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 be seen from positions on the Earth's surface north of latitude 30°N (roughly the latitude of New Orleans). The apparent orbit of component B around component A, as obtained from numerous telescopic observations over the past 100 years, is shown in Figure 41. It is an extremely elongated orbit, since it is seen almost edge-on from the Earth. The actual orbit has an eccentricity of 0.52 and a semimajor axis of 23.2 astronomical units; hence, at their closest approach (periastron), A and B are separated by 11.2 astronomical units while their separation at apastron (greatest separation) is 35.3 astronomical units. The approximate distances at which ecospheres exist around A and B are shown to scale in the figure. Although there is no theoretical method of determining the stability of orbits of planetary bodies in multibody systems, application of the stability limits obtained from the restricted three-body problem suggests that habitable planets (if any) orbiting within the ecospheres of A and B should have highly stable orbits.
For example, if A and B were on circular orbits around their common center of gravity, taking their mass ratio as 0.45 and placing them at such a distance from each other (9.73 astronomical units) that their new angular velocity was equal to their actual angular velocity at periastron, then planetary orbits of A should be stable within a radius of 2.68 astronomical units, and planetary orbits of B should be stable within a radius of 2.34 astronomical units. From Figure 26 (see page 72), it may be seen that complete ecospheres of both A and B fall well inside these stability boundaries.
The larger component of the system, a Centauri A, is a star very similar to the Sun. Its spectral class is given as G4 (or sometimes as GO); its apparent visual magnitude, as 0.09; its absolute visual magnitude, as 4.5; and its mass, as about 1.08 solar masses. It has a probability of possessing a habitable planet of approximately 0.054.
Component B is somewhat smaller, of spectral class K1 (or K5); it has an apparent visual magnitude of 1.38, an absolute visual magnitude of 5.9 (or 6.1), and a mass of 0.88 solar mass. Thus its probability of possessing one habitable planet is 0.057. Component C, also called Proxima
Centauri, is 2.2 degrees away from the other two, as seen from the Earth. It is a small flare star of visual magnitude 10.68, too small to be considered the center of a system in which a habitable planet could be found.
The probabilities are extremely sensitive to the values of stellar mass derived from the astronomical data. It is clear that none of the values of stellar mass that have been determined so far is very precise. As better data are obtained, future revisions to the mass determinations will necessitate some changes in the probability figures given.
Of the remaining stars in the list of the most promising candidates, relevant information is given below.
Epsilon Eridani, located in the sky at almost 10 degrees south of the projected plane of the Earth's equator (declination, 9° 48'), can be seen from any part of the Earth's surface, except from a small region around the North Pole. It is an isolated star (no companion has ever been detected) of spectral class K2, although it is sometimes classed as K0. Its apparent visual magnitude is 4.2, its parallax is 0.303 second, and its distance from the Earth is 10.8 light-years. Therefore, its absolute visual magnitude is 6.2. From this it may be deduced that its mass, which can not be measured directly, is about 0.80 the mass of the Sun. The probability that there is a habitable planet orbiting within its ecosphere is here calculated to be 3.3 per cent.
Tau Ceti, fairly close to e Eridani in the night sky (in a neighboring constellation), can be seen from any point on the Earth's surface, except from the arctic regions. Both e Eridani and r Ceti were recently "listened to" by means of a radio telescope during the course of Project Ozma, an attempt to detect intelligence-bearing radio signals directed toward our Sun by possible intelligent inhabitants of planets of these stars. This was based on a suggestion of Cocconi and Morrison (1959) of Cornell University, who selected a frequency of 1420 megacycles (wave length, 21 centimeters) as the optimum for a high signal-to-noise ratio. An attempt to detect radio signals from e Eridani and r Ceti was made in the spring of 1960 by using the 85-foot diameter radio telescope of the National Radio Astronomy Observatory at Green Bank, West Virginia. Results were negative. That no signals were detected is not surprising, since the reverse experiment conducted on planets orbiting around e Eridani or r Ceti but directed toward our Sun would also have given negative results. We are not sending out signals on a wave length of 21 centimeters that "they" could detect. The joint probability that e Eridani and r Ceti have one habitable planet between them is only about 7 per cent, while the probability that a given habitable planet will be inhabited by intelligent beings is difficult to estimate.
Like e Eridani, r Ceti is apparently an isolated star. Its spectral class is variously given as G8, G4, and KO; its absolute visual magnitude, as 6.02, 5.8, and 5.9; and its distance, as 12.2, 11.8, and 11.2 light-years. Its apparent visual magnitude is 3.65. Based on an absolute visual magnitude of 6.02, its mass is estimated to be 0.82 solar mass. The probability that it has one habitable planet in orbit about it is here calculated to be 3.6 per cent.
