## N

Positions of the nearest stars that have significant probabilities of possessing at least one habitable planet coordinates of right ascension and declination. Figure 40. Positions of the nearest stars that have significant probabilities of possessing at least one habitable planet coordinates of right ascension and declination. chance of the existence of a habitable planet in the Alpha Centauri system. Because of its location close to the south celestial pole, Alpha Centauri can not...

## Iii

1 10 100 1000 Mt, total mass, relative to Earth's mass Figure 17. The changing composition of an accreting body after it becomes large enough to capture light gases (illustrative). 1 10 100 1000 Mt, total mass, relative to Earth's mass Figure 17. The changing composition of an accreting body after it becomes large enough to capture light gases (illustrative). This is an all-or-nothing model as far as gas capture is concerned accordingly, the gas fractions shown are probably quite high and the...

## Probability That A Given Star Will Have At Least One Habitable Planet In Orbit Around It

Having computed all the essential probabilities for all primaries in the stellar mass range of interest, we have completed all preliminaries to calculating the probabilities that a given star will have habitable planets in orbit about it and the total number of habitable planets in the Galaxy (see page 82 for definitions) From previous sections we have seen that PD and PA depend on the properties of the primary star, and we have adopted the following values Pp 1.0, Pt 0.81, PM 0.19, Pe 0.94, PR...

## What Is Habitable

For present purposes, we shall enlarge on our definition of a habitable planet (a planet on which large numbers of people could live without needing excessive protection from the natural environment) to mean that the human population must be able to live there without dependence on materials brought from other planets. In other words, a planet that is habitable can supply all of the physical requirements of human beings and provide an environment in which people can live comfortably and...

## Probability That The Planets Orbital Eccentricity Is Sufficiently

An inspection of the values of orbital eccentricity among the bodies of planetary size in the solar system suggests that these eccentricities are distributed according to an empirical probability curve having the approximate form P 1 (1 e)N, where N is equal to about 13. In this assumed distribution, P is the cumulative probability that the eccentricity is less than any assigned value. Undoubtedly, other empirical distribution curves could be found that would fit the observational data more...

## Special Properties Of Binary Star Systems

So far, we have been concerned almost entirely with planets revolving about isolated stars. As will be discussed later, however, a large fraction of all stars exist in multiple stellar systems binary, triple, and quadruple. The most common type of multiple system is the binary one that is, where more than two stars are gravitationally associated, the other members are far removed from the first two so far away that they can have little effect (from our point of view) on the binary system. Thus,...

## Index

The letter preceding a page number indicates tabular material. Absolute visual magnitude, 86 Acceleration, tolerance for high levels of, 11-13, t 2 Accretion hypothesis, 27 Accretion process, 60-61 ALLEN, c. w., 95, 106, 115 Allen's star tables, number of habitable planets in, 106, 108-109 Alpha Centauri absolute visual magnitude of, 86-87 orbit of, 111-112 probability of habitable planet in, 110-111 ASIMOV, ISAAC, 126 Atelectasis, 17 Atmospheres bodies without extensive, 30-31 classifications...

## The Brightest Star In Cassiopeia

Alpha Centauri, our nearest neighbor in space beyond the solar system and one of the brightest stars of the Southern Hemisphere, is actually a triple star system. The two larger stars, A and B, revolve about each other in a period of 80 years, at a mean separation of about 23 astronomical units an astronomical unit (A.U.) is the mean distance between the Sun and the Earth the third member, Alpha Centauri C, some 10,000 astronomical units removed, revolves about the pair A, B in a period of the...

## Probability That The Planet Has A Suitable Mass

The number-versus-mass distribution for the planets of the solar system also exhibits a pattern of regularity. The smaller individuals among the planets are more numerous than the larger ones, as is the case with stars, asteroids, and meteorites. This is apparent from Figure 10 (see page 33). Under the assumption that the planets of our solar system represent a fair sampling of such objects, it is now possible to construct a probability HABITABLE PLANETS FOR MAN Table 13. Probability That a...

## Introduction to General Planetology

About 50 years ago, it was recognized by E. Hertzsprung (1911) and H. N. Russell (1914) that the stars may be grouped into families by spectral type (temperature) and luminosity (absolute magnitude). Since the time of their discovery of some of the fundamental stellar properties, many general relationships between mass, luminosity, age, diameter (density), temperature, spectral type, composition, internal conditions, nuclear reactions, et cetera, have been deduced for stars. Ideas are...

## An Appreciation of the Earth

We take our home for granted most of the time. We complain about the weather, ignore the splendor of our sunsets, the scenery, and the natural beauties of the lands and seas around us, and cease to be impressed by the diversity of living species that the Earth supports. This is natural, of course, since we are all products of the Earth and have evolved in conformity with the existing environment. It is our natural habitat, and all of it seems very commonplace and normal. Yet how different our...

## Probability That At Least One Planet Orbits Within The Ecosphere Of A Given Star

As discussed earlier, primaries having masses greater than 0.88 solar mass have complete ecospheres, while those with masses between 0.72 and 0.88 solar mass have narrowed ecospheres because of the rotation-retarding effects of the primary at the inner edge of the ecosphere. To estimate the probability that there will be at least one planet orbiting within an ecosphere, it is necessary to know something about the spacing of planets in planetary systems. Unfortunately there is only one planetary...

## Atmospheric Composition And Pressure

A habitable planet must, of course, have a breathable atmosphere and this now can be rather completely specified in terms of its component gases and their concentrations or partial pressures (see Figure 6). As far as we know, the only essential ingredients of a breathable atmosphere are oxygen and minor amounts of water vapor. The inspired partial pressure of oxygen must lie between two extreme limits a lower limit below which hypoxia is encountered and an upper limit above which oxygen...

## Satellite Relationships

The tidal braking forces of a primary also apply to satellites. If h2 of a satellite on a planet is greater than 2.0 but that of the primary is less than 2.0, one would expect to find the planet's rotation halted with respect to the satellite but continuing with respect to the primary. The planet's solar day and synodic month would be of the same length. For this condition to be compatible with habitability, however, the period would have to be such as to produce a solar day less than 96 hours...

## Other Requirements

The above-stated requirements for conditions of temperature, light, gravity, atmospheric composition and pressure, and water are probably the major human requisites yet there are many others. These will be stated briefly. As indicated earlier, other life forms must be present, since ultimately all human food supplies and probably the very existence of free oxygen in the atmosphere depend on photosynthesis in green plants. There must also be an absence of inimical intelligent beings in prior...

## Bibliography

AFP 160-6-1, Threshold Limit Values for Toxic Chemicals. U.S. Department of the Air Force, February 16, 1959. Allen, C. W. Astrophysical Quantities. London The Athlone Press, 1955. Arend, S. Com. VOb. R. Belg., No. 20 1950 . Asimov, Isaac. 'Our Lonely Planet, Astounding Science Fiction, 62, No. 3 November, 1958 , pp. 127-137. Beadle, G. W. The Place of Genetics in Modern Biology, Eng. Sei., 23, No. 6 March, 1960 , pp. 11-17. Berger, R. The Proton Irradiance of Methane,...