Extraterrestrial Civilizations (19 page)

We might estimate that a star should have at least ⅓ the mass of the Sun (which means it would have to be of spectral class M2 at least) before a planet in its ecosphere would be suitable for life.

Nor is the matter of tidal effect the only problem with midget stars. The width of an ecosphere depends on how much energy a star is radiating. A massive, luminous star has an ecosphere far out in space and one that is very deep; deeper than the entire width of our Solar system. A midget star has an ecosphere that is close in on itself and is very shallow. The chance of a planet’s happening to form within so shallow an ecosphere is vanishingly small.

Finally, stars smaller than spectral class M2 are very often “flare stars.” That is, flares of unusually bright and hot gas periodically burst out on its surface. This happens on all stars, even on our Sun, for instance. On the Sun, however, such a flare would only add a small and bearable fraction to the ordinary Solar output of light and heat. The same flare on a dim midget star would increase its light and heat output by up to 50 percent. A planet receiving a proper amount of energy from the midget star would receive far too much under flare conditions. The star’s role as incubator would be carried out in too irregular a fashion to be compatible with life.

Between tidal effects, shallowness of ecosphere, and periodic flares, the exclusion of midget stars from further consideration in connection with extraterrestrial intelligence is triply justified.

If the stars with too much mass to serve as adequate incubators for life, those more massive than spectral class F2, make up a small fraction of all the stars, this is not the case for the stars that are less massive than spectral class M2 and also don’t serve as adequate incubators for life. Midget stars are very common. More than two-
thirds of the stars in our Galaxy, and presumably in any galaxy, are too small to be suitable for life.

Between spectral classes F2 and M2 are the stars that range in mass from 1.4 times that of the Sun to 0.33 times that of the Sun. At the upper end of this range, the lifetime of the stars is barely enough to give intelligence a fair chance to evolve. At the lower end of this range, a planet barely escapes tidal effects of too serious a nature.

Within the range, though, are the “Sunlike stars,” which, all other things being equal, are suitable incubators for life. While these Sunlike stars do not make up a majority of the stars in the sky, they are not really few in number, either. Perhaps 25 percent of all the stars in the Galaxy are sufficiently Sunlike in character to serve as adequate incubators of life.

That gives us our third figure:

A star may be Sunlike and yet still not be a suitable incubator for life. It may have properties, other than its mass and luminosity, that make it impossible for an Earthlike planet to circle it.

A star may be like the Sun in every apparent respect, for instance, and yet have as a companion not a planet or a group of planets, but another star. The presence of two stars in close association may conceivably ruin the chances for an Earthlike planet to circle either one.

The possibility of multiple stars did not dawn on astronomers until about two centuries ago. After all, our Sun is a star without stellar companions and that made it seem a natural condition. When the stars were recognized to be other suns, they, too, were assumed to be single. To be sure, there are stars that are close together in the sky. For instance, Mizar, the middle star in the handle of the Big Dipper, has a fainter star, Alcor, very near it. Such “double stars” were taken, however, to be single stars lying nearly in the same direction from the
Earth but at radically different distances. In the case of Mizar and Alcor, this turned out to be true.

In the 1780s, William Herschel began to make a systematic study of double stars in the hope that the brighter (and presumably closer) one might move slightly and systematically with reference to the dimmer (and presumably more distant) one. This motion might reflect the motion of the Earth about the Sun and be the star’s “parallax.” From this, the star’s distance could be determined, something that had not yet been done.

Herschel did find motions among such stars, but never of the kind that would indicate the presence of a parallax. Instead, he found some double stars to be circling about a mutual center of gravity. These were true double stars, bound to each other gravitationally, and were called binary stars, from a Latin word meaning
in pairs
.

By 1802, Herschel was able to announce the existence of many such binary stars, and they are now known to be very common among the stars of the Universe. Among the bright and familiar stars, for instance, Sirius, Capella, Procyon, Castor, Spica, Antares, and Alpha Centauri are all binaries.

In fact, more than two stars might be held together gravitationally. Thus, the Alpha Centauri binary (which are referred to as Alpha Centauri A and Alpha Centauri B) have a very distant companion, Alpha Centauri C, some 1,600,000,000,000 kilometers (one trillion miles) from the center of gravity of the two other stars. A binary star system may also be gravitationally bound to another binary star system, the two pairs of stars circling a common center of gravity. Systems of five or even six stars are known.

In every case, though, where more than two stars are involved in a multiple system, the stars exist in relatively close pairs widely separated from companion singles or other binaries.

In other words, suppose that there were a planet about Star A, which is a member of a binary system. Star B might be close enough to have some important effect on the planet. It might add its own radiation to that of Star A in different amounts at different times. Or else its gravitational pull might introduce irregularities into the planet’s orbit that might not have existed otherwise.

On the other hand, if the A-B binary had, associated with it, a third star, or another binary, or both a star and a binary—all would be so far off that they would simply be stars in the sky without
particular influence on the development of life on the planet.

From the standpoint of this book, therefore, let us talk only of binaries.

There is nothing puzzling about the existence of binaries.

When an initial nebula condenses to form a planetary system, one of the planets may, by the chance of the turbulence, attract enough mass to become a star itself. If, in the course of the development of our own Solar system, Jupiter had accumulated perhaps 65 times as much mass as it did, the loss of that mass to the Sun would not have been particularly significant. The Sun would still have very much the appearance it now has, while Jupiter would be a dim “red dwarf” star. The Sun would then be part of a binary system.

