Drake equation

The Drake equation (sometimes called the Green Bank equation or the Green Bank Formula) is an equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy. It is used in the fields of exobiology and the Search for ExtraTerrestrial Intelligence (SETI). The equation was devised by Frank Drake, Emeritus Professor of Astronomy and Astrophysics at the University of California, Santa Cruz.

Contents

History

In 1960, Frank Drake conducted the first search for radio signals from extraterrestrial civilizations at the National Radio Astronomy Observatory in Green Bank, West Virginia. Soon thereafter, the National Academy of Sciences asked Drake to convene a meeting on detecting extraterrestrial intelligence. 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:

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, of course, was aimed at the radio search, and not to search for primordial or primitive life forms.

— Frank Drake[1]

This meeting established SETI as a scientific discipline. The meeting's dozen participants — astronomers, physicists, biologists, social scientists, and industry leaders — became known as the "Order of the Dolphin". The Green Bank meeting has been commemorated by a plaque at the site.

The Drake equation is closely related to the Fermi paradox in that Drake suggested that a large number of extraterrestrial civilizations would form, but that the lack of evidence of such civilizations (the Fermi paradox) suggests that technological civilizations tend to disappear rather quickly. This theory often stimulates an interest in identifying and publicizing ways in which humanity could destroy itself, and then counters with hopes of avoiding such destruction and eventually becoming a space-faring species. A similar argument is the Great Filter,[2] which notes that since there are no observed extraterrestrial civilizations, despite the vast number of stars, then some step in the process must be acting as a filter to reduce the final value. According to this view, either it is very hard for intelligent life to arise, or the lifetime of such civilizations must be relatively short.

Carl Sagan, a great proponent of SETI, quoted the formula often and as a result the formula is sometimes mislabeled as "The Sagan Equation."

The equation

The Drake equation states that:

N = R^{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L \!

where:

N = the number of civilizations in our galaxy with which communication might be possible;

and

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.[3]

R factor

One can question why the number of civilizations should be proportional to the star formation rate, though this makes technical sense. (The product of all the terms except L tells how many new communicating civilizations are born each year. Then you multiply by the lifetime to get the expected number. For example, if an average of 0.01 new civilizations are born each year, and they each last 500 years on the average, then on the average 5 will exist at any time.) The original Drake Equation can be extended to a more realistic model, where the equation uses not the number of stars that are forming now, but those that were forming several billion years ago. The alternate formulation, in terms of the number of stars in the galaxy, is easier to explain and understand, but implicitly assumes the star formation rate is constant over the life of the galaxy.

Alternative expression

The number of stars in the galaxy now, N*, is related to the star formation rate R* by

N^{\ast} = \int_0^{T_g} R^{\ast}(t) dt , \,\!

where Tg = the age of the galaxy. Assuming for simplicity that R* is constant, then N^{\ast} = R^{\ast} \cdot T_g and the Drake equation can be rewritten into an alternate form phrased in terms of the more easily observable value, N*.[4]

N = N^{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L / T_g \,\!

Elaborations

As many observers have pointed out, the Drake equation is a very simple model that does not include potentially relevant parameters. David Brin states:

[The Drake Equation] merely speaks of the number of sites at which ETIs spontaneously arise. It says nothing directly about the contact cross-section between an ETIS and contemporary human society.[5]

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. These include the number of times a civilization might re-appear on the same planet, the number of nearby stars that might be colonized and form sites of their own, and other factors.

Colonization

Brin has proposed generalizing the Drake Equation to include additional effects of alien civilizations colonizing other star systems. Each original site expands with an expansion velocity v, and establishes additional sites that survive for a lifetime L'. The result is a more complex set of 3 equations.[5]

Reappearance number

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 nr 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 (1+nr), 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 nr may be relatively high. On the other hand, if it is generally by permanent uninhabitability, such as stellar evolution, then nr may be almost zero.

In the case of total life extinction, a similar factor may be applicable for fℓ, 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 practice such activities as the purposeful and regular transmission of interstellar messages. For this reason, he suggested introducing the METI factor (Messaging to Extra-Terrestrial Intelligence) to the classical Drake Equation. The factor is defined 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.

