Drake equation

File:Dr. Frank Drake.jpg

Frank Drake

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy.<ref name="Drake 1961">Physics Today 14 (4), 40–46 (1961). Template:Cite journal</ref><ref name="Burchell"/><ref>Template:Cite journal</ref>

The equation was formulated in 1961 by Frank Drake, not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at the first scientific meeting on the search for extraterrestrial intelligence (SETI).<ref name="December 2002">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life.<ref name="December 2002"/> It is more properly thought of as an approximation than as a serious attempt to determine a precise number.

Criticism related to the Drake equation focuses not on the equation itself, but on the fact that the estimated values for several of its factors are highly conjectural, the combined multiplicative effect being that the uncertainty associated with any derived value is so large that the equation cannot be used to draw firm conclusions.

Equation

The Drake equation is:<ref name="Drake 1961" />

<math display="block">N = R_* \cdot f_\mathrm{p} \cdot n_\mathrm{e} \cdot f_\mathrm{l} \cdot f_\mathrm{i} \cdot f_\mathrm{c} \cdot L</math>

where

• Template:Math = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone);

and

• Template:Math = the average rate of star formation in our Galaxy.
• Template:Math = the fraction of those stars that have planets.
• Template:Math = the average number of planets that can potentially support life per star that has planets.
• Template:Math = the fraction of planets that could support life that actually develop life at some point.
• Template:Math = the fraction of planets with life that go on to develop intelligent life (civilizations).
• Template:Math = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
• Template:Math = the length of time for which such civilizations release detectable signals into space.<ref name="NOVA">

{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="SETI-Drake-Equation">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

This form of the equation first appeared in Drake's 1965 paper.<ref>Template:Cite book</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

History

In September 1959, physicists Giuseppe Cocconi and Philip Morrison published an article in the journal Nature with the provocative title "Searching for Interstellar Communications".<ref name="Cocconi 1959"> Template:Cite journal</ref><ref name="history">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 cm (1,420.4 MHz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe, and they reasoned that other intelligences might see this as a logical landmark in the radio spectrum.

Two months later, Harvard University astronomy professor Harlow Shapley speculated on the number of inhabited planets in the universe, saying "The universe has 10 million, million, million suns (10 followed by 18 zeros) similar to our own. One in a million has planets around it. Only one in a million million has the right combination of chemicals, temperature, water, days and nights to support planetary life as we know it. This calculation arrives at the estimated figure of 100 million worlds where life has been forged by evolution."<ref name="SydneyMorningHerald"> Template:Cite news</ref>

Seven months after Cocconi and Morrison published their article, Drake began searching for extraterrestrial intelligence in an experiment called Project Ozma. It was the first systematic search for signals from communicative extraterrestrial civilizations. Using the Template:Convert dish of the National Radio Astronomy Observatory, Green Bank in Green Bank, West Virginia, Drake monitored two nearby Sun-like stars: Epsilon Eridani and Tau Ceti, slowly scanning frequencies close to the 21 cm wavelength for six hours per day from April to July 1960.<ref name="history"/> The project was well designed, inexpensive, and simple by today's standards. It detected no signals.

Soon thereafter, Drake hosted the first search for extraterrestrial intelligence conference on detecting their radio signals. The meeting was held at the Green Bank facility in 1961. The equation that bears Drake's name arose out of his preparations for the meeting.<ref name="Astrobiology Magazine">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it's going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms.{{#if:Frank Drake|{{#if:|}}

— {{#if:|, in }}Template:Comma separated entries

}}

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The ten attendees were conference organizer J. Peter Pearman, Frank Drake, Philip Morrison, businessman and radio amateur Dana Atchley, chemist Melvin Calvin, astronomer Su-Shu Huang, neuroscientist John C. Lilly, inventor Barney Oliver, astronomer Carl Sagan, and radio-astronomer Otto Struve.<ref name="Wende"> Template:Cite news</ref> These participants called themselves "The Order of the Dolphin" (because of Lilly's work on dolphin communication), and commemorated their first meeting with a plaque at the observatory hall.<ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite encyclopedia</ref>

