This calculator determines the numbers of suitable stars, planets, life-bearing planets, and civilizations which might arise in a galaxy. The algorithm includes numerous factors that can influence the number of advanced, species-wide civilizations which might arise in a galaxy during the era when life and civilization can arise. Click here for general information and on the topics for more specific information.
In the first part of the form, enter your favorite dimensions of the habitable zone in the galaxy. It is modeled as a flat ring and dimensioned in light-years.
Then enter the values for calculating the number of civilizations. All entries except the first are non-dimensional fractions which multiply against the preceding value. They are entered as floating point fractions greater than 0 and not exceeding 1. Note that each level of development has a life zone factor to reflect the likelihood of tighter conditions with higher levels of development.
Results are presented in a printable format showing the most significant digits. The number of stars, planets, and civilizations are shown along with the average separation of the closest stars in their category.
This calculator was created by Jerome L. Wright. It may be freely used with acknowledgment. Permission is granted to establish links to this page or to run this calculator and report from another site provided no alterations other than data entries are made to these pages. The selection of default values was influenced by the book Rare Earth, by Peter Ward and Donald Brownlee.
© 2006 Jerome L. Wright
The algorithm includes two simple assumptions: a star can give rise to only one life-bearing planet, a life-bearing planet can give rise to only one technological civilization. The algorithm is dimensionally correct and every term is potentially knowable (both factors in contrast to a popular equation), so there is a physical basis for estimates.
There are no rigid interpretations for the computational factors. The values which a user inputs must reflect how he interprets the factors. For example, I interpret intelligent life as being equivalent to mammalian creatures, still a long way, evolutionarily speaking, from beings intelligent enough to create a civilization, so I chose values which I think are consistent with that interpretation.
Several factors, such as star type, metalicity (relative content of heavy elements), life zone, and planetary spin come under tighter requirements for higher levels of life. These changes need to be accounted for at each level to give better quality estimates of developments at the different levels. Doing so lets us estimate the cases where life reached some level, but did not progress further. Estimates of this type are important in our search for life beyond the Solar System.
The planetary life zone factor is a complex one. Not only does the tolerance of life for thermal variance diminish with higher life forms, but the irradiance given out by stars varies across the life span of the stars. The output from Sol has increased about 30% since its formation. Life on Earth successfully got through those changes so far, but how many other planets would fail to maintain the critical thermal balance to sustain complex life? A major part of keeping this balance is having the chemical makeup of the atmosphere change in concert with the irradiance. Along with the irradiance, internal heating of the planet by nuclear fission, which requires adequate metalicity, appears to be essential. The internal heating maintains surface temperatures and keeps the interior molten to sustain a protective magnetic field and maintain plate tectonics, but it must not be high enough to have frequent supervolcanos.
The values of some factors can influence the nearby factors. For example, the life window factor is an estimate of evolutionary opportunity occurring within the window. It does not assure success. A very narrow window will reduce the probability of evolutionary success.
The evolutionary success factor means effective progress by natural selection and successfully getting past environmental catastrophes such as large body impacts, supervolcanos, and celestial radiation events.
A wet, rocky planet in a life zone is a candidate for further development which might lead to intelligent life and civilization if everything goes well. A wet body outside the life zone, but with radionucleotide or gravitational heating, might host unicellular life, possibly even some multicellular forms, but would probably be a dead end for any further development. Thus, life might arise in multiple sites within a stellar system, but to the best of our knowledge only a wet, rocky planet in the life zone could support intelligent life and civilization. This calculator is designed for evaluation of wet, rocky planets with atmospheres, a strong reliance on stellar irradiance, and an ability to maintain suitable thermal conditions.
This calculator is obviously influenced by our knowledge of one case: that of Earth and its life. This is life as we know it. We also need to stretch our imaginations to try to think about life as we donít know it.
Galactic Habitable Zone
A galactic habitable zone is the region of a galaxy where life is most likely to arise and continue its existence. Regions closer to the center will have too much radiation to allow life. Regions farther from the center will have inadequate metalicity to support life, or even to support the formation of rocky planets. Further, parts of the habitable zone can be sterilized by radiation events such as star formations and supernovas, so life might not be spread uniformly through the zone.
