A
recent paper published in the journal Astrobiology has got quite a
bit of online news coverage. Newsweek covered the paper with an
article entitled, “Intelligent Alien Life Almost Certainly Existed
Somewhere Else, Study Says.” The Newsweek article then says, “The
new equation estimates a one in 10 billion trillion chance that
humans are the only intelligent species to have ever existed.” But
the paper actually estimates no such thing. A careful examination of
the paper will show that it doesn't really clarify the likelihood of
extraterrestrial intelligence.

Authored
by A. Frank and W. T. Sullivan III, the paper is entitled “A New
Empirical Constraint on the Prevalence of Technological Species in
the Universe,” although, as we will see, the paper actually
presents no such constraint. The basic idea of the paper is to kind
of present a simpler version of the famous Drake equation used to
estimate the number of planets with extraterrestrial intelligence. The paper
mentions a form of the Drake equation which says that the total
number of technological species that have evolved in the universe is
equal to the product of the following thing:

- the total number of stars
- the fraction of those stars that form planets
- the average number of planets in the habitable zone of a star with planets
- the probability that a habitable zone planet develops life
- the probability that a planet with life develops intelligence
- the probability that a planet with intelligent life develops technology

The
paper attempts to simplify this equation into a simpler equation that
says that the total number of technological species that have
evolved in the universe is equal to the product of:

- the total number of habitable zone planets
- the total ‘‘biotechnical’’ probability that a given habitable zone planet has ever evolved a technological species (here “given” means “average” or “randomly selected”)

(The
latter probability should not be confused with the “total
biotechnical probability that

*at least one*habitable zone planet has ever evolved a technological species,” which is presumably a probability of 100%, since we exist.)
The
paper estimates 2 X 10

^{22 }(twenty billion trillion) as the total number of stars in the observable universe. Using that figure, the paper estimates that if the total ‘‘biotechnical’’ probability that a given habitable zone planet has ever evolved a technological species is greater than about 1 in 2.5 X 10^{-24}(two chances in a trillion trillion), then mankind is unlikely to be the first technological species to appear in the universe.
So
what is that telling us? Nothing we didn't already know. We already
knew our galaxy has about 200 billion stars, and that there
are billions of other galaxies. Multiply 200 billion by, say, 50
billion, and you have a number similar to 2 X 10

^{24 }(two trillion trillion). So if the chance of intelligent life appearing on a particular planet is greater than about 1 in a trillion trillion, there will be many planets with intelligent life. But if the chance of intelligent life appearing on a particular planet is much smaller than about 1 in a trillion trillion, then we might be the only planet with intelligent life. Anybody could have done that math without reading this paper.
Why,
then, are the authors claiming to have reached a “new empirical
constraint” on the number of technological species in the universe?
Their paper really adds nothing. Since the Drake equation was created
back in the 1960's, there has been only one relevant development.
That development is that the “fraction of those stars that form
planets” and the “average number of planets in the habitable
zone of a star with planets” (referred to above) have been
clarified – we now know these numbers are pretty high. But that
development has been a gradual thing that has been decades in the
making, so it's not like Frank and Sullivan should be having some
sudden “Aha!” moment relating to that.

Far
from having been clarified, probabilities in regard to the evolution
of civilized life on other planets are still about as unclear as
anything could be. We simply have no basis for estimating “the
total biotechnical probability that a given habitable zone planet has
ever evolved a technological species.” Part of the problem is that
such a probability must be computed by considering all of these
probabilities:

- the probability that self-replicating molecules would ever appear on a random planet in the habitable zone of a star
- the probability that a genetic code would somehow arise on this planet
- the probability that cells and proteins would ever appear
- the probability that microscopic life would ever evolve into highly organized and coordinated large organisms
- the probability that intelligent life capable of building civilizations would ever appear if large organisms had appeared

Basically
nothing has happened during the past 50 years to clarify any of these
probabilities. If there is not some type of teleological fine-tuning
or extraordinary factor or undiscovered cosmic laws which makes these
probabilities high on many different planets, then it is
all-too-possible that some of these probabilities may be virtually
zero. Some of these items on this list (particularly the items at
the top of this list) look pretty much like miracles, and really
nothing has been done to establish that they are things that would
happen in even one case in a trillion trillion trillion planets.
Accordingly, it is very possible that the “total biotechnical
probability that a given habitable zone planet has ever evolved a
technological species” could be some probability that is virtually
zero. In such a case, man would be alone in the universe. (Again,
the probability discussed here is the chance of a technological
species on an average planet, not the known probability that it has
evolved on one planet.)

I
may note that Newsweek completely errs when its article states,
"The new equation estimates a one in 10 billion trillion chance that
humans are the only intelligent species to have ever existed.” In
fact, the study doesn't even estimate a likelihood that humans are
the only intelligent species to have ever existed. It merely states
that if the chance of an intelligent species existing on an average
planet is greater than 1 in 2.5 X 10

^{-24}(two chances in a trillion trillion), then other intelligent species should have existed. But the study does not estimate either that the likelihood of man being alone in the universe is only 1 in 10 billion trillion, not does it estimate that such a likelihood is less than 50%.
But
in the press release for the study, Frank tries to spice up things by
suggesting an arbitrary probability not even mentioned in the study.
That's a good way to gin up interest, but not so good from the
standpoint of describing your paper that never mentioned such a
thing. Frank says this:

*Think of it this way. Before our result you'd be considered a pessimist if you imagined the probability of evolving a civilization on a habitable planet were, say, one in a trillion. But even that guess, one chance in a trillion, implies that what has happened here on Earth with humanity has in fact happened about a 10 billion other times over cosmic history!*

This
probability of 1 in a trillion is never mentioned in the study, nor
is an estimate of 10 billion other civilizations. As for the
estimate of 10 billion other civilizations, this is just a case of
“picking a number out of a hat.” The same thing used to be done
by the late astronomer Carl Sagan, who used to estimate that there
were 100 million civilizations in our galaxy. Such estimates are
purely arbitrary guesses, very much “picking a number out of a
hat,” because we do not understand the likelihood of life appearing
by chance on a planet in the habitable zone. There is no basis for
suggesting a probability of 1 in a trillion for a civilization
appearing on a habitable planet. Given the difficulties of life
getting starting from chemicals through chance events, it is still
quite possible that the chance of life appearing on a random planet
may be much less than 1 in a trillion, and even much less than 1 in
10 billion trillion – perhaps as low in 1 a billion trillion
quadrillion quintillion. In the latter case we would be alone in the
universe.

Long
story short, the Astrobiology study basically says that if we are
alone in the universe, then it must be really, really hard for life
to get started on a planet and evolve to a state where intelligent
beings exist. But we already knew that. What the study does not do is
give us any basis for estimating the number of extraterrestrial
civilizations that exist, nor does it even show a likelihood that
extraterrestrials have existed. There may well be reasons for
thinking we are not alone, reasons pertaining to the desire of a
Creator to avoid a “only one flower in the huge desert” type of
universe, or reasons relating to sightings of UFO's, or reasons
relating to a general but debatable philosophical principle that we
should tend to avoid believing that incredibly improbable things
happened locally. But the Astrobiology study does not provide any
such reason, other than the “there's so many places where life
could evolve” reason that has already been known for decades.

Contrary
to the insinuation of the Astrobiology paper (and its press release),
our scientists still don't have any idea whether the rest of the
universe is an empty desert or a universe teeming with life.

*We're still clueless about ETs*