Historical estimates of the parameters

Considerable disagreement on the values of most of these parameters exists, but the values used by Drake and his colleagues in 1961 were:

Drake's values give N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 10,000 = 10.

The value of R* is determined from considerable astronomical data, and is the least disputed term of the equation; fp is less certain, but is still much firmer than the values following. The value of ne is based on our own solar system, and assumes that two planets had the possibility of having life. This not only has problems with anthropic bias, but also is inconsistent with a fl of one unless life on Mars is ever discovered. Also, the discovery of numerous gas giants in close orbit with their stars has introduced doubt that life-supporting planets commonly survive the creation 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.

Geological evidence from the Earth suggests that fl 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). Also 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. In addition, 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 fl 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.

Similar arguments of bias can be made regarding fi and fc by considering the Earth as a model: intelligence with the capacity of extraterrestrial communication occurs only in one species in the 4 billion year history of life on Earth. If generalized, this means only relatively old planets may have intelligent life capable of extraterrestrial communication. 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.

fi, fc and L, like fl, are also guesses. Estimates of fi 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. 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 terms.

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.

By plugging in apparently plausible values for each of the parameters above, the resultant value of N can be made greater than 1. This has provided considerable motivation for the SETI movement. However, we have no evidence for extraterrestrial civilizations. This conflict is often called the Fermi paradox, after Enrico Fermi who first asked about our lack of observation of extraterrestrials, and motivates advocates of SETI to continually expand the volume of space in which another civilization could be observed.

Some computations of the Drake equation, given different assumptions:

Current estimates (see below):

R* = 7/year, fp = 0.5, ne = 2, fl = 0.33, fi = 0.01, fc = 0.01, and L = 10,000 years
N = 7 × 0.5 × 2 × 0.33 × 0.01 × 0.01 × 10,000 = 2.31 (so two communicative civilizations exist in our galaxy at any given time, on average, plus two hundred more that are not trying to communicate).

But a pessimist might equally well believe that suitable planets are rare, life seldom becomes intelligent, and intelligent civilizations do not last very long:

R* = 10/year, fp = 0.5, ne = 0.01, fl = 0.13, fi = 0.001, fc = 0.01, and L = 1000 years
N = 10 × 0.5 × 0.01 × 0.13 × 0.001 × 0.01 × 1000 = 0.000065 (we are almost surely alone in our galaxy, but many other galaxies host life).

Alternatively, making some more optimistic assumptions, assuming that planets are common, life always arises when planets are favorable, 10% of civilizations become willing and able to communicate, and then spread through their local star systems for 100,000 years (a very short period in geologic time):

R* = 20/year, fp = 0.5, ne = 2, fl = 1, fi = 0.1, fc = 0.1, and L = 100,000 years
N = 20 × 0.5 × 2 × 1 × 0.1 × 0.1 × 100,000 = 20,000 (there are quite a few civilizations, and the closest one would be about 350 light years away based on N/R^2=1/r^2 where R is the radius on the galaxy and 1/r^2 is the average area that contains one galaxy).

Current estimates of the parameters

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.[6]

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,[7] 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.[9] 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.[10] The Kepler mission, from its initial data, estimates that about 34% of stars host at least one planet.[11]

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 et al.[8] 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.[12][13] Since about 200 planetary systems are known, this very roughly estimates n_e > 0.005. 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.[14] Given the closeness of Gliese 581, and the number of stars examined to the level of detail needed to find such planets, they estimated εEarth, 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, 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.[15]
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.[17]

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[18] and from this infer a tiny value for fi. Those who favor higher values note the generally increasing complexity of life[19] and conclude that the eventual appearance of intelligence might be inevitable,[20] 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.[21] 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.[22] 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.