Stars & Planets
Estimates of the number of stars in our galaxy usually range from 100 to 400 billion. The number in the Andromeda Galaxy might be as many as 600 billion. Stars in small galaxies might be low in metalicity, reducing the possibility of life arising there.
The number of stars in the galactic habitable zone is a modest fraction of the stars in the galaxy. The density of stars is very high in the central region, which makes the region hazardous to life and eliminates many stars from being candidates for supporting life.
A similar situation exists in the dense clusters of stars which exist in the habitable zone. Life is unlikely to be sustainable within dense clusters because of radiation hazards and instabilities with planetary orbits.
Single star systems offer the best stability of planetary orbits. Multiple star systems are not likely to have stable orbits, especially if the primaries have eccentric orbits, which is the case with Alpha Centauri, the stellar system closest to Sol. It is highly unlikely that a star would have more than one planet in a life zone, so talking about a star harboring advanced life implies a planet harboring advanced life. Actually, it is not very likely that a star would have even one planet in a life zone suitable for land-dwelling organisms.
Adequate metalicity is necessary for formation of rocky planets, planetary warmth, plate tectonics, protective magnetic field, and metabolism of organisms. People arriving from other stars might be able to build homes at any of these stars, even if no rocky planets exist, by using the resources of small bodies.
The range of star types which could support indigenous life is uncertain. Stars substantially more massive than Sol will be too short lived and too energetic. Stars substantially less massive than Sol will have very narrow life zones capable of sustaining complex life, so it is likely that most of them would not have planets in their life zones. Sol-like stars can be found throughout the galactic habitable zone, but not in great numbers. About 95% of all stars are less massive than Sol. Progressively more complex life forms will have narrower ranges of star types which could support them. The default values show about 500 million suitable stars in our Home Galaxy habitable zone, yielding about 1 million within 2000 light-years of Sol.
The existence of rocky planets in the inner stellar system with stable, near-circular orbits is essential for the development of complex life as we know it. No planets of this type beyond the Solar System have been found as yet. Even if not supporting indigenous life forms, many of these planets could be usable by people who might be able to travel to the stellar system.
The rocky inner system planets which could support even simple life as we know it must be wet and must be within the life zone compatible with simple life. The orbits must be reasonably close to circular to avoid lethal extremes of heating and freezing. Rotation periods and spin axis orientations are important, but do not need to be as benign as those of Earth.
The planets must be wet during the time when they have suitable temperatures to support the emergence of life. The stars have lifetimes of roughly 10 billion years, but their output increases throughout that span. The increasing output will eventually sterilize the planets, so simple life forms have a window of time in which they can arise and exist. Mars is an example of a planet which had, at most, a very narrow window of opportunity in which life could emerge, then it lost atmosphere, water, and warmth, ending the window of opportunity (if it ever existed).
With wet conditions and suitable environmental chemistry, pre-life chemicals can form and cellular life may then also form. The formation of functional cells is a complex process, but may have a relatively high probability of success if the environment is right and a billion years or so is available for development. These cells can consume life-sustaining molecules in the environment, develop photosynthetic capabilities, or consume other cells to sustain their existence. They may survive only while wet conditions exist, but they do not necessarily need to be immersed in water. They could survive underground where they are protected from dessication. Where seas or lakes exist, they might form mats of photosynthetic, undifferentiated cells. These might be called slime worlds, where only the simplest of organisms exist. All planets that develop higher forms of life will pass through a phase like this. Earth existed in this phase for roughly 3 billion years.
The development of multicellular life implies differentiation of cells to perform specialized functions. A sponge is an entity which minimally meets the definition of multicellular life. Any planet that develops the equivalent of sponges and jellyfish has arrived at the multicellular level. Life there might advance on to more complex forms such as worms, fish, and insects. Photosynthetic cells might advance to become vascularized plants.
The environmental requirements for multicellular life are somewhat more stringent than for unicellular life. The tighter requirements on star type, metalicity, life zone, and planetary chemistry reduce the number of stellar systems at this level.
The window in which multicellular life could emerge is necessarily narrower than that for unicellular life. Unicellular life must endure and improve before multicellular life can develop, which uses up part of the life window.