Usefulness

File:C G-K - DSC 0421.jpg

The Allen Telescope Array for SETI

The Drake equation results in a summary of the factors affecting the likelihood that we might detect radio-communication from intelligent extraterrestrial life.<ref name="Burchell">Template:Cite journal</ref><ref name="NOVA"/><ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The last three parameters, Template:Math, Template:Math, and Template:Mvar, are not known and are very difficult to estimate, with values ranging over many orders of magnitude (see Template:Section link). Therefore, the usefulness of the Drake equation is not in the solving, but rather in the contemplation of all the various concepts which scientists must incorporate when considering the question of life elsewhere,<ref name="Burchell"/><ref name="December 2002"/> and gives the question of life elsewhere a basis for scientific analysis. The equation has helped draw attention to some particular scientific problems related to life in the universe, for example abiogenesis, the development of multi-cellular life, and the development of intelligence itself.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Within the limits of existing human technology, any practical search for distant intelligent life must necessarily be a search for some manifestation of a distant technology. After about 50 years, the Drake equation is still of seminal importance because it is a 'road map' of what we need to learn in order to solve this fundamental existential question.<ref name="Burchell"/> It also formed the backbone of astrobiology as a science; although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories. Some 50 years of SETI have failed to find anything, even though radio telescopes, receiver techniques, and computational abilities have improved significantly since the early 1960s. SETI efforts since 1961 have conclusively ruled out widespread alien emissions near the 21 cm wavelength of the hydrogen frequency.<ref>SETI: A celebration of the first 50 years. Keith Cooper. Astronomy Now. 2000</ref>

Estimates

Original estimates

There is considerable disagreement on the values of these parameters, but the 'educated guesses' used by Drake and his colleagues in 1961 were:<ref name="Drake 1961" /><ref> Template:Cite book</ref><ref> Template:Cite journal Note: This reference has a table of 1961 values, claimed to be taken from Drake & Sobel, but these differ from the book.</ref>

• Template:Math = 1 yr⁻¹ (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative)
• Template:Math = 0.2 to 0.5 (one fifth to one half of all stars formed will have planets)
• Template:Math = 1 to 5 (stars with planets will have between 1 and 5 planets capable of developing life)
• Template:Math = 1 (100% of these planets will develop life)
• Template:Math = 1 (100% of which will develop intelligent life)
• Template:Math = 0.1 to 0.2 (10–20% of which will be able to communicate)
• Template:Math = somewhere between 1000 and 100,000,000 years

Inserting the above minimum numbers into the equation gives a minimum N of 20 (see: Range of results). Inserting the maximum numbers gives a maximum of 50,000,000. Drake states that given the uncertainties, the original meeting concluded that Template:Math, and there were probably between 1000 and 100,000,000 planets with civilizations in the Milky Way Galaxy.

Current estimates

This section discusses and attempts to list the best current estimates for the parameters of the Drake equation.

Rate of star creation in this Galaxy, Template:Math

Calculations in 2010, from NASA and the European Space Agency indicate that the rate of star formation in this Galaxy is about Template:Solar mass of material per year.<ref name=Robitaille>Template:Cite journal</ref><ref name="The Drake Equation"> Template:Cite book</ref> To get the number of stars per year, we divide this by the initial mass function (IMF) for stars, where the average new star's mass is about Template:Solar mass.<ref>Template:Cite journal</ref> This gives a star formation rate of about 1.5–3 stars per year.

Fraction of those stars that have planets, Template:Math

Analysis of microlensing surveys, in 2012, has found that Template:Math may approach 1—that is, stars are orbited by planets as a rule, rather than the exception; and that there are one or more bound planets per Milky Way star.<ref name="bbc.co.uk"> Template:Cite news</ref><ref name="Nature-20120111"> Template:Cite journal</ref>

Average number of planets that might support life per star that has planets, Template:Math

In November 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy.<ref name="NYT-20131104">Template:Cite newsTemplate:Cbignore</ref><ref name="PNAS-20131031">Template:Cite journal</ref> 11 billion of these estimated planets may be orbiting sun-like stars.<ref name="LATimes-20131104">Template:Cite news</ref> Since there are about 100 billion stars in the galaxy, this implies Template:Math is roughly 0.4. The nearest planet in the habitable zone is Proxima Centauri b, which is as close as about 4.2 light-years away.