70 Ophiuchi A is the more massive component of the system 70 Ophiuchi. This system consists of two stars revolving about each other with a period of 87.85 years in an orbit with an eccentricity of 0.50. Its distance from the Earth is given variously as 17.3, 16.4, and 16.5 light-years. The parallax given by van de Kamp (1958), tt — 0.199, corresponds to 16.4 light-years. The semimajor axis of the binary orbit is 22.8 astronomical units; thus at periastron the two components are separated by 11.4 astronomical units; at apastron they are 34.2 astronomical units apart. No third companion has been established for the 70 Ophiuchi system, although dark companions are suspected to exist. The apparent visual magnitude is 4.19. Component A of spectral class K1 has an absolute visual magnitude of 5.7 (or 5.8), a mass of about 0.90 solar mass, and hence a 5.7 per cent probability of possessing one habitable planet. According to van de Kamp (1958), the spectral class of A is K0, and its apparent visual magnitude is 5.09.
The less massive component B is of spectral class K5 with an absolute visual magnitude of 7.3 (or 7.4 or 7.5). Its mass is about 0.65 solar mass; thus it could possess a habitable planet only if it had a large, close satellite to preserve its rotation rate, van de Kamp gives the spectral class of B as K4. Planets revolving at ecosphere distance should have stable orbits in this binary system.
As seen from the system of 70 Ophiuchi, the Sun would appear as a third-magnitude star in the constellation Orion, not far from the belt.
Eta Cassiopeiae A is the larger component of the binary system ry Cassiopeiae. This system, at a distance of 18.0 light-years from the Earth, has a period of the order of 500 years and an orbital eccentricity of 0.53. The semimajor axis is about 70 astronomical units. The existence of a third component is not well established. Apparent visual magnitude is 3.54. Component A is of spectral class F9, has an absolute visual magnitude of 4.87 (very close to that of the Sun), and a mass of about 0.94 solar mass. Its probability of having one habitable planet is 5.7 per cent.
The smaller component B is of class K6, absolute visual magnitude 8.7, and mass 0.58 solar mass; thus its probability of having a habitable planet is very small.
Our Sun, as seen from this system, would appear to be imbedded in the Southern Cross.
Sigma Draconis, the most northerly star in this list (declination, 69° 29'), appears to be an isolated star. At a distance of 18.2 light-years, with an apparent visual magnitude of 4.72, it is in spectral class G9. Its absolute visual magnitude is 6.01, and its mass is about 0.82 solar mass. There is a probability of approximately 3.6 per cent that it has a habitable planet.
36 Ophiuchi A is the most massive member of a system that lies almost directly between us and the center of our Galaxy. Apparently its orbital elements have not yet been established. According to Allen (1955), A and B are separated by 4 seconds of arc, while component C is over 12 minutes of arc away from A-B. The system is about 18.2 light-years away and has an apparent visual magnitude of 5.17. Component A, in spectral class K2, has an absolute visual magnitude of 6.4 and a mass of about 0.77 solar mass. Thus it has a probability of about 2.3 per cent of having a habitable planet. Component B, in spectral class Kl, has an absolute visual magnitude of 6.5 and a mass of about 0.76 solar mass. Its probability of having a habitable planet is 0.020. Component C, in spectral class K6, has an absolute visual magnitude of 7.8 and a mass of about 0.63. The probability of its having a habitable planet is very small.
HR 7703 A is the larger member of its system, which is 18.6 light-years away in the southern constellation Sagittarius and consists of two stars separated by 8 seconds of arc. The orbital elements have not yet been determined. Apparent visual magnitude is 5.24. Component A is of spectral class K2, with absolute visual magnitude of 6.5, estimated mass of 0.76 solar mass, and a 2-per-cent probability of having one habitable planet. The smaller component B is of spectral class M5, has an absolute visual magnitude of 12.7, and is too small to possess a habitable planet.
Delta Pavonis, even more southerly than a Centauri, can not be seen by observers on the Earth's surface north of latitude 23°N. It is apparently an isolated star of spectral class G7. Its apparent visual magnitude is 3.67; its distance from our Sun, 19.2 light-years; its absolute visual magnitude, 4.9; and its mass, 0.98 solar mass. On the basis of this mass value, it has a 5.7-per-cent probability of possessing one habitable planet.
82 Eridani, another apparently isolated star, can be seen from latitudes south of 46°N. Its spectral class is G5; its apparent visual magnitude is 4.3; its distance from our Sun is 20.9 light-years; its absolute visual magnitude is 5.3; and its deduced mass is 0.91 solar mass. Its probability of possessing a habitable planet is 5.7 per cent.
Beta Hydri is the most southerly star on the list of best candidates within 22 light-years from the Sun. It is 21.3 light-years away and is an isolated G1 star with an apparent visual magnitude of 2.90, an absolute visual magnitude of 3.8, and a deduced mass of 1.23 solar masses. It has a 3.7-per-cent probability of possessing a habitable planet.
HR 8832, in the constellation Cassiopeia, is too faint to be seen with the naked eye except under unusually good viewing conditions. It is an isolated star in spectral class K3 with an apparent visual magnitude of 5.67. Located at a distance of 21.4 light-years from the Earth, its absolute visual magnitude is 6.69; its mass is 0.74 solar mass; and its probability of possessing a habitable planet is 1.1 per cent.
The combined probability of the existence of at least one habitable planet in the whole volume of space out to a distance of 22 light-years from the Sun is about 43 per cent.
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