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.[23]

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

James Kasting, in his book "How To Find A Habitable Planet", gives the equation as N=N_g \cdot f_p \cdot n_e \cdot f_l \cdot f_i \cdot f_c \cdot f_L, where the first term on the right hand side of the equation is the number of stars in the galaxy. He estimates the first three terms at 4 × 109. He then uses Carl Sagan's figures for the next three terms, disclaiming responsibility, and arrives at approximately 10 to the seventh power as an estimate, not considering the final term, f_L, which is the fraction of a planet's lifetime during which it supports a technical civilization. He notes that this is the most uncertain factor in the equation.[24]

Criticism

Criticism of the Drake equation follows mostly from the observation that several terms in the equation are largely or entirely based on conjecture. Thus the equation cannot be used to draw firm conclusions of any kind. As Michael Crichton, a science fiction author, stated in a 2003 lecture at Caltech:[25]

The problem, of course, is that none of the terms can be known, and most cannot even be estimated. The only way to work the equation is to fill in with guesses. [...] As a result, the Drake equation can have any value from "billions and billions" to zero. An expression that can mean anything means nothing. Speaking precisely, the Drake equation is literally meaningless...

Another objection is that the very form of the Drake equation assumes that civilizations arise and then die out within their original solar systems. If interstellar colonization is possible, then this assumption is invalid, and the equations of population dynamics would apply instead.[26]

One reply to such criticisms[27] is that even though the Drake equation currently involves speculation about unmeasured parameters, it was not meant to be science, but 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.[28]

In fiction

The Drake equation and the Fermi paradox have been discussed many times in science fiction, including both serious takes in stories such as Frederik Pohl's Hugo award-winning "Fermi and Frost", which cites the paradox as evidence for the short lifetime of technical civilizations—that is, the possibility that once a civilization develops the power to destroy itself (perhaps by nuclear winter), it does. Optimistic results of the equation along with unobserved extraterrestrials also serves as backdrop for humorous suggestions such as Terry Bisson's classic short story "They're Made Out of Meat," that there are many extraterrestrial civilizations but that they are deliberately ignoring humanity.[29]

Ff^2 (MgE)-C^1 Ri^1 ~ \cdot ~ M=L/So.\
Drake has gently pointed out, however, that a number raised to the first power is merely the number itself. A poster with both versions of the equation was seen in the Star Trek: Voyager episode "Future's End."

