What are the odds of alien life? The Drake equation.
We don't know the answers to a lot of questions, for example:
How many alien societies exist, and are detectable? Even though we don't know how to answer such
a question, we can at least try to figure it out with a little help from our beloved...Math.
First, we have to have a pretty good idea about how the universe works, and of course
about the star and planetary formation, as well as conditions for life as we know it.
This means we have to study and collect a lot of data. Luckily for us, we – humans
- aren't so bad. Physics, astronomy, chemistry, biology and all-natural sciences offer us
the hints for the mathematical set of parameters that will give us an equation to calculate
the number of alien societies that exist and are detectable.
Second, one has to sit down and think about which parameters should appear in the equation,
and which not. Do you think it's difficult? I think so.
But luckily for us, in 1961 scientists Drake came up with a famous equation, that estimated
the number of transmitting societies in the Milky Way Galaxy.
Follow me to know more about Drake Equation. I promise you will not regret it!
Are there civilizations out there listening? We don't yet know the answer, but Dr Frank
Drake, president of the SETI Institute, came up with an equation that allows us to
make an estimate by multiplying seven quantities related to the prevalence of life.
Before getting into it, here's some historical context.
Drake organized a historic meeting at Greenbank in November 1961 to discuss what would soon
be called SETI, the search for extraterrestrial intelligence. Most sources agree that 10 individuals
attended this meeting, including Drake himself, and in particular Philip Morrison, Carl Sagan
was both in attendance. Drake said that he came up with the equation a few days before
the meeting, largely as a way to come up with an agenda.
Each term in his equation was meant to serve as a discussion point for the attendees.
Now we don’t know precisely the form that Drake wrote down at the meeting, but the canonical
formalism was first published by Drake in 1965. After the original meeting, the participants
dubbed themselves “the order of the dolphin”, inspired by attendee John Lilly’s research
on dolphin communication. The meeting and equation were even commemorated at green bank
with their plaque which is still there and worth checking out if you like Astro tourism.
Clearly then, there was a sense that this was a rather historic meeting. But what exactly
is the drake equation? The Drake equation estimates N, the number
of transmitting societies in the Milky Way galaxy.
Let's take a look at it: (to the videomaker: SHOW DRAKE's EQUATION).
The terms are defined as follows: N= The number of civilizations in the Milky
Way galaxy whose electromagnetic emissions are detectable.
R*= The rate of formation of stars suitable for the development of intelligent life (number
per year). fp= The fraction of those stars with planetary
systems. ne= The number of planets, per solar system,
with an environment suitable for life. fl= The fraction of suitable planets on which
life actually appears. fi= The fraction of life-bearing planets on
which intelligent life emerges. fc= The fraction of civilizations that develop
a technology that produces detectable signs of their existence.
L= The average length of time such civilizations produce such signs (years).
Currently, there are 100 scientists at the SETI Institute, working on nearly 100 research
questions. But each of these topics can be related to one of the terms in the Drake Equation.
Looking closer, the anatomy of the Drake equation can be grouped into just three types of variables.
We have a rate as the first term that has units of inverse time, for example, the rate
of stars is x per century or y per year. It tells us how many stars are forming in the
Milky Way in a year or in a century. Next, we have a bunch of fractions and numbers
in quantities without any units. Okay, since we have a rate multiplied by a
bunch of numbers, then the first six terms must all represent a rate when combined, but
the Drake equation calculates N. And N isn’t a rate.
To get rid of the rate, we have to multiply by a time which in this case is L, the mean
lifetime of civilizations.
That's all. Simple.
Elegant. The Drake equation. But...can we say something more?
Of course, we can. Can you see what's the problem with the equation?
I bet you see it.
The problem is...what are the parameters' values?
Of course, we can go ahead and plug in guesses for these values and that’s actually what
the original attendees did - arriving at a final estimate of N between 1, 000 and 100
million. But let me emphasize in the clearest possible
terms. Those are just guesses, j.
Our problem's that is not possible to evaluate the formula in a conventional sense, because
we simply don’t know the values of most of these terms.
Humans are ignorant. We have to deal with that. As we said, we don't have all the answers:
we have just a few of them. In the case of the Drake Equation, we don't
know the values of the terms, and anyone can guess - and your guess is as good as mine.
Even a child can use it, and here we have a recipe for disaster! We end up with a situation
where lots of people can come up with their answers and claim them as a result.
There have been enough previous studies on this topic to fill a small horde of hard drives
in many ways. The Drake equation is a victim of its own success. It seems that every month
or so we hear about a new study that plugs numbers into the equation and arrives at a
different answer. None of this teaches us anything because at
the end of the day we still don’t know most of the terms even worse.
Also, the press releases in particular give the public a false sense that we know the
answer or that these arbitrary estimates somehow represent evidence.