The consensus at the Green Bank meeting was that Template:Math had a minimum value between 3 and 5. Dutch science journalist Govert Schilling has opined that this is optimistic.<ref name=schilling2011 /> Even if planets are in the habitable zone, the number of planets with the right proportion of elements is difficult to estimate.<ref name="Trimble">Template:Cite journal</ref> Brad Gibson, Yeshe Fenner, and Charley Lineweaver determined that about 10% of star systems in the Milky Way Galaxy are hospitable to life, by having heavy elements, being far from supernovae and being stable for a sufficient time.<ref> Template:Cite journal</ref>

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. So-called hot Jupiters may migrate from distant orbits to near orbits, in the process disrupting the orbits of habitable planets.

On the other hand, the variety of star systems that might have habitable zones is not just limited to solar-type stars and Earth-sized planets. It is now estimated that even tidally locked planets close to red dwarf stars might have habitable zones,<ref> Template:Cite journal</ref> although the flaring behavior of these stars might speak against this.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The possibility of life on moons of gas giants (such as Jupiter's moon Europa, or Saturn's moons Titan and Enceladus) adds further uncertainty to this figure.<ref>Template:Cite journal</ref>

The authors of the rare Earth hypothesis propose a number of additional constraints on habitability for planets, including being in galactic zones with suitably low radiation, high star metallicity, and low enough density to avoid excessive asteroid bombardment. They also propose that it is necessary to have a planetary system with large gas giants which provide bombardment protection without a hot Jupiter; and a planet with plate tectonics, a large moon that creates tidal pools, and moderate axial tilt to generate seasonal variation.<ref name="RareEarth">Template:Cite book</ref>

Fraction of the above that actually go on to develop life, Template:Math

Geological evidence from the Earth suggests that Template:Math may be 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, without assuming that the underlying distribution of Template:Math is the same for all planets in the Milky Way, there are zero degrees of freedom, permitting no valid estimates to be made. If life (or evidence of past life) were to be found on Mars, Europa, Enceladus or Titan that developed independently from life on Earth it would imply a value for Template:Math close to 1. While this would raise the number of 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.<ref> Template:Cite journal</ref> 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."<ref> Template:Cite journal</ref> 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".

In 2020, a paper by scholars at the University of Nottingham proposed an "Astrobiological Copernican" principle, based on the Principle of Mediocrity, and speculated that "intelligent life would form on other [Earth-like] planets like it has on Earth, so within a few billion years life would automatically form as a natural part of evolution". In the authors' framework, Template:Math, Template:Math, and Template:Math are all set to a probability of 1 (certainty). Their resultant calculation concludes there are more than thirty current technological civilizations in the galaxy (disregarding error bars).<ref>Template:Cite journal</ref><ref>Template:Cite news</ref>

Fraction of the above that develops intelligent life, Template:Math

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 Template:Math.<ref name="Ernst Mayr on SETI"> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Likewise, the Rare Earth hypothesis, notwithstanding their low value for Template:Math above, also think a low value for Template:Math dominates the analysis.<ref>Rare Earth, p. xviii.: "We believe that life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake or Sagan envisioned. However, complex life—animals and higher plants—is likely to be far more rare than commonly assumed."</ref> Those who favor higher values note the generally increasing complexity of life over time, concluding that the appearance of intelligence is almost inevitable,<ref name="acampbell.ukfsn.org"> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref> Template:Cite book</ref> implying an Template:Math 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 extinction events have raised the possibility that life on Earth is relatively fragile. Research on any past life on Mars is relevant since a discovery that life did form on Mars but ceased to exist might raise the estimate of Template:Math but would indicate that in half the known cases, intelligent life did not develop.

Estimates of Template:Math 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 tens 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.

There has been quantitative work to begin to define <math>f_\mathrm{l} \cdot f_\mathrm{i}</math>. One example is a Bayesian analysis published in 2020. In the conclusion, the author cautions that this study applies to Earth's conditions. In Bayesian terms, the study favors the formation of intelligence on a planet with identical conditions to Earth but does not do so with high confidence.<ref name="Kipping2020"> Template:Cite journal</ref><ref name="ColumbiaPR"> {{#invoke:citation/CS1|citation |CitationClass=web }} </ref>

Planetary scientist Pascal Lee of the SETI Institute proposes that this fraction is very low (0.0002). He based this estimate on how long it took Earth to develop intelligent life (1 million years since Homo erectus evolved, compared to 4.6 billion years since Earth formed).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}Template:Cbignore</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Fraction of the above revealing their existence via signal release into space, Template:Math

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 human presence. (See Arecibo message, for example). There is considerable speculation why an extraterrestrial civilization might exist but choose not to communicate. However, deliberate communication is not required, and calculations indicate that current or near-future Earth-level technology might well be detectable to civilizations not too much more advanced than present day humans.<ref> Template:Cite journal</ref> By this standard, the Earth is a communicating civilization.

