My Values for the Drake Equation
N = R* fp ne fl fi fc L
The Factors and Proposed Values
R* = 7 stars/year
The rate of formation of stars in the Milky Way galaxy:
Based on the most current NASA and European Space Agency estimates the Milky Way's star formation is approximately 7 stars per year.1
fp = 0.6
The fraction of those stars with planets:
As each of the factors of the Drake equation are progressively considered we get into more and more inconclusive and debatable estimates for their values.
The fraction of R* star systems that contain planets is dependent upon many factors itself. For example, the estimated fraction of sun-like star systems that
contain planets is probably much higher than brown or red dwarf stars, of which there are many more in the galaxy. (New studies are showing, however, that
habitable white dwarf star systems may be in the billions in our galaxy). The fraction of giant stars with planets may be higher still than sun-like systems.
There are other more exotic forms of star systems that have been found to have planets orbiting them or that may contain them, but have not yet been found.
Some of these include pulsar and neutron stars, black holes, binary and other multiple star systems, and proto-star systems with planets that have broken free
from their original star systems and have subsequently become lone stragglers plying the ocean between the stars.
With all of this in mind, it is very difficult to estimate the fraction of stars with planets, since the data supporting this factor is very dynamic and is
constantly being updated. The primary instrument that is being used for this purpose is the Kepler space probe. It is currently observing about 100,000 stars
and as of this writing, it has possibly found about 706 new planets.
Astronomers have announced that at least 40% of sun-like stars have "low-mass" planets. 2 The percentage may be much higher, since many of these
planets were detected with the help of more massive planets orbiting the parent star and therefore the lower mass planets are more difficult to detect and the
proportion of lower-mass planets may be larger than the high-mass (Jupiter-like) planets.
My conservative and tentative estimate, therefore, will be 60%.
ne = 2
The number of earth-like planets in each planet-bearing star system:
Again, there are many factors involved with determining this number. Planets may not be the only habitable locations within a star system as promising new research
is showing. Some moons of planets may be included in this number, as can comets. In our solar system alone, controversial as they may be, Titan (a moon of Saturn);
Europa, Enceladus, Callisto, and Io (all moons of Jupiter); and even some comets, have been proposed as being habitable. Mars, of course, has been proposed as well
as early Venus (and its current cloud layers) and the clouds of Jupiter and Saturn.
If seriously considering these possibilities it would make 5 out of the 8 planets in our solar system potentially habitable locations at one point or another in their
lifetimes and about 6 other extra-planetary locations that could be habitable. This makes the total proposed habitable solar system locations to be 11!
However, I'll take a more conservative estimate on this factor to be 2 locations within planet-bearing star systems that are (or ever were) habitable.
fl = 0.5
The number of habitable earth-like planets (or locales) where life does develop:
The fact that we only have one confirmed case of life developing on an earth-like locale (here on earth) makes it is very difficult to extrapolate from this single instance.
Astrophysicist Paul Davies of Arizona State University has recently championed the idea of searching for a "shadow biosphere" in which a second case of life, completely
different from our familiar terrestrial biology, has developed on earth. This would confirm that there is an almost certainty for the development of life in earth-like
conditions rather than a very rare phenomenon. There has been tantalizing possible evidence recently found of a shadow biology existing right now on the earth:
The "Red Rain of Kerala"
The Indian state of Kerala between July and September 2001 had been soaked with a mysterious "red rain". After some analysis the red material
in the rain had been found to contain cells that moved. Some scientists have claimed that these cells were extraterrestrial in origin.3
A thin layer of manganese-enriched material coats many rocks of the desert. The high manganese level is anomalous considering that only biochemical
processes are known to accumulate this element in the earth's crust and material composition of the rocks that host these varnishes are very
Nanobacteria and Nanobes
Nanobacteria and "nanobes" are objects that are only a few hundred nanometers across and are therefore about ten to a hundred times smaller than the smallest
bacterium; close to the size of viruses. Due to their sizes, conventional terrestrial biology seems to have ruled them out as to originating here. They have
been found to be of different shapes and some have been found to contain anomalous DNA sequences. They've been found in blood, rocks, and oil wells.5
Two complications to these proposals are the possibilities that there have been "cross-contaminations" of microbes between planets, such as between Earth and Mars
and, even more controversial, was an idea championed by Sir Fred Hoyle called "panspermia" in which extraterrestrial micro-organisms have "seeded" star systems
throughout the galaxy.
Because there has never been any conclusive evidence of another origin of life elsewhere or on earth, I will take a middle-of-the-road estimate and say that there is
a 50% chance that life will develop on an earth-like locale, if given enough time. Besides, the most convincing argument that I've seen made for the likelihood of life
almost certainly developing in earth-like conditions is the fact that life on earth started almost immediately after the period on earth known as "the late heavy bombardment",
a time on earth when conditions for life were extremely inhospitable (massive numbers and high frequency of meteorite impacts, large-scale volcanism, hot surface temperature, etc.).
The earliest observed evidence for life was about 3.86 billions years ago, which happens to be an almost immediate event after conditions were ripe. At the moment, that's
enough evidence to convince me of the ease at which life can develop.
fi = 0.001
The fraction of planets (or other life-bearing locales) where life exists that intelligence develops:
This is yet another factor that is very difficult, if not impossible, to determine with available current evidence. Much of the determination hinges on whether intelligence is
something that benefits species generally and therefore is considered a convergent type of evolution. Wings and eyes are organ systems that are traditionally used as illustrative
examples of convergent evolution. They are both systems that are very beneficial for species survival and are therefore found in many disparate species. They have independently
evolved many different times. Wings, for example, are found in birds, insects, mammals and even fish, all of which are completely unrelated organisms.