The real thing is that we just know the first two numbers in the Drake equation, R* and
fp, but that’s about it. The rate of star formation is something we
can absolutely measure and pin down through the detection and study of young stellar objects.
We can estimate that the current rate of star formation is about 2 per year.
The fraction of stars with planets is also eminently observable, and recent estimates
from Kepler and microlensing studies suggest that it is close to 1.
Planets seem to be a natural byproduct of star formation.
The next term is the average number of planets around each star capable of supporting life,
and this is where we enter terra incognito. Take, for example, our Solar System.
There are numerous definitions of the habitable zone that lead to anywhere from one, just
the Earth, up to three: Venus, Earth and Mars. But clearly, Venus is pretty different from
Earth. Yes, maybe it might be possible for it to
support life, but intelligent life? We don't know.
Our surveys can count how many Earth-sized planets are in the habitable zones of their
stars. We have no way with these surveys, at least
to distinguish between venuses and earth’s or indeed presumably other types of planets
that we’ve simply never seen before, and this doesn’t even touch on other criteria,
like plate-tectonics, obliquity, magnetic fields, etc...
So really we don’t know this number, but we can actually make progress if we just redefine
it. If we consider it to be the number of Earth-sized
planets in the temperate zone of their stars, that’s tractable.
That’s a number we can potentially measure, but by casting a larger net like this we would
end up making the other term smaller since it’s now defined over a wider span of worlds,
but at least this makes N an observable quantity. Having said all of that, we actually can’t
even agree on how many Earth-sized planets are in Earth-like orbits anyway, with modern
estimates from the same data set varying by a factor of a hundred.
Nevertheless, in principle, this is a number we can hope to measure with near-future surveys.
Now, as we move on to the subsequent parameters in Drake’s equation, our ignorance only
gets worse. The uncertainty here is so extreme it’s
not even possible to define it in a meaningful sense.
So any calculation you see about the Drake equation has very simply guessed all of the
terms except for maybe the first two. The perpetual torrent of articles claiming
to estimate N in the drake equation has frustrated and exacerbated many astronomers to the point
where now, where they see such articles, many astronomers either ignore them or actively
disparage them. The Drake equation has been abused so many
times with the media circus, being dragged out and glammed up for another round of misleading
headlines. Nowadays the Drake equation is now often marked
and ridiculed in many academic circles. Many astronomers not only don’t trust the
drake equation, they really hate it! But here’s the thing. Yes, there are undoubtedly
flaws with the drake equation, but much of the bad reputation that it offers is a product
of its past abuses and not its original formulation. All of this is undoubtedly largely fueled
by the media circus that tends to come along for the ride.
In the last part of the video, we try to provide a list of the most reliable estimates of the
parameters that appear in the Drake equation. R * - star formation rate in the Milky Way
The most recent calculations by NASA and ESA indicate that the current star formation rate
in our galaxy is 7 stars per year.
f p - the fraction of such stars that own planets
Recent research on exoplanets has shown that 40% of Sun-like stars have planets, and the
real fraction could be much greater, as only considerably more massive planets than Earth
can be detected with current technology. The Kepler mission of NASA started on 6 March
2009, it should provide an estimate more accurate in the number of planets per star located
in a habitable zone. In fact, it can count on instruments much more sensitive than in
the past, capable of detecting planets as large as the Earth with orbital periods of
up to one year. Even if we know the number of planets that
are in a habitable zone, however, determining how many of them have the right combination
of factors can be difficult. Furthermore, if the hypothesis of the rarity of the Earth
is valid, the set of characteristics verified on Earth may not be common at all, and the
number n e could be extremely low, tending towards zero.
f l - the fraction of them that actually develops life
We don't know very much about it. It must be greater than 0.13 on planets older
than one billion years.
f i - the fraction of them that actually develop intelligent life
This value remains particularly controversial. Those who speak out in favour of a low value,
point out that of the billions of species that have existed on Earth only one has become
intelligent. Those who are in favour of higher values ​​emphasize instead that the complexity
of living beings tends to increase along the course of evolution, and conclude that the
appearance of intelligent life, sooner or later, is almost inevitable.
f c - the fraction of them that is able and decides to communicate
Although much speculation has been made, no concrete data is available on this parameter.
L - the average duration of the communicative phase of each of these civilizations
Some values were proposed: 420 years, 304 years.
Others, such as astrobiologist David Grinspoon, have suggested that once a civilization has
developed it could overcome all threats posed to its survival, and then survive for an indefinite
period, bringing L to the order of billions of years.
Values-based on the quoted estimates,
R * = 7 / year, f p = 0.5, n e = 2, f l = 0.33, f i = 0.01, f c = 0.1, e L = 10 000 years
give as a result N = 7 × 0.5 × 2 × 0.33 × 0.01 × 0.1 × 10
000 = 23.1 existing extraterrestrial civilizations capable
of communicating in our galaxy .
Drop a line below so we know if this is the result you expected!