Another question is what percentage of civilizations in the galaxy are close enough for us to detect, assuming that they send out signals. For example, existing Earth radio telescopes could only detect Earth radio transmissions from roughly a light year away.<ref>Template:Cite journal</ref>

Lifetime of such a civilization wherein it communicates its signals into space, Template:Math

Michael Shermer estimated Template:Math as 420 years, based on the duration of sixty historical Earthly civilizations.<ref name="Why ET Hasn’t Called"> Template:Cite journal</ref> Using 28 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 is 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 does not matter for the result, since such a civilization turnover could be described as an increase in the reappearance number rather than increase in Template:Math, 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 enough, it might overcome all threats to its survival. It will then last for an indefinite period of time, making the value for Template:Math potentially billions of years. If this is the case, then he proposes that the Milky Way Galaxy may have been steadily accumulating advanced civilizations since it formed.<ref name="David Grinspoon 2004"> Template:Cite book</ref> He proposes that the last factor Template:Math be replaced with Template:Math, where Template:Math is the fraction of communicating civilizations that become "immortal" (in the sense that they simply do not die out), and Template:Math representing the length of time during which this process has been going on. This has the advantage that Template:Math 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 hypothesized 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.<ref name="GoldsmithOwen"> Template:Cite book</ref>

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. Paleobiologist Olev Vinn suggests that the lifetime of most technological civilizations is brief due to inherited behavior patterns present in all intelligent organisms. These behaviors, incompatible with civilized conditions, inevitably lead to self-destruction soon after the emergence of advanced technologies.<ref name=vinn2024>Template:Cite journal</ref>

An intelligent civilization might not be organic, as some have suggested that artificial general intelligence may replace humanity.<ref>Template:Cite news</ref>

Range of results

As many skeptics have pointed out, the Drake equation can give a very wide range of values, depending on the assumptions,<ref>"The value of Template:Mvar remains highly uncertain. Even if we had a perfect knowledge of the first two terms in the equation, there are still five remaining terms, each of which could be uncertain by factors of 1,000." from Template:Cite journal, or more informally, "The Drake Equation can have any value from "billions and billions" to zero", Michael Crichton, as quoted in Template:Cite book, p. 13</ref> as the values used in portions of the Drake equation are not well established.<ref name="schilling2011">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=renamed_from_2011_on_20160211035411>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=schilling2009>Template:Cite news</ref> In particular, the result can be Template:Math, meaning we are likely alone in the galaxy, or Template:Math, implying there are many civilizations we might contact. One of the few points of wide agreement is that the presence of humanity implies a probability of intelligence arising of greater than zero.<ref name="Dean">Template:Better source needed {{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

As an example of a low estimate, combining NASA's star formation rates, the rare Earth hypothesis value of Template:Math,<ref>Rare Earth, page 270: "When we take into account factors such as the abundance of planets and the location and lifetime of the habitable zone, the Drake Equation suggests that only between 1% and 0.001% of all stars might have planets with habitats similar to Earth. [...] If microbial life forms readily, then millions to hundreds of millions of planets in the galaxy have the potential for developing advanced life. (We expect that a much higher number will have microbial life.)"</ref> Mayr's view on intelligence arising, Drake's view of communication, and Shermer's estimate of lifetime:

Template:Math,<ref name=Robitaille/> Template:Math,<ref name="RareEarth"/> Template:Math,<ref name="Ernst Mayr on SETI"/> Template:Math^{[Drake, above]}, and Template:Math years<ref name="Why ET Hasn’t Called"/>

gives:

Template:Math

i.e., suggesting that we are probably alone in this galaxy, and possibly in the observable universe.

