Drake equation
The Drake equation is used to estimate the number of technical, intelligent civilizations in our galaxy, the Milky Way. It was developed by Frank Drake, an American astrophysicist and at a conference in Green Bank, USA, presented in November 1961; it is therefore also known as the Green Bank formula or SETI equation. The formula is often used to look at considerations relating for extraterrestrial life. Most of the factors of the equation are unknown.
Preview
On sulfur and silicon -based life is not factored into the equation, since it is not exactly predictable, whether and under what conditions such a life can arise. The Drake 's considerations relate to life, which develops under certain conditions with respect to the ratios of nitrogen, carbon and other uncertainties. The human species is regarded as proof that it can work. The system and the planet on which such a life is to develop must meet certain astronomical and physical-chemical conditions according to this theory:
The central star must have an appropriate circumstellar habitable zone. This is the case for stars of spectral types F to M and the luminosity class V. Thus, on the one hand planets can form suitable chemical, on the other hand, these planets are protected from too-frequent cosmic catastrophes such as supernova explosions, the system must be located in the galactic habitable zone. So enough radioactive elements are available in order to keep a carbonate - silicate cycle as CO2 source in motion, the planet must before the end of the cosmic habitable age form ( which is, however, continue for 10 to 20 billion years).
In addition to these recognized as general conditions, there are some limitations which, although held in astrobiology due to the development of our earth for probably, but they are not necessarily recognized as essential. For example, it is assumed that the axis of rotation should not be tilted too much, so there are no major seasonal differences. A moon in the correct size stabilizes the tilt of the axis of rotation and thus the climate, but also a planet with high or even chaotic axial inclination may be habit of attacking.
Equation
Indicates the potential number of extraterrestrial civilizations in the galaxy that communicate technically capable and would be willing.
Factors
Average rate of star formation per year in our galaxy: The mean rate of star formation is relatively well estimated by empirical observations such as by the Hubble Space Telescope and is estimated 4-19. When viewing is to be noted that a star average magnitude is required. Stars that are larger and more luminous than the sun, consume their energy in less than a billion years, allowing for the development of life on suitable planets there is insufficient time. It is therefore searched for stars that are similar to our sun, since it is assumed that the development of life as on Earth takes about a billion years. About 70 percent of the stars are low luminosity red dwarfs. Although these stars have a life cycle, which is higher by one order of magnitude than that of the sun, while its luminosity, mass and gravitational force is much lower, making the Habitable Zone is located very close to the central star and planet in this zone would thus be exposed to strong tidal forces. In addition, red dwarfs are prone to strong changes in solar activity, which would be detrimental to the development of life.
Furthermore, approximately every second emergence as a double or multiple star system. It is two or more stars orbiting each other, more specifically revolve around their common center of gravity. Physical simulations have shown that planets in such systems have a very unstable path and crash sooner or later in one of the suns or are thrown completely out of the system ( three-and many-body problem). An exception are planets that are so far from their suns, that the attraction of the two stars on the planet acts like a single star and the planet thus again a more stable path has ( two-body problem ). The probability that a multiple star system over time has planets, was long considered very low, but planetary companions are now several double stars have been found.
Proportion of stars with planetary systems: How many stars in our galaxy have a planetary system? Observations show that about half of all stars have planetary systems like our sun. Since 1995 with very sensitive detectors by measuring the radial velocity of solar-like stars than 1,000 extrasolar planets discovered (as of 2013). With increasing accuracy of the instruments, new methods and better resolution telescopes even more precise measurements will be possible. In particular, the mission of the space telescope Kepler has multiplied the number of discovered smaller planets. Before that could extrasolar planets, especially the very high mass (several Jupiter masses) and / or are very close to their star to be found. In both cases, there is likely to be very inhospitable living conditions.
