Drake Equation - Discussion and Current Estimates

Discussion and Current Estimates

This section discusses, then 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.

f_p = 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. 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. However, a new study suggests that f_p may approach 1 -- that is, that virtually every star has at least one planet.

n_e = 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 et al. 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 . In 2010, researchers announced the discovery of Gliese 581 g, a 3.1 Earth-mass planet 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 Gliese 581, and the number of stars examined to the level of detail needed to find such planets, they estimated ε_Earth, or the fraction of stars with Earth-like planets, as 10-20%. However, other research has put the existence of Gliese 581 g, the basis for this estimate, into question.

Using different criteria, Brad Gibson, Yeshe Fenner, and Charley Lineweaver 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 6 March 2009. Unlike previous searches, it is sensitive to planets as small as Earth, and with orbital periods as long as a year. Since it looks at a larger sample, about 150,000 stars, the ongoing Kepler mission should eventually provide a fairly solid estimate of the number of planets per star that are found in the habitable zone. Estimates from partial data include that 34±14 percent of FGK stars are predicted to have at least one terrestrial, habitable-zone planet, and that at least 5.4% of all stars host a terrestrial planet.

Even if planets are in the habitable zone, however, the number of planets with the right proportion of elements may be difficult to estimate. 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 n_e would be infinitesimal.

Also, the discovery of numerous gas giants in close orbit with their stars has introduced doubt that life-supporting planets commonly survive the formation of their stellar systems. In addition, most stars in our galaxy are red dwarfs, which flare violently, mostly in X-rays—a property not conducive to life as we know it (simulations also suggest that these bursts erode planetary atmospheres). The possibility of life on moons of gas giants (such as Jupiter's moon Europa, or Saturn's moon Titan) adds further uncertainty to this figure.

Estimates of f_i have been affected by discoveries that the solar system's orbit is circular in the galaxy, at such a distance that it remains out of the spiral arms for hundreds of millions of years (evading radiation from novae). Also, Earth's large moon may aid the evolution of life by stabilizing the planet's axis of rotation.

f_l = the fraction of the above that actually go on to develop life

Geological evidence from the Earth suggests that f_l may be very high; life on Earth appears to have begun around the same time as favorable conditions arose, suggesting that abiogenesis may be relatively common once conditions are right. However, this evidence only looks at the Earth (a single model planet), and contains anthropic bias, as the planet of study was not chosen randomly, but by the living organisms that already inhabit it (ourselves). From a classical hypothesis testing standpoint, there are zero degrees of freedom, permitting no valid estimates to be made. If life were to be found on Mars that developed independently from life on Earth it would imply a value for f_l close to one. While this would improve the degrees of freedom from zero to one, there would remain a great deal of uncertainty on any estimate due to the small sample size, and the chance they are not really independent.

Countering this argument is that there is no evidence for abiogenesis occurring more than once on the Earth—that is, all terrestrial life stems from a common origin. If abiogenesis were more common it would be speculated to have occurred more than once on the Earth. Scientists have searched for this by looking for bacteria that are unrelated to other life on Earth, but none have been found yet. It is also possible that life arose more than once, but that other branches were out-competed, or died in mass extinctions, or were lost in other ways. Biochemists Francis Crick and Leslie Orgel laid special emphasis on this uncertainty: "At the moment we have no means at all of knowing" whether we are "likely to be alone in the galaxy (Universe)" or whether "the galaxy may be pullulating with life of many different forms.". As an alternative to abiogenesis on earth, they proposed the hypothesis of directed panspermia, which states that earth life began with "microorganisms sent here deliberately by a technological society on another planet, by means of a special long-range unmanned spaceship" (Crick and Orgel, op.cit.).

In 2002, Charles H. Lineweaver and Tamara M. Davis (at the University of New South Wales and the Australian Centre for Astrobiology) estimated f_l 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.

f_i = 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 f_i. 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 f_i approaching 1. Skeptics point out that the large spread of values in this factor and others make all estimates unreliable. (See criticism).

In addition, while it appears that life developed soon after the formation of Earth, the Cambrian explosion, in which a large variety of multicellular life forms came into being, occurred a considerable amount of time after the formation of Earth, which suggests the possibility that special conditions were necessary. Some scenarios such as the Snowball Earth or research into the extinction events have raised the possibility that life on Earth is relatively fragile. Again, the controversy over life on Mars is relevant since a discovery that life did form on Mars but ceased to exist would affect estimates of these factors.

Again this model has a large anthropic bias and there are still zero degrees of freedom. Note that the capacity and willingness to participate in extraterrestrial communication has come relatively "quickly", with the Earth having only an estimated 100,000 year history of intelligent human life, and less than a century of technological ability.

f_c = the fraction of the above that release detectable signs of their existence into space

For deliberate communication, the one example we have (the Earth) does not do much explicit communication, though there are some efforts covering only a tiny fraction of the stars that might look for our presence. (See Arecibo message, for example). For other civilizations, there is considerable speculation why a civilization might exist but choose not to communicate, but no hard data. However, deliberate communication is not required, and calculations indicate that current or near-future Earth-level technology might well be visible to civilizations not too much in advance of our own. By this standard the Earth is a communicating civilization.

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 factor L be replaced with f_IC*T, where f_IC 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.

It has also been pointed out that, once a civilization has learned of a more advanced one, its longevity could increase because it can learn from the experiences of the other.

The astronomer Carl Sagan speculated that all of the terms, except for the lifetime of a civilization, are relatively high and the determining factor in whether there are large or small numbers of civilizations in the universe is the civilization lifetime, or in other words, the ability of technological civilizations to avoid self-destruction. In Sagan's case, the Drake equation was a strong motivating factor for his interest in environmental issues and his efforts to warn against the dangers of nuclear warfare.