It’s Professor Dave, let’s find some aliens.
Throughout this series we’ve achieved a pretty good sense of the size of the universe.
Whereas the ancients thought it was just the solar system and a sphere of bright lights
rotating around us, we eventually understood the true scope of the Milky Way galaxy, with
its several hundred billion stars, many of which probably have one or more planets just
like ours does.
Then about a century ago, we realized that our galaxy was just one of many billions of
other galaxies, and our universe grew to an unfathomable size.
We know of one place for sure that intelligent life developed, that’s here on earth.
But is there life anywhere else?
No matter how unlikely you think abiogenesis is, which is life being generated spontaneously
from non-living matter, even if you think it’s a one in a billion shot, there are
billions of worlds in our galaxy alone, and when you combine infinitesimal odds with impossibly
large numbers, you get near certain probabilities for even the unlikeliest of things.
In a nearly endless universe, there are nearly endless worlds, and it would be hard to believe
that we are really the only ones.
How many intelligent civilizations are out there, trying to understand the cosmos just
as we are?
Where are they?
What do they look like?
What are their problems?
What do they want?
What do they fear?
Do they have art?
Will we ever meet?
I can’t imagine what could possibly be more fascinating than these questions, and there
are a lot of scientists who agree with me, which is why they decided to study astrobiology.
This is the study of life in the universe, where it might exist and in what conditions,
as well as how we might be able to detect it.
Essentially, the search for aliens.
Let’s talk a little bit about what astrobiologists have come up with, and some of the best candidates
we know of for worlds that may already harbor complex life forms.
First of all, what are the conditions necessary for life to exist?
If we think about life on earth, we know that liquid water was absolutely critical.
This was the medium within which the first organic molecules underwent endless chemical
reactions until stumbling upon a self-replicating structure by chance.
No solvent, no chemistry, no life.
So it seems that liquid water is a must.
And in order for liquid water to exist on a planet, the planet must be within the so-called
habitable zone around its star.
That is the range of distances from the star within which any ambient surface temperature
ought to be between the melting point and boiling point of water, which allows for liquid
water to exist on the surface of the planet.
But even in outlining these two simple criteria, if we think critically, we might realize that
we are already getting overly presumptuous.
Must a world truly be within the habitable zone of its star for liquid water to exist?
We already know for sure that this is not the case.
Remember when we talked about the Jovian and Saturnian systems, we saw that many of the
moons of Jupiter and Saturn orbit their planet closely enough that the considerable tidal
forces distort them such that there is a lot of friction inside these moons.
That generates a lot of heat, enough heat to melt ice and get liquid water.
We know with reasonable certainty that a number of these worlds contain oceans of liquid water,
most notably Europa and Enceladus, and so it seems that solar radiation is not the only
way to get an ocean.
Furthermore, who is to say that a liquid medium for life has to be water?
Water works very well because it’s amphoteric.
It can act as an acid or a base, participating in a variety of acid-base reactions that are
biochemically relevant, allowing for biosynthesis.
Would something like ammonia work just as well?
Ammonia could hypothetically perform the same function as water, it would just result in
a kind of life that is stable at a dramatically different pH range.
But we certainly can’t say that it is impossible.
To get even a little more abstract, does life even need a liquid medium at all?
Could life arise in clouds of interstellar dust that are rich in organic material?
It seems difficult to imagine how it would work, but we can’t rule it out with certainty.
Once we realize that life could potentially arise in a variety of different conditions,
we begin to understand that life might be utterly commonplace.
Given our knowledge of the history of Mars, it is not outrageous to believe that simple
unicellular life may have existed there in the distant past, and may continue to exist
beneath its surface.
Mars is literally our next door neighbor, and we also identified multiple moons in the
outer solar system that seem ideal for life.
With so many possibilities just in our tiny little nook of the galaxy, it seems that the
Milky Way must be teeming with life.
Now as to how often simple forms of life evolve to the intelligence of human civilization
or greater is another story altogether.