On the other hand, with larger values for each of the parameters above, values of Template:Math can be derived that are greater than 1. The following higher values that have been proposed for each of the parameters:

Template:Math,<ref name=Robitaille/> Template:Math,<ref name="bbc.co.uk"/> Template:Math,<ref name="W. von Bloh, C.Bounama, M. Cuntz, and S. Franck. 2007 1365">

Template:Cite journal</ref><ref name="F. Selsis, J.F. Kasting, B. Levrard, J. Paillet, I. Ribas, and X. Delfosse. 2007 1373"> Template:Cite journal</ref> Template:Math,<ref name="Lineweaver, C. H. & Davis, T. M. 2002 293–304"> Template:Cite journal</ref> Template:Math,<ref name="acampbell.ukfsn.org"/> Template:Math^{[Drake, above]}, and Template:Math years<ref name="David Grinspoon 2004"/> Use of these parameters gives:

Template:Math

Monte Carlo simulations of estimates of the Drake equation factors based on a stellar and planetary model of the Milky Way have resulted in the number of civilizations varying by a factor of 100.<ref> Template:Cite journal</ref>

Possible former technological civilizations

In 2016, Adam Frank and Woodruff Sullivan modified the Drake equation to determine just how unlikely the event of a technological species arising on a given habitable planet must be, to give the result that Earth hosts the only technological species that has ever arisen, for two cases: (a) this Galaxy, and (b) the universe as a whole. By asking this different question, one removes the lifetime and simultaneous communication uncertainties. Since the numbers of habitable planets per star can today be reasonably estimated, the only remaining unknown in the Drake equation is the probability that a habitable planet ever develops a technological species over its lifetime. For Earth to have the only technological species that has ever occurred in the universe, they calculate the probability of any given habitable planet ever developing a technological species must be less than Template:Val. Similarly, for Earth to have been the only case of hosting a technological species over the history of this Galaxy, the odds of a habitable zone planet ever hosting a technological species must be less than Template:Val (about 1 in 60 billion). The figure for the universe implies that it is extremely unlikely that Earth hosts the only technological species that has ever occurred. On the other hand, for this Galaxy one must think that fewer than 1 in 60 billion habitable planets develop a technological species for there not to have been at least a second case of such a species over the past history of this Galaxy.<ref>Template:Cite news</ref><ref>Template:Cite episode</ref><ref>Template:Cite news</ref><ref>Template:Cite journal</ref><ref>Bioverse: How the Cellular World Contains the Secrets to Life's Biggest Questions William B Miller Jr. Template:ISBN p50</ref>

Modifications

As many observers have pointed out, the Drake equation is a very simple model that omits potentially relevant parameters,<ref> Template:Cite journal</ref> and many changes and modifications to the equation have been proposed. One line of modification, for example, attempts to account for the uncertainty inherent in many of the terms.<ref> Template:Cite journal</ref> Combining the estimates of the original six factors by major researchers via a Monte Carlo procedure leads to a best value for the non-longevity factors of 0.85 1/years.<ref>Template:Cite journal</ref> This result differs insignificantly from the estimate of unity given both by Drake and the Cyclops report.

Others note that the Drake equation ignores many concepts that might be relevant to the odds of contacting other civilizations. For example, David Brin states: "The Drake equation merely speaks of the number of sites at which ETIs spontaneously arise. The equation says nothing directly about the contact cross-section between an ETIS and contemporary human society".<ref name="GS"> Template:Cite journal</ref> Because it is the contact cross-section that is of interest to the SETI community, many additional factors and modifications of the Drake equation have been proposed.

Colonization

It has been proposed to generalize the Drake equation to include additional effects of alien civilizations colonizing other star systems. Each original site expands with an expansion velocity Template:Mvar, and establishes additional sites that survive for a lifetime Template:Mvar. The result is a more complex set of 3 equations.<ref name="GS"/>

Reappearance factor

The Drake equation may furthermore be multiplied by how many times an intelligent civilization may occur on planets where it has happened once. Even if an intelligent civilization reaches the end of its lifetime after, for example, 10,000 years, life may still prevail on the planet for billions of years, permitting the next civilization to evolve. Thus, several civilizations may come and go during the lifespan of one and the same planet. Thus, if Template:Math is the average number of times a new civilization reappears on the same planet where a previous civilization once has appeared and ended, then the total number of civilizations on such a planet would be Template:Math, which is the actual reappearance factor added to the equation.

The factor depends on what generally is the cause of civilization extinction. If it is generally by temporary uninhabitability, for example a nuclear winter, then Template:Math may be relatively high. On the other hand, if it is generally by permanent uninhabitability, such as stellar evolution, then Template:Math may be almost zero. In the case of total life extinction, a similar factor may be applicable for Template:Math, that is, how many times life may appear on a planet where it has appeared once.

