxy and around 10% of them
have an Earth-sized planet situated around them.
It turns out that most stars have at least one planet so this is hardly surprising.
If we say that around half of these Earth-sized planets are found within the star’s habitable
zone, we can multiply these fractions together to give us the fraction of appropriate stars
with habitable planets around them.
This works out to be 0.01 or 1% of all stars.
However this doesn’t factor in other requirements that must also be met—in particular, an
atmosphere and the right eccentricity.
So I’ll just lower f_hp by a factor of two to give us 0.005.
Next, we have the fraction of those planets where life will emerge.
As I’ve mentioned before, we don’t have much of a clue when it comes to the probability
of abiogenesis occurring.
Given that these planets will literally have billions of years in which life can arise,
I’ll say that 50% of these habitable planets will spawn simple life.
As for what number of these life-bearing planets will develop into more sophisticated and intelligent
forms of life, this is also incredibly uncertain.
I’m going to be a bit optimistic here and say that 10% of these planets spawn complex,
intelligent life and I’ll also say that it’s inevitable for each case of intelligent
life to develop the necessary technology to communicate over interstellar distances.
Finally, the average lifetime of radio-transmitting civilisations.
I’ll say that this value is on average around 12,000 years.
We divide this value by the age of the Milky Way—around 10 billion years—to get our
final fraction to complete the equation.
According to these estimates for each factor, we can expect to find upwards of 90 civilisations
in our galaxy alone.
If there’s anything to gain from this episode, it’s that, in terms of the search for extraterrestrial
life and whether we are alone, we just don’t know.
And it’s okay to admit that.
We are, however, making a lot of progress.
Twenty years ago, we hadn't detected any exoplanets; now more than 4000 are confirmed.
If we are truly alone in the universe, it becomes ever more important that we take care
of ourselves and this planet.
It would be a tragedy if the only form of life with consciousness, the only species
capable of art, love and language, were to become extinct before it had a chance to truly
live and flourish.
Comment down below your thoughts on whether you think we are alone in the universe.
Make sure you subscribe for more content on astronomy and futurism.
If you enjoyed this video check out my most recent video on the Fermi paradox and interstellar
travel, a previous video or perhaps my playlist on black holes.
Thank you so much for watching, have a nice day.