This section attempts to list best current estimates for the parameters of the Drake equation.
R* = the rate of star creation in our galaxy
Latest calculations from NASA and the European Space Agency indicate that the current rate of star formation in our galaxy is about 7 per year.
fp = the fraction of those stars that have planets
It is known from modern planet searches that at least 40% of sun-like stars have planets, and the true proportion may be much higher, since only planets considerably larger than Earth can be detected with current technology.[8] Infra-red surveys of dust discs around young stars imply that 20-60% of sun-like stars may form terrestrial planets. Microlensing surveys, sensitive to planets further from their star, see planets in about 1/3 of systems examined–a lower limit since not all planets are seen. The Kepler mission, from its initial data, estimates that about 34% of stars host at least one planet.
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
Marcy note that most of the observed planets have very eccentric orbits, or orbit very close to the sun where the temperature is too high for earth-like life. However, several planetary systems that look more solar-system-like are known, such as HD 70642, HD 154345, Gliese 849 or Gliese 581. There may well be other, as yet unseen, earth-sized planets in the habitable zones of these stars. Also, the variety of solar systems that might have habitable zones is not just limited to solar-type stars and earth-sized planets; it is now believed that even tidally locked planets close to red dwarfs might have habitable zones, and some of the large planets detected so far could potentially support life
In early 2008, two different research groups concluded that Gliese 581 d may possibly be habitable. Since about 200 planetary systems are known, this very roughly estimates ne > 0.005. In 2010, researchers announced the discovery of Gliese 581 g, a 3.1 Earth-mass planet in near the middle of the habitable zone of Gliese 581, and a strong candidate for being the first known Earth-like habitable planet. Given the closeness of the planet’s star, and the number of stars examined to the level of detail needed to find such planets, they estimate eEarth, or the fraction of stars with Earth-like planets, as 10-20%.
Using different criteria, Lineweaver has also determined that about 10% of star systems in the Galaxy are hospitable to life, by having heavy elements, being far from supernovae and being stable for a sufficient time.
NASA’s Kepler mission was launched on March 6, 2009. Unlike previous searches, it is sensitive to planets as small as Earth, and with orbital periods as long as a year. If successful, Kepler should provide a much better estimate of the number of planets per star that are found in the habitable zone.
Even if planets are in the habitable zone, however, the number of planets with the right proportion of elements may be difficult to estimate.[16] Also, the Rare Earth hypothesis, which posits that conditions for intelligent life are quite rare, has advanced a set of arguments based on the Drake equation that the number of planets or satellites that could support life is small, and quite possibly limited to Earth alone; in this case, the estimate of ne would be infinitesimal.
fl = the fraction of the above that actually go on to develop life
In 2002, Charles H. Lineweaver and Tamara M. Davis (at the University of New South Wales and the Australian Centre for Astrobiology) estimated fl as > 0.13 on planets that have existed for at least one billion years using a statistical argument based on the length of time life took to evolve on Earth.
fi = the fraction of the above that actually go on to develop intelligent life
This value remains particularly controversial. Those who favor a low value, such as the biologist Ernst Mayr, point out that of the billions of species that have existed on Earth, only one has become intelligent and from this infer a tiny value for fi. Those who favor higher values note the generally increasing complexity of life and conclude that the eventual appearance of intelligence might be inevitable, implying an fi approaching 1. Skeptics point out that the large spread of values in this term and others make all estimates unreliable. (See criticism).
fc = the fraction of the above that are willing and able to communicate
There is considerable speculation why a civilization might exist but choose not to communicate, but there is no hard data.
L = the expected lifetime of such a civilization for the period that it can communicate across interstellar space
In an article in Scientific American, Michael Shermer estimated L as 420 years, based on compiling the durations of sixty historical civilizations. Using twenty-eight civilizations more recent than the Roman Empire he calculates a figure of 304 years for “modern” civilizations. It could also be argued from Michael Shermer’s results that the fall of most of these civilizations was followed by later civilizations that carried on the technologies, so it’s doubtful that they are separate civilizations in the context of the Drake equation. In the expanded version, including reappearance number, this lack of specificity in defining single civilizations doesn’t matter for the end result, since such a civilization turnover could be described as an increase in the reappearance number rather than increase in L, stating that a civilization reappears in the form of the succeeding cultures. Furthermore, since none could communicate over interstellar space, the method of comparing with historical civilizations could be regarded as invalid.
David Grinspoon has argued that once a civilization has developed it might overcome all threats to its survival. It will then last for an indefinite period of time, making the value for L potentially billions of years. If this is the case, then the galaxy has been steadily accumulating advanced civilizations since it formed. He proposes that the last term L be replaced with fIC*T, where fIC is the fraction of communicating civilizations become “immortal” (in the sense that they simply don’t die out), and T representing the length of time during which this process has been going on. This has the advantage that T would be a relatively easy to discover number, as it would simply be some fraction of the age of the universe.
Values based on the above estimates,
R* = 7/year, fp = 0.5, ne = 2, fl = 0.33, fi = 0.01, fc = 0.01, and L = 10000 years
result in
N = 7 × 0.5 × 2 × 0.33 × 0.01 × 0.01 × 10000 = 2.31
Even if there are only three or four civilizations per galaxy, there are billions and billions of galaxies. So the odds that other advanced civilizations similar to our exist in the universe are quite high.
Harvard physicist and SETI leader Paul Horowitz boldly stated in a 1996 interview with TIME Magazine, “Intelligent life in the universe? Guaranteed. Intelligent life in our galaxy? So overwhelmingly likely that I’d give you almost any odds you’d like.”
So where are they?
This paradox, first articulated by nuclear physicist Enrico Fermi in 1950, asks the following questions: If extraterrestrials are so common, why haven’t they visited? Why haven’t they communicated with us? Or, finally, why haven’t they left behind some trace of their passage?