Given that the physical and chemical conditions are suitable within the life window, there are still some other, often random, factors in the evolution of life to multicellular organisms. From the terrestrial case, we know that if evolution progresses through development of sea worms, then a stage has been reached which has the potential to evolve on to land organisms and intelligence.
The move onto land by multicellular organisms requires a suitable physical environment as well as successful evolutionary development of the characteristics which enable living on land. The physical factors mentioned above must meet even more stringent requirements. In addition, the planet must has an atmosphere with suitable chemistry, which is able to provide protection against hard radiation as well as harmful wavelengths of the starís light. Both plants and animals must successfully transition to land to create an ecosystem. The minimal level of success is simple vascular plants and insect-like organisms (exoskeletal) or endoskeletal animals which have some land mobility.
The type of star, its metalicity, and planetary chemistry requirements are more restrictive for an ecosystem to develop on land. The planetís orbit must be close to circular and in a relatively tight life zone to support the ecosystem. In addition, the atmosphere must vary in composition in a manner to compensate for increasing irradiance to maintain temperate conditions. The spin of the planet must have a favorable tilt and rotation rate and must remain stable against perturbations, which requires the presence of a large satellite sufficiently close to the planet.
The window for complex life on the surface of Earth is narrow, no more than about 10% of its existence. Other planets which are not as physically and chemically favorable for life on land, such as lacking a large satellite, would have life windows which are much narrower--perhaps only 1% or so of the planetís existence, if they exist at all. A 1% window is roughly 100 million years, not much time in which to get from land-crawling fish to mammalian-like creatures and on to high intelligence and civilization.
Land-dwelling creatures face more hazards than sea-dwelling creatures; they are more affected by celestial collisions, radiation, supervolcanos, and increasing irradiance. They must survive these and progress successfully through natural selection to achieve intelligence. I think of intelligent creatures as having adaptive behavior, some ability to understand their environment, and some ability to plan their activities. I think squirrels and cats meet this criteria, while chickens are very doubtful.
Evolutionary success in achieving intelligent life necessarily includes successful evolution of the entire ecosystem to provide the environment within which intelligent life could exist. This is a long way from the ecosystem condition of minimal multicellular life. This also includes getting through environmental catastrophes like those which beset Earth during the time leading to the development of intelligent life.
Dinosaurs dominated Earth for about 135 million years. An unlikely collision event 65 million years ago allowed mammals to dominate Earth. If that event had not happened, the dinosaurian domination might never have ended and Earth would possibly never have supported a civilization. If the event had happened 100 or 200 million years later, it might have been too late for some mammals to become intelligent enough to create civilization--the window of opportunity for development of high intelligence and civilization would possibly be closed by a seriously deteriorating thermal condition due to increasing irradiance.
Creating a civilization obviously requires highly intelligent people who have evolved from minimally intelligent creatures within the window of opportunity. This is a continuation of the evolutionary process described above, with all of the hazards of land-dwelling existence. It is worth noting that the most intelligent creatures in Earthís oceans are ones that evolved from land-dwelling mammals. They also lack external appendages which would allow them to manipulate objects.
Some people could have sufficient intelligence to develop a rudimentary civilization, but might lack the intellect, societal flexibility, or resources necessary to develop a scientific-technological civilization.
Having an intelligence close to that of modern humans does not assure success in developing civilization. Essentially modern humans existed about 70,000 years ago, but did not found civilization until about 7000 years ago. Obviously there were some social developments needed. It took nearly all of that 7000 years to reach technological civilization with scientific knowledge of cause and effect. Substantially more social progress is needed to spread civilization throughout a stellar system and achieve social stability to assure continuance of the civilization. For a civilization to survive across geological ages and through the death of its star, even more progress must be made. Our civilization, along with our species, still faces numerous threats to its existence.
The end of this chain of calculations is the number of stars at which civilizations might have developed during the time in which they could have developed, which is roughly the first 30 to 40 billion years of a galaxyís existence. It seems likely that the span from 10 billion to 20 billion years of age would be the most prolific time for the emergence of technological civilizations. Our galaxy is close to 13 billion years old.
If the final number is less than 1, it does not mean we do not exist. If the value is 0.1 for example, that means about 1 galaxy in 10 would develop a technological civilization.