So, is intelligence something that is a convergent phenomenon in evolution? This is debatable and is an idea where evidence can be found to support both sides of the argument. Those on the
pro side mention the fact that intelligence of a sufficiently advanced form has occurred independently several times in the history of life on earth. For example, several extant bird
species, primates, cetaceans, and some invertebrates have all acquired intelligence of an advanced form.
Those who favor the view that intelligence is an extremely rare occurrence cite the fact that it has appeared in only one of the three major groupings of life: the eukaryotes. Of the
eukaryotes it has only occurred in a small branch: the animals. And within the animals, it has occurred only several times out of the many millions of different species that have evolved.
However, they say, the bacteria and the archaea had existed on earth for an immense time before the eukaryotes started even forming multicellular organisms. It also seems that the
archaea and the bacteria account for a much larger number of organisms that have ever lived on earth. Intelligence, they say, is more of a statistical fluke in the evolution of life than
it is an inevitable consequence and is, therefore, not a convergent phenomenon.
Based on these findings I feel compelled to give the fraction of planets where life exists, and that have developed intelligence, a low number.
fc = 0.5
The fraction of intelligent species that eventually develop a "high technology" communicative ability:
I believe that given enough time, a high intelligence species will eventually produce high technology, including those technologies associated with radio communication.
Even though homo-sapiens have been the only species that have gone that far on this planet, there's no reason to believe that it could not have happened to other species
on earth if we were not here and given enough time. Several other extant species are already known to possess the ability to use tools for their own purposes and even to
teach the skills to use them to others within their group. Several other primates make and use tools fashioned from their surroundings as well as some birds, cetaceans,
elephants, sea otters, octopuses and even ants!6, 7
With the evidence that many disparate species of animals on earth have learned tool-making and usage, I believe it lends a lot of credence to the idea that with enough
intelligence high technology and science eventually become, when "stumbled upon" or when discovered from innate curiosity, very beneficial for the survival and defense of
Some criticism to this idea comes from those who say that there have been many civilizations on earth that have come and gone without building their technology to any
meaningfully advanced stage. While this may be true, it is also very evident that technology has a definite trend throughout history towards one that is certainly advanced.
While there may be fits and starts along the way, there will come a civilization within that species that reaches a stage where its technology is sufficiently advanced for
How long they stay at that stage and how long their radio (or similar) technology stays "on the air" or inadvertently advertises leaked signals remains debatable.
So, given this evidence I am moderately optimistic about the prospects of civilizations eventually reaching a radio-communicative stage. I give the fraction 50%.
L = 2,000,000 years
The lifetime of high-tech communicative civilizations:
With so many potentially catastrophic events that can happen to destroy life on earth, it can be very easy to become pessimistic about the likelihood of an advanced
civilization to render its survival lifetime short. On earth alone, our very delicate biological selves face, among other things, global warming, comet and meteorite
impaction, deadly worldwide micro-biological outbreaks, pollutants, ozone depletion, massive droughts, genocides and wars, and other events that we may not be able to
anticipate (see my page on some of these matters.)
However, there is also some great optimism for the future that I can foresee if humanity has the drive and ambition to approach it. I believe wholeheartedly that,
as a species, we are very close to a major evolutionary change of our own making: transhumanism, artificial intelligence, and eventually "posthumanism". Machine intelligence
will be our cultural handoff to our descendents. They will essentially be immortal, tough, and consist of an intelligence that will be of a power many magnitudes beyond ours.
I believe that once that stage is reached the survival, and therefore the lifetime, of the civilization will be indefinite.
The sticking points in this admittedly speculative vision are the stages preceding the post-human era. Will a civilization out-maneuver the potential threats inherent in its
double-edged technology, such as nuclear weaponry? Will it navigate through the treacherous waters of its political, religious, philosophical and ethical threats and reach the
technology necessary to pass into its post-biological stage? These are all very serious questions to consider in order to make any kind of educated guesses on the lifetime of a
Michael Shermer of The Skeptics Society has calculated the lifetime of human civilizations to be only a few hundred years due to their instability. Frank Drake, the "father" of
SETI has estimated a civilization's lifetime to be about 10,000 years, while Carl Sagan was more pessimistic. I believe, as I mentioned, that it all depends on if the civilization
ever reaches that second (post-biological) stage. If, and when, it does its lifetime will extend into an indefinite future (many millions - Eons - or billions of years). However,
not every civilization will reach that second stage. Many will self-destruct or will be the unfortunate victims of a natural catastrophe. I will give a positive and encouraging
high-end number for the average lifetime of an advanced extraterrestrial civilization to be 2 million years.
My Conclusions for N
N = 4200
advanced extraterrestrial civilizations in the Milky Way galaxy
How close are the civilizations?
Given that N = 4200 advanced extraterrestrial civilizations existing
now in the Milky Way galaxy we can calculate approximately how close the nearest one is to us. Using the
assumptions that the civilizations are spread evenly throughout the volume of the galaxy (which is far from clear),
it is estimated that the nearest civilization will be:
1,490 light-years away from earth
Calculate your own number of civilizations
Enter your own values for the Drake Equation and see what your assumptions come up with.
Notes and References
1. Jan. 5, 2006. Milky Way Churns Out Seven New Stars Per Year, Scientists Say
2. Oct. 19, 2009. Astronomers have announced a haul of planets found beyond our Solar System.
3. Wikipedia; Red Rain of Kerala
4. Dec. 1, 2006. A Shadow Biosphere
5. Nanobacteria and Nanobes- Are They Alive?
6. Wikipedia: Tool Use by Animals
7. V. S. Banschbach , A. Brunelle, et al.
Tool use by the forest ant Aphaenogaster rudis: Ecology and task allocation. Nov. 17, 2006.