Earlier this week we reported on the
astounding revelation that
22% of sunlike stars in the Milky
Way are orbited by potentially habitable, Earth-sized worlds.
Given that there may be billions upon
billions of life-friendly planets out there, it's time to revise the
numbers in
the Drake Equation and estimate how
many communicable
alien civilizations may be out there.
The Drake Equation goes like this:
R * fp *
ne * fl * fi * fc * L = N
...where:
R = the average rate of star
formation in our galaxy
fp = the fraction of those stars
that have planets
ne = the average number of
planets that can potentially support life per star that has
planets
fl = the fraction of planets
that could support life that actually develop life at some
point
fi = the fraction of planets
with life that actually go on to develop intelligent life
(civilizations)
fc = the fraction of
civilizations that develop a technology that releases
detectable signs of their existence into space
L = the length of time for which
such civilizations release detectable signals into space
We don't know the precise numbers to
fill in these variables, but we're getting a better idea.
Let's go with the following assumptions:
Average
rate of star formation in our galaxy:
Drake originally went with the
number 1 - a very conservative estimate - but it's probably
closer to 7, so we'll go with that (R=7).
Average
number of planets that can potentially support life per star
that has planets:
Here's where we get to plug in
the new data.
According to the new study,
1-in-5 sunlike stars host an Earth-sized planet in the
habitable zone.
Depending on the total number of
stars in the Milky Way, that could be as high as 11 billion
planets, or a figure of roughly 20%. But there's also red
dwarfs to consider.
Recent surveys have shown that
upwards of 40% of these dim stars host Earth-like planets in
their habitable zones. That means there's as many as 40-60
billion habitable planets orbiting red dwarfs. That said, we
do not know if red dwarfs can harbor life.
These systems have poor magnetic
fields, are tidally locked, and experience poor levels of
"good" radiation. So let's prepare the equation to account
for both possibilities. So our figures will be 34% (rl=0.34)
in the optimistic case and 4% (fl=0.04) in the pessimistic
case.
Drake himself gave values
between 1-5.
Fraction
of planets that could support life that actually develop
life at some point:
This is a tough one, and we
haven't got a clue.
Drake thought it was 100%, but
that can't possibly be right. Still, life on Earth started
almost immediately once the proper conditions were
established, so it's probably not a figure that's close to
zero.
Fraction
of planets with life that actually go on to develop
intelligent life (civilizations):
Another tough one.
There have been billions of
species on Earth, yet only one has developed the capacity
for radio communication. It's not immediately obvious that
evolution favors human-like intelligence, preferring instead
other sorts of adaptations. What's more, it not obvious that
all human-like intelligences go on to form technological
civilizations.
Most estimates place this figure
somewhere between 50 and 100% - but that seems absurdly
high. Let's go with something more reasonable, like 1-in-10
(fi=0.1).
Fraction
of civilizations that develop a technology that releases
detectable signs of their existence into space:
This one's probably quite high.
Even pre-atomic civilizations are capable of this. Let's go
with a figure of 80% (fc = 0.8).
Drake himself said this figure
should be between 10-20%.
Length
of time for which such civilizations release detectable
signals into space:
This variable is challenging to
assess because we ourselves are about to go radio silent.
But there's also the possibility
that technological civs are short-lived owing to existential
risks, including nuclear war, artificial super-intelligence,
and molecular assembling nanotechnology. Humanity could
conceivably wipe itself off the intergalactic map at some
point in the coming decades.
But should we survive our
technologies and the accompanying technological Singularity,
we could be around for a tremendously long period
of time - something on the order of millions of years.
But because that's highly
speculative, and because the Fermi Paradox and the Great
Filter hypothesis tells us otherwise, let's go with 200
years (L=200).
Drake gave values between 1,000
and 100,000 years.
Okay, with that out of the way, let's do
some math:
7 x
1 x 0.34 x 0.13 x 0.1 x 0.8 x 200 = 5
So, if both sun-like stars and red
dwarfs host habitable planets, we get a figure of five communicable civilizations in
our galaxy (including ourselves).
This implies that, probabilistically
speaking, our nearest radio-capable neighbor is 22,000 lightyears
away. That's highly discouraging, to say the least. But it gets
considerably worse if we exclude red dwarfs.
In that case, our equation reads:
7 x
1 x 0.04 x 0.13 x 0.1 x 0.8 x 200 = 0.58
Which is a tough pill to swallow
considering that we're here!
What this might imply, however - aside
from the fact that our variables may be wrong - is that we may in
fact be the only ones in the Milky Way right now who are spewing
radio signals out into the cosmos.
As for Frank Drake, he came up with
values for N ranging between 1,000 and 100,000
civilizations in the Milky Way.