See also

Footnotes

  1. ^ "The Drake Equation Revisited: Part I". http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=610. 
  2. ^ Robin Hanson (1998). "The Great Filter — Are We Almost Past It?". http://hanson.gmu.edu/greatfilter.html. 
  3. ^ "PBS NOVA: Origins - The Drake Equation". Pbs.org. http://www.pbs.org/wgbh/nova/origins/drake.html. Retrieved 2010-03-07. 
  4. ^ Michael Seeds, Horizons: Exploring the Universe, Brooks/Cole Publishing Co., 10th edition, ISBN 978-0-495-11358-4
  5. ^ a b G. D. Brin (1983). "The Great Silence - the Controversy Concerning Extraterrestrial Intelligent Life". Quarterly Journal of the Royal Astronomical Society 24: 283–309. Bibcode 1983QJRAS..24..283B. 
  6. ^ "Milky Way Churns Out Seven New Stars Per Year, Scientists Say". Goddard Space Flight Center, NASA. http://www.nasa.gov/centers/goddard/news/topstory/2006/milkyway_seven.html. Retrieved 2008-05-08. 
  7. ^ "Scientists announce planet bounty". BBC. 2009-10-19. http://news.bbc.co.uk/1/hi/sci/tech/8314581.stm. Retrieved 2009-10-19. 
  8. ^ a b Marcy, G.; Butler, R.; Fischer, D.; Vogt, Steven et al. (2005). "Observed Properties of Exoplanets: Masses, Orbits and Metallicities". Progress of Theoretical Physics Supplement 158: 24–42. arXiv:astro-ph/0505003. Bibcode 2005PThPS.158...24M. doi:10.1143/PTPS.158.24. http://ptp.ipap.jp/link?PTPS/158/24. 
  9. ^ "Many, Perhaps Most, Nearby Sun-Like Stars May Form Rocky Planets". http://www.nasa.gov/mission_pages/spitzer/news/spitzer-20080217.html. 
  10. ^ John Chambers (23 September 2010). "Extrasolar planets: More giants in focus". Nature 467 (467): 405–406. Bibcode 2010Natur.467..405C. doi:10.1038/467405a. PMID 20864987. 
  11. ^ Borucki, William J.; Koch, David G; Basri, Gibor; Batalha, Natalie; Brown, Timothy M.; et. al. (1 February 2011). "Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data" (PDF). arXiv. http://arxiv.org/ftp/arxiv/papers/1102/1102.0541.pdf. Retrieved 2011-02-16.  Although called 'candidates' in the paper, other references show most of these candidates will turn out to be real planets.
  12. ^ W. von Bloh, C.Bounama, M. Cuntz, and S. Franck. (2007). "The habitability of super-Earths in Gliese 581". Astronomy & Astrophysics 476 (3): 1365. Bibcode 2007A&A...476.1365V. doi:10.1051/0004-6361:20077939. 
  13. ^ F. Selsis, J.F. Kasting, B. Levrard, J. Paillet, I. Ribas, and X. Delfosse. (2007). "Habitable planets around the star Gliese 581?". Astronomy & Astrophysics 476 (3): 1373. Bibcode 2007A&A...476.1373S. doi:10.1051/0004-6361:20078091. 
  14. ^ Steven S. Vogt, R. Paul Butler, Eugenio J. Rivera, Nader Haghighipour, Gregory W. Henry, Michael H. Williamson (2010). "The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581". arXiv:1009.5733 [astro-ph.EP]. 
  15. ^ "One tenth of stars may support life". New Scientist. 2004-01-01. http://www.newscientist.com/article/dn4525-one-tenth-of-stars-may-support-life.html. Retrieved 2010-05-12. 
  16. ^ Trimble, V. (1997). "Origin of the biologically important elements.". Orig Life Evol Biosph. 27 (1–3): 3–21. doi:10.1023/A:1006561811750. PMID 9150565. 
  17. ^ Lineweaver, C. H. & Davis, T. M. (2002). "Does the rapid appearance of life on Earth suggest that life is common in the universe?". Astrobiology 2 (3): 293–304. arXiv:astro-ph/0205014. Bibcode 2002AsBio...2..293L. doi:10.1089/153110702762027871. PMID 12530239. 
  18. ^ "Ernst Mayr on SETI". http://www.planetary.org/explore/topics/search_for_life/seti/mayr.html. 
  19. ^ Bonner, J.T. (1988). The evolution of complexity by means of natural selection. Princeton Univ Press. ISBN 0691084947. 
  20. ^ "Review of Life's Solution by Simon Morris". http://www.acampbell.ukfsn.org/bookreviews/r/morris.html. 
  21. ^ "Why ET Hasn’t Called". Scientific American. August 2002. http://www.michaelshermer.com/2002/08/why-et-hasnt-called/. 
  22. ^ Lonely Planets, David Grinspoon (2004)
  23. ^ Goldsmith, Donald; Tobias Owen (1992). The Search for Life in the Universe (2 ed.). Addison-Wesley. pp. 415. 
  24. ^ Kastings, James. "How To Find A Habitable Planet" Princeton University Press, Princeton, 2010.
  25. ^ Michael Crichton. "Aliens cause Global Warming". http://www.michaelcrichton.com/speech-alienscauseglobalwarming.html. 
  26. ^ Jack Cohen and Ian Stewart (2002). Evolving the Alien. John Wiley and Sons, Inc., Hoboken, NJ.  Chapter 6, What does a Martian look like?
  27. ^ Jill Tarter, The Cosmic Haystack Is Large, Skeptical Inquirer magazine, May 2006.
  28. ^ Amir Alexander. "The Search for Extraterrestrial Intelligence: A Short History - Part 7: The Birth of the Drake Equation". http://www.planetary.org/explore/topics/seti/seti_history_07.html. 
  29. ^ "They're made out of Meat, by Hugo and Nebula Winner Terry Bisson". Baetzler.de. http://baetzler.de/humor/meat_beings.html. Retrieved 2010-03-07. 
  30. ^ The Making of STAR TREK by Stephen E. Whitfield and Gene Roddenberry, Ballantine Books, N. Y., 1968

References

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