METI factor

Alexander Zaitsev said that to be in a communicative phase and emit dedicated messages are not the same. For example, humans, although being in a communicative phase, are not a communicative civilization; we do not practise such activities as the purposeful and regular transmission of interstellar messages. For this reason, he suggested introducing the METI factor (messaging to extraterrestrial intelligence) to the classical Drake equation.<ref>

{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> He defined the factor as "the fraction of communicative civilizations with clear and non-paranoid planetary consciousness", or alternatively expressed, the fraction of communicative civilizations that actually engage in deliberate interstellar transmission.

The METI factor is somewhat misleading since active, purposeful transmission of messages by a civilization is not required for them to receive a broadcast sent by another that is seeking first contact. It is merely required they have capable and compatible receiver systems operational; however, this is a variable humans cannot accurately estimate.

Biogenic gases

Astronomer Sara Seager proposed a revised equation that focuses on the search for planets with biosignature gases.<ref name="NYT-20161207">Template:Cite news</ref> These gases are produced by living organisms that can accumulate in a planet atmosphere to levels that can be detected with remote space telescopes.<ref name="equation">{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

The Seager equation looks like this:<ref name=equation />Template:Refn

<math display=block>N = N_* \cdot F_\mathrm{Q} \cdot F_\mathrm{HZ} \cdot F_\mathrm{O} \cdot F_\mathrm{L} \cdot F_\mathrm{S}</math>

where:

Template:Math = the number of planets with detectable signs of life

Template:Math = the number of stars observed

Template:Math = the fraction of stars that are quiet

Template:Math = the fraction of stars with rocky planets in the habitable zone

Template:Math = the fraction of those planets that can be observed

Template:Math = the fraction that have life

Template:Math = the fraction on which life produces a detectable signature gas

Seager stresses, "We're not throwing out the Drake Equation, which is really a different topic," explaining, "Since Drake came up with the equation, we have discovered thousands of exoplanets. We as a community have had our views revolutionized as to what could possibly be out there. And now we have a real question on our hands, one that's not related to intelligent life: Can we detect any signs of life in any way in the very near future?"<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

Carl Sagan's version of the Drake equation

American astronomer Carl Sagan made some modifications<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> in the Drake equation and presented it in the 1980 program Cosmos: A Personal Voyage.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The modified equation is shown below

<math display="block">N = N_\mathrm{*} \cdot f_\mathrm{p} \cdot n_\mathrm{e} \cdot f_\mathrm{l} \cdot f_\mathrm{i} \cdot f_\mathrm{c} \cdot f_\mathrm{L}</math><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> where

• Template:Math = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone);

and

• Template:Math = Number of stars in the Milky Way Galaxy
• Template:Math = the fraction of those stars that have planets.
• Template:Math = the average number of planets that can potentially support life per star that has planets.
• Template:Math = the fraction of planets that could support life that actually develop life at some point.
• Template:Math = the fraction of planets with life that go on to develop intelligent life (civilizations).
• Template:Math = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
• Template:Math = fraction of a planetary lifetime graced by a technological civilization

Criticism

Criticism of the Drake equation is varied. Firstly, many of the terms in the equation are largely or entirely based on conjecture.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Star formation rates are well-known, and the incidence of planets has a sound theoretical and observational basis, but the other terms in the equation become very speculative. The uncertainties revolve around the present day understanding of the evolution of life, intelligence, and civilization, not physics. No statistical estimates are possible for some of the parameters, where only one example is known. The net result is that the equation cannot be used to draw firm conclusions of any kind, and the resulting margin of error is huge, far beyond what some consider acceptable or meaningful.<ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Others point out that the equation was formulated before our understanding of the universe had matured. Astrophysicist Ethan Siegel, said:

The Drake equation, when it was put forth, made an assumption about the Universe that we now know is untrue: It assumed that the Universe was eternal and static in time. As we learned only a few years after Frank Drake first proposed his equation, the Universe doesn’t exist in a steady state, where it’s unchanging in time, but rather has evolved from a hot, dense, energetic, and rapidly expanding state: a hot Big Bang that occurred over a finite duration in our cosmic past.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>{{#if:|{{#if:|}}

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One reply to such criticisms<ref> Template:Cite journal</ref> is that even though the Drake equation currently involves speculation about unmeasured parameters, it was intended as a way to stimulate dialogue on these topics. Then the focus becomes how to proceed experimentally. Indeed, Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference.<ref>