Number of planets in the ecosphere: The Ecosphere is the area in the planetary system in which not preclude the formation of life from the outset the physical conditions. A planet may, depending on the size of the sun, do not be away too close and not too far from its star. If it is too far away, it is too cold; he is too close, it is too hot and the solar wind blows away the atmosphere. In our solar system, Venus, Earth and Mars are in the ecosphere. 2007, two exoplanets were first discovered, which could be in the habitable zone: HD 209458 b, and the institution designated by its discoverers as Earthlike Planet Gliese 581 c. Whether the conditions but are really friendly to life there, is controversial among scientists.
Statistical analysis of data from the Kepler telescope suggest that there are several billion Earth-sized planets in the ecosphere to sun-like stars in the Milky Way.
Percentage of planets with life: How many planets in the ecosphere created life? There is no scientifically verifiable numbers for this factor, because so far there is only the example of our solar system. For the future it is expected with more sensitive telescopes on exoplanets, the signs of life, to be able, for example, oxygen in the atmosphere, are looking for.
Percentage of planets with intelligent life: If evolved on a planet of life, so it does not develop into intelligent life itself. Also for this factor, there is no scientifically verifiable numbers. There can be only our solar system as an example. This raises the question of how intelligence is defined.
Percentage of planets with an interest in interstellar communication: How many intelligent civilizations might be interested in communicating with other individuals? Only if you are interested in communication, it is possible to find them for us. It is believed that extraterrestrial intelligent beings also go on the search for life.
Lifetime of a technological civilization in years: As a technical civilization is called a civilization that is able to receive and transmit a signal into space radio signal from space. Life on the planet is threatened by external and internal factors. A complete destruction can be triggered by events that have resulted in Earth's history several times to mass extinction. These include drastic climate changes, for example, by massive volcanic eruptions and impacts from comets or asteroids. It could also mean self- destruction of a technological civilization and the destruction of a technical intelligent civilization by another species such as a virus.
Since the lifetime of stars is limited, the life of a civilization in the respective solar system is also limited. Civilizations outside of solar systems would be upgraded to sufficient sun -independent energy sources; However, Perpetua mobilia are impossible according to the laws of thermodynamics.
Uncertainties
Destined for the validity of the Drake Equation, the uncertainties of the individual factors. Especially for the last four factors there are at best very far stray guesses about the correct value. Thus, the estimated from the product of uncertain factors total number of intelligent civilizations is extremely inaccurate.
The Drake equation applies only to our galaxy, the Milky Way, which is a barred spiral galaxy. This barred spiral type is correct to the present knowledge about 2 /3 of the galaxies located in the universe. Under the assumption that the observable universe today is home to approximately 50-100 billion galaxies of similar type, the value from the Drake equation for the entire universe by a corresponding factor would have to be multiplied. Thus, although the estimated total number of possible civilizations increased significantly, but remains so far due to insufficient data from other galaxies still extremely inaccurate. The estimates also determined based on the extrapolation of only presumed similarities of the output data in all galaxies.
The Drake equation explicitly refers not only to the theoretically possible number of civilizations, but on the practical possibility of contacts. Since the next more galaxy, the Andromeda nebula, 2.5 million light-years away is already not get these and all other practical contact into account.
Criticism, discussion and extensions
The biologist Ernst Mayr has pointed out that there had been merely developed under the 50 billion species that has brought forth the earth an intelligence.
1983 beat David Brin, an expanded Drake equation.
Published in 2010 the astronomer and technical director of the IAA Claudio Maccone a more complex version of the equation, the Statistical Drake Equation.
Astrophysicist Martin Elvis adapted in 2013 to meet the Drake equation to first estimates on the possible number of asteroids that could be used for space mining come into question.
Models
On the above-mentioned Green Bank conference three models were presented for the Drake equation.
Although this information, given the above enormous uncertainties can not be refuted, going from various later sources of much smaller values for the second and third model. Firstly, the ecosphere is much narrower, if you already incorporates the principle possibility of complex life here. On the other hand require the above models that most likely arises sometime life when conditions are favorable for a long time for this.
The American astronomer Carl Sagan and exobiologist estimated the number of civilizations to ten.