It is even quite possible that the original nebula might condense more or less equally about two centers to form stars of roughly equal mass, each smaller than our Sun, as in the case of the 61 Cygni binary system; or each roughly equal in size to our Sun, as in the case of the Alpha Centauri binary system; or each larger than the Sun, as in the Capella binary system.

The two stars might, if they are of different mass, have radically different histories. The more massive star may leave the main sequence, expand to a red giant, and then explode. Its remnants would then condense to a small, dense star, while the less massive companion star remains on the main sequence. Thus, Sirius has as a companion a white dwarf, a small, dense remnant of a star that once exploded. Procyon also has a white dwarf as a companion.

The total number of binaries in the Galaxy (and presumably in the Universe generally) is surprisingly large. Over the nearly two centuries since their discovery, the estimate of their frequency has steadily risen. At the moment, judging from the examples of those stars close enough to ourselves to be examined in detail, it would seem that anywhere from 50 to 70 percent of all stars are members of a binary system. In order to arrive at a particular figure, let us take an average and say that 60 percent of all stars and, therefore, of all Sunlike stars, too, are members of a binary system.

If we assume that any Sunlike star can form a binary with a star of any mass, then, keeping in mind the proportions of stars of various masses, we could venture a reasonable division of the 75 billion Sunlike stars in the Galaxy as follows:

Ought we now to eliminate the 45 billion Sunlike stars involved in binary systems as unfit incubators for life?

Certainly, it would seem that we can omit the 2 billion Sunlike stars that form binaries with giant stars. In their case, long before the Sunlike star has reached an age where intelligence might develop on some planet circling it, the companion star would explode as a supernova. The heat and radiation of a nearby supernova is quite likely to destroy any life on the planet that already existed.

What about the remaining 43 billion Sunlike stars forming a part of binaries?

In the first place, can a binary system possess planets at all?

We might argue that if a nebula condenses into two stars, the two will be twice as effective in picking up debris as one would be. Any planetary material that might escape one would be picked up by the other. In the end, therefore, there would be two stars and no planets.

That this is not necessarily so is demonstrated by the star 61 Cygni, the first whose distance from Earth was determined, in 1838, and that is now known to be 11.1 light-years from us.

61 Cygni, as I have said earlier, is a binary star. The two component stars, 61 Cygni A and 61 Cygni B, are separated by 29 seconds of arc as viewed from Earth (a separation about 1/64 the width of the full Moon).

Each of the component stars is smaller than the Sun, but each is large enough to be Sunlike. 61 Cygni A has about 0.6 times the mass of the Sun, and 61 Cygni B about 0.5 times the mass. The former has a diameter of about 950,000 kilometers (600,000 miles) and the latter a diameter of about 900,000 kilometers (560,000 miles). They are separated by an average distance of about 12,400,000,000 kilometers (7,700,000,000 miles), or a little over twice the average distance between the Sun and Pluto, and they circle each other about their center of gravity once in 720 years.

If we imagined the planet Earth circling one of the 61 Cygni stars at the same distance it now circles the Sun, the other 61 Cygni
star would appear in the night sky at various times as a bright, starlike object, showing no visible disc, delivering no significant amount of radiation, and producing no significantly interfering gravitational effect.

Indeed, we might easily imagine each 61 Cygni star as possessing a planetary system nearly as extensive as the Sun’s, each without interference from the other.
^*
In this particular case, we need not resort entirely to speculation. The very first planetary object about another star for which some evidence was obtained involved 61 Cygni. From the manner in which the separation of the two stars changed in a wobbly manner as they circled each other, the presence of a third body, 61 Cygni C, was deduced. From the extent of the wobble, it was thought to be a large planet some eight times the mass of Jupiter.

Soviet astronomers at the Pulkovo Observatory near Leningrad have studied the orbits of the 61 Cygni stars with care, have measured the irregularities of the wobble itself, and have suggested, in 1977, that
three
planets are involved. They feel that 61 Cygni A has two large planets, one with 6 times the mass of Jupiter and one with 12 times the mass, while 61 Cygni B has one large planet with 7 times the mass of Jupiter.

These are very borderline observations. The tiny changes in the motion of the 61 Cygni stars can just barely be made out, and the chance that insignificant errors of measurement or interpretation have produced them is all too likely.

For what it’s worth, however, and until something better comes along, it implies that both stars of a binary system (both stars being Sunlike stars) have planets—large planets at least. If large planets exist, however, it doesn’t take much of a strain to suppose the existence of a large collection of smaller planets, satellites, asteroids, and comets—all too small to leave detectable marks on the wobble.

Of course, some binary systems are separated by smaller distances than the 61 Cygni stars.

Consider the two stars of the Alpha Centauri binary system.
Alpha Centauri A has a mass 1.08 times that of the Sun, and Alpha Centauri B a mass 0.87 times that of the Sun. The two stars are separated by an average distance of 3,500,000,000 kilometers (2,200,000,000 miles). They revolve about the center of gravity in quite elliptical orbits, however, and are much closer to each other at some times than at others. The maximum distance between the two stars is 5,300,000,000 kilometers (3,400,000,000 miles) and the minimum distance between the two is 1,700,000,000 kilometers (1,050,000,000 miles).