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|CitationClass=web }}</ref>

Fermi paradox

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} A civilization lasting for tens of millions of years could be able to spread throughout the galaxy, even at the slow speeds foreseeable with present-day technology. However, no confirmed signs of civilizations or intelligent life elsewhere have been found, either in this Galaxy or in the observable universe of 2 trillion galaxies.<ref name="Conselice">Template:Cite journal</ref><ref name="NYT-20161017">Template:Cite newsTemplate:Cbignore</ref> According to this line of thinking, the tendency to fill (or at least explore) all available territory seems to be a universal trait of living things, so the Earth should have already been colonized, or at least visited, but no evidence of this exists. Hence Fermi's question "Where is everybody?".<ref name="OSTI-19850301"> Template:Cite report</ref><ref> Template:Cite news</ref>

A large number of explanations have been proposed to explain this lack of contact; a book published in 2015 elaborated on 75 different explanations.<ref> Template:Cite book</ref> In terms of the Drake Equation, the explanations can be divided into three classes:

• Few intelligent civilizations ever arise. This is an argument that at least one of the first few terms, Template:Math, has a low value. The most common suspect is Template:Math, but explanations such as the rare Earth hypothesis argue that Template:Math is the small term.
• Intelligent civilizations exist, but we see no evidence, meaning Template:Math is small. Typical arguments include that civilizations are too far apart, it is too expensive to spread throughout the galaxy, civilizations broadcast signals for only a brief period of time, communication is dangerous, and many others.
• The lifetime of intelligent, communicative civilizations is short, meaning the value of Template:Mvar is small. Drake suggested that a large number of extraterrestrial civilizations would form, and he further speculated that the lack of evidence of such civilizations may be because technological civilizations tend to disappear rather quickly. Typical explanations include it is the nature of intelligent life to destroy itself, it is the nature of intelligent life to destroy others, they tend to be destroyed by natural events, and others.

These lines of reasoning lead to the Great Filter hypothesis,<ref> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref> which states that since there are no observed extraterrestrial civilizations despite the vast number of stars, at least one step in the process must be acting as a filter to reduce the final value. According to this view, either it is very difficult for intelligent life to arise, or the lifetime of technologically advanced civilizations, or the period of time they reveal their existence must be relatively short.

An analysis by Anders Sandberg, Eric Drexler and Toby Ord suggests "a substantial ex ante (predicted) probability of there being no other intelligent life in our observable universe".<ref>Template:Cite arXiv</ref>

In popular culture

File:Europa Clipper commemorative plate.jpg

Commemorative plate on Europa Clipper

The equation was cited by Gene Roddenberry as supporting the multiplicity of inhabited planets shown on Star Trek, the television series he created. However, Roddenberry did not have the equation with him, and he was forced to "invent" it for his original proposal.<ref>The Making of Star Trek by Stephen E. Whitfield and Gene Roddenberry, New York: Ballantine Books, 1968</ref> The invented equation created by Roddenberry is:

<math display="block">Ff^2 (MgE)-C^1 Ri^1 \cdot M=L/So </math>

Regarding Roddenberry's fictional version of the equation, Drake himself commented that a number raised to the first power is just the number itself.<ref>Template:Cite book</ref>

A commemorative plate on NASA's Europa Clipper mission, which launched October 14, 2024, features a poem by the U.S. Poet Laureate Ada Limón, waveforms of the word 'water' in 103 languages, a schematic of the water hole, the Drake equation, and a portrait of planetary scientist Ron Greeley on it.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }} Template:PD-notice</ref>

The track Abiogenesis on the Carbon Based Lifeforms album World of Sleepers features the Drake equation in a spoken voice-over.

See also

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• The Search for Life: The Drake Equation, BBC documentary

Further reading

• Template:Cite book
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• Template:Cite book

External links

• Interactive Drake Equation Calculator
• Frank Drake's 2010 article on "The Origin of the Drake Equation"
• "Only a matter of time, says Frank Drake". A Q&A with Frank Drake in February 2010
• Template:Cite magazine
• Macromedia Flash page allowing the user to modify Drake's values from PBS's Nova
• "The Drake Equation", Astronomy Cast episode #23; includes full transcript
• Animated simulation of the Drake equation. (Template:Webarchive)
• "The Alien Equation", BBC Radio program Discovery (22 September 2010)
• "Reflections on the Equation" (PDF), by Frank Drake, 2013