It seems to me…
There has been considerable discussion regarding the possibility of other intelligent life in the universe (yes, that implies “intelligent” life actually exists here on Earth) and, if it does, why it has so far not been detected. Much of the discussion originated with the Drake Equation proposed by Dr. Frank Drake at the University of California – Santa Cruz in 1961 which attempted to estimate the number of life-supporting communicating planetary systems in the Universe[i].
There have been many legitimate criticisms of the equation and almost everyone considering it has modified it in some way. While not an astronomer, I’m entitled to my own opinions on values for the most commonly included parameters.
We have very little knowledge of the life development process, especially higher intelligence, on other extraterrestrial systems. In reality, any attempt to make that assessment at some point actuality becomes an estimate of survivability of life on our own planet as it provides the only known example with which we are familiar.
The number of stars in the galaxy, N*, is related to the star formation rate R* by
where Tg = the age of the galaxy. It normally is assumed that R* is constant so that permitting the equation to be rewritten into an alternate form phrased in terms of the more easily observable value, N*.
N = N* • fp • ne • fl • fi • fc • L/Tg
The Drake equation then can be stated in a simpler form:
N = R* • fp • ne • fl • fi • fc • L
N = the number of civilizations in our galaxy with which communication might be possible;
R* = the average rate of star formation per year 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 the above that actually go on to develop life at some point.
fi = the fraction of the above that actually go on to develop intelligent life.
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.
Here are my estimates for the Drake equation parameters (I’ve also included Frank Drake’s values for comparison):
R* = the rate of star creation in our galaxy.
About 7 per year. By this estimate there should be between 100-400 billion. (Drake: 10/year )
fp = the fraction of those stars that have planets.
20-60 percent of sun-like stars may form terrestrial planets: 0.2 > fp < 0.6. There should be about 50 billion planets. (Drake: 0.5)
ne = the average number of planets (satellites may perhaps sometimes be just as good candidates) that can potentially support life per star that has planets.
The fraction of stars with Earth-like planets is estimated to be from greater than 0 to 10-20 percent. I actually prefer a value of 0.0005 > ne < 0.45 or about 500 million habitable planets. (Drake: 2)
fl = the fraction of the above that actually go on to develop life.
Estimated to be greater than 0.13 since I’m choosing my own values: 0.1 > fl ≤ 0.5. (Drake: 2)
fi = the fraction of the above that actually go on to develop intelligent life
Value estimates for fi are the most controversial of any in the equation (and make any resulting estimate questionable). I’m relatively optimistic. 0.3 > fi < 0.6 seems reasonable. (Drake: 0.01)
fc = the fraction of the above that are willing and able to communicate.
Drake’s value of 0.01 probably is about the best we can do. I’ll be a bit more optimistic: 0.01 ≥ fc ≤ 0.02. (Drake: 0.01)
L = the expected lifetime of such a civilization for the period that it can communicate across interstellar space
It does not matter how long a civilization is able to communicate across interstellar space – only how long any of the transmissions are of a type we are able to identify as containing intelligence. Given that our significant analog electromagnetic transmissions began around 1920 but now are converting to a digital form much more difficult to detect, a reasonable estimate might be as low as 100 years: 100 ≥ L ≤ 500. (Drake: 10,000)
The Milky Way galaxy is estimated to be about 100,000 light years across. As the closest star, other than our Sun, is Proxima Centauri at 4.24 light years, any detected signal therefore would have originated at some point from 4.24 up to 104 light years in our past.
Values based on my best estimates,
R* = 7 (7 stars formed per year, on the average over the life of the galaxy).
fp = 0.4 (40 percent of all stars formed will have planets).
ne = 2 (stars with planets will have 2 planets capable of developing life).
fl = 0.13 (13 percent of these planets will develop life).
fi = 0.01 (1 percent of which will be intelligent life).
fc = 0.01 (1 percent of which will be able to communicate).
L = 100 (detectable transmissions will last 100 years).
With a result of
N = 7 × 0.4 × 2 × 0.13 × 0.01 × 0.01 × 100 = 0.0073
My estimate is 0.000021 ≥ N ≤ 5.67 with a preferred value of 0.0073. (Drake: 2.31)
This does not imply extraterrestrial life does not exist, it merely indicates the improbability of ever detecting it.
It has been repeatedly pointed out that since most of the values in the equation are purely estimates not based on known probabilities, any value relative to the actual number of planets containing a species capable of interplanetary communications is purely speculative and essentially useless for scientific purposes. Still, it is interesting to consider why we never have detected any other sign of intelligent life…
I’m convinced we are not alone. We just are not sufficiently advanced to detect anyone else that is out there.
That’s what I think, what about you?