It might be the case that only one in a billion times does a world with basic forms of life
produce a civilization capable of creating the technology needed to understand and explore
the universe.
So how many such civilizations exist in our galaxy?
Five?
Five thousand?
Or maybe we are truly alone.
What are the odds?
It may seem absurd to try to calculate the odds of such things, but that is precisely
what astronomer Frank Drake tried to do in 1961, with his Drake equation.
It outlines a way to calculate N, the number of active, intelligent civilizations in the
Milky Way, available for communication.
So what are these variables?
First we have the average rate of star formation.
It’s difficult to conceive of life without a parent star, so that’s an important variable.
Next is the fraction of stars that have planets.
Then, of those stars with planets, the fraction of them that can potentially support life.
Of those, the fraction that actually develop life.
Of those, the fraction that develop intelligent life.
Of those, the fraction that develop sophisticated technology with which to potentially communicate
with us, and are perhaps actively sending signals through the galaxy.
And finally, the length of time that such civilizations send such signals, which can
be interpreted as the lifetime of the civilization.
This equation is obviously overflowing with speculation, as these variables are difficult
to estimate without huge amounts of uncertainty.
Therefore, it was not meant to be used to arrive at a rigorous value, but rather to
provoke thought and discussion regarding the potential for alien life.
It highlights key variables that are involved in such a search, and begs for their contemplation.
And if we plug in a variety of estimates for these variables, the equation frequently yields
results that tell us that it is rather unlikely that we are the only intelligent civilization
in the galaxy, let alone the universe.
There are those who are a little more pessimistic.
Our star is a population one star, so there have been older stars capable of sustaining
life for billions of years.
If intelligent civilizations have existed for many millions of years longer than we
have, it is likely they would have begun to colonize other worlds, a process that would
move at an exponential rate, and it would seem like even one hyperintelligent race would
quickly inhabit the entire galaxy.
So in short, where is everybody?
The fact that we can’t see or hear anyone is the essence of the Fermi paradox, which
suggests we might be alone.
Does the fact that we can’t see or hear anyone mean that no one is there?
Not necessarily.
Perhaps they are there, but rather than colonizing the galaxy, they have decided to retreat into
their own technology, existing solely in virtual worlds and therefore no longer making any
cosmic noise.
Perhaps they are indeed everywhere but are leaving us alone, watching from afar to see
if we become a civilization that is worthy of their attention.
Or maybe, just maybe, we really are the only folks in town.
In the meantime, amidst all the speculation, an actual search is in progress.
This search is multi-faceted, largely focused on searching for radio signals.
This effort is called SETI, the search for extra-terrestrial intelligence.
We’ve been listening since the 60s to see if aliens are trying to say hello, but there
is just so much sky to listen to, and we can only listen in one spot at a time.
Beyond this, there is the search for exoplanets.
These are planets outside of our solar system, orbiting other stars in our galaxy.
These had been postulated for quite some time, but we had not actually detected one scientifically
until 1988.
Fast forward a few decades, and we have detected nearly four thousand exoplanets distributed
amongst nearly three thousand systems.
How is that we find these things?
Well, we detect them in a number of ways, by measuring the difference in brightness
of the star when a planet passes in front of it, by the wobble induced on the star as
the star and its planets orbit around their collective center of mass, or other methods still.
What does this data tell us?
First, we see that about one in every five stars like our sun has an earth-sized planet
in its habitable zone.
Think for a moment about how many billions of planets similar to ours that must exist
just in the Milky Way.
Is there life on these planets?
Could we one day colonize them ourselves?
Incredibly, there is an exoplanet orbiting Proxima Centauri, which is the single closest
star to ours, at just over four light years away.
The possibilities for the next hundred thousand years of human effort are beginning to unfold.
What will we do as our civilization grows, as we become capable of interplanetary, interstellar,
or even intergalactic travel?
To examine this, let’s move forward and talk about our future in space.