THE SOUTHWORTH PLANETARIUM
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Founded January 1970
       "Yes, we know who won last night!"


THE DAILY ASTRONOMER
Monday, February 8, 2016
The Drake Equation

The Drake Equation is described as the perfect blend of mathematics
and dart throwing. Of all the myriad equations one encounters in
astronomy, it is the one most reliant on pure guesswork.  Developed by
SETI founder Frank Drake, the Drake Equation's aim is to estimate how
many communicative alien civilizations are lurking out there in our
home galaxy.  A 'communicative alien civilization' is one capable of
transmitting information about itself to others through
electromagnetic radiation or other means.    We humans have been a
communicative race ever since Tesla and Marconi managed to manipulate
radio waves in the latter 19th century.    By galactic time scales,
we've been announcing ourselves for a couple microsceconds.  Of
course, we don't know if we're the very first civilization in the
galaxy capable of such transmission, or merely the most recent arrival
to the Milky Way's vast communication network.


The Drake Equation tries to answer this question with a combination of
stellar astrophysics and simple speculation.    The equation states
that the number of communicative civilizations is estimable when one
multiples the rate of suitable star formation, the fraction of the
aforementioned stars that have planets, the percentage of those
planets suitable for life, the actual number of suitable worlds
actually harboring life; the fraction of those life-bearing worlds
that produce intelligent life and civilizations; the number of those
civilizations that transmit 'detectable' transmissions into space and
the duration of time during which they emit these signals.



Ladies and gentlemen, most of those variables are currently unknown.
We have to rely on our instincts, which are not always trustworthy,
and our knowledge of ourselves, which is quite limiting, to address
most factors within this equation.  Whatever number the final
estimation yields will be as conjectural as the values we selected for
our calculations.     However,  we need less guesswork for the first
two factors, the star formation rate and the percentage of stars
harboring planets.



Astronomers believe that three 'solar masses' of stars form each year
in the Milky Way Galaxy.   A solar mass equals the mass of our Sun.
This value, "three solar masses," refers to the combined sum of all
stars born.    That could equal three stars as massive as the Sun, or
a greater number of less massive stars or even a more massive star.
One should be quite careful with this statistic as it is an average.
A year is negligible on astronomical time scales and star formation
requires millions of those years.  Moreover, we need to have
'suitable' stars, those with lifespans long enough to allow life to
develop.   We know that Earth needed every minute of four billion
years to go through the painstakingly slow process of cooking simple
molecules into complex life forms.     The most massive stars are
fleeting, with life cycles on the order of tens of millions of years:
hardly enough time for a planet to cool, let alone labor through the
biochemical processes life's development necessitates.
Fortunately, most stars exist for billions of years (red dwarfs, the
most common stars, live for a couple trillion years.).



The second factor, pertaining to planets, was all guesswork when Frank
Drake introduced his equation in 1961.    The only planets then known
to us were those within our own solar system.    Now, in 2016,
astronomers have cataloged more than two thousand* exo-planets,
planets in orbit around other star systems.  These outer worlds
represent a small sampling of the myriad planets scattered around the
Milky Way.       During the first decades after Drake developed this
formula, humans didn't know if the galaxy contained only a smattering
of planets or a proliferation.    We're now confident that planets
abound in the Milky Way, with a population perhaps  exceeding that of
the stars, themselves.   Even amongst these 2000 worlds, astronomers
have found a few "Earth-like" planets, those that are "rocky," not
highly massive, and located within a narrow region where temperatures
are neither excessively high (Mercury) nor low (Mars).
Scientists have not detected life on these "super-Earths," of course,
and they don't expect to find any.  Simply because the worlds 'could'
be life-bearing doesn't mean they are.



One might think that this information would lend insight into the next
factor: the percentage of other worlds that could harbor life.
Based on present findings, one could be tempted to state that ratio is
perhaps four out of a thousand, more or less.  The problem is that our
sample is far too small to allow for such conclusions.    Secondly,
giant planets, those with masses equal to or exceeding that of
Jupiter, are disproportionately represented.  Astronomers have been
finding exo-planets since 1992 and during most of the ensuing two
decades, they employed detection methods capable of finding only the
largest planets.   Only recently, as with the launch of the Kepler
spacecaft in 2009, have astronomers developed techniques enabling them
to locate smaller, Earth-like worlds.   The big boys take up most of
the room in our exo-planet catalog, at least for now.   We'll need
many more years to ascertain a more accurate small world-big world
ratio.

The remaining factors are anyone's guess.    We know that life thrives
on this planet and that a certain species developed a
radio-transmitting civilization.   This gives us a 1-1 correlation -
life did produce a civilization in one instance.   However, that is
the only instance on record.   How are we to know how long life
persists on other worlds?    Perhaps it starts, evolves, and then some
catastrophe annihilates all life forms.   Our planet has sustained
many powerful assaults throughout its history, not just the one 65
million years ago.   The geological record is punctuated by mass
extinctions.    Maybe we're fortunate in that some life forms have
always survived these widespread wipe-outs.  Other planets might not
be so lucky.    As we have only one data point -Earth- we cannot make
any conclusions at all about life's progression or resiliency on other
worlds.

We know that the galaxy contains hundreds of billions of stars; we
also suspect that the galaxy also harbors billions of planets.
These two values -the first certain, the other inferred- bode well for
humanity's search for extraterrestials.  Perhaps only by establishing
contact with such aliens will we be in a better position to determine
if life is rare in the galaxy or is commonplace.









*2065 confirmed planets catalogued as of February 8, 2016