Extraterrestrial Civilizations - Isaac Asimov [65]
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).
Suppose we imagined Alpha Centauri B circling our Sun exactly as it, in fact, circles Alpha Centauri A. If we plotted Alpha Centauri B’s orbit relative to the Sun, it would follow an elliptical path that would carry it well beyond the orbit of Neptune at its farthest recession from the Sun, and nearly as close as the orbit of Saturn as its nearest approach.
Under such circumstances, neither star could have a very extensive planetary system of the sort the Sun has now. Planets at the distance of Jupiter or the other giants, circling either star, would be interfered with by the gravitational influence of the other star and would have unstable orbits.
On the other hand, an inner planetary system might still exist. If Alpha Centauri B were circling our Sun as it circles Alpha Centauri A, we on Earth could scarcely tell the difference with our eyes closed. Alpha Centauri B would be a bright, starlike object in the sky, which at its closest approach would be 5,000 times brighter than our full Moon and 1/100 as bright as our Sun. It would add anywhere from 0.1 percent to 1 percent to the heat we receive from the Sun, depending on what part of its orbit it was in, and we could live with that. Nor would its gravitational influence affect Earth’s orbit in any significant way.
For that matter, Alpha Centauri B could have an inner planetary system, too. A planet circling in its ecosphere (which would of course be closer to itself than the ecosphere is to either Alpha Centauri A or the Sun) would not be seriously interfered with by its somewhat larger companion.
As in the case of the 61 Cygni system, both Alpha Centauri A and Alpha Centauri B would have what we might call a “useful ecosphere,” one in which an Earthlike planet could orbit without serious interference from the companion in terms of either radiation or gravitation.
Robert S. Harrington of the U.S. Naval Observatory in 1978 reported the results of high-speed computer studies of orbits about binary stars.
If a Sunlike star is part of a binary system, and if the separation between the two stars is at least 3.5 times the distance of the ecosphere from the Sunlike star, then it is a useful ecosphere. In the case of our own Solar system, it would mean that the Sun could have a companion at a distance