Extraterrestrial Civilizations - Isaac Asimov [54]
Then, too, if the observer were as far away as one of the nearest stars, the Sun’s wobble would be too small to measure accurately or even, perhaps, to detect.
Would it be possible to turn the tables? Could we look at some other star and detect a wobble in its path and from that deduce that it had a planet or planets?
Undoubtedly in some cases, for it was done as long ago as 1844.
In that year the German astronomer Friedrich Wilhelm Bessel (1784–1846) noted a wobble in the motion of the bright star Sirius. From that wobble, he deduced the presence of an unseen companion that had 2/5 the mass of Sirius.
As it happens, we now know that Sirius is 2.5 times as massive as our Sun. The companion, therefore, has just about the mass of our Sun. So it is not a planet, actually, but a full-sized star that is dim and hard to see because it happens to be very compact.*To find a companion star is easy by comparison to finding a companion planet, however. A planet is so small in mass compared with the star it circles that the center of gravity between itself and the star is that much closer to the center of the star. The star therefore makes a very tiny wobble indeed.
Can such a wobble ever be measured?
Possibly, if the conditions are right.
First, the star must be as close to us as possible, so that the wobble is as large in appearance as possible.
Second, the star must be a small one, certainly smaller than our Sun, so that its mass predominates as little as possible. The center of gravity is then comparatively far from the star’s center and this star makes a comparatively large wobble.
Third, the star must have a large planet, at least as large as Jupiter, so that the planetary mass will be large enough to drag the center of gravity far enough away from the small star it circles to force a comparatively large wobble on the star.
This triple requirement of a nearby small star with a large planet cuts down the possibilities enormously. If the chance of planetary formation is small, then it would be too much to ask of coincidence that a planetary system should just happen to exist around a small nearby star, and that the planetary system should just happen to include a planet at least as large as Jupiter.
On the other hand, if we search small nearby stars and do happen to find evidence of an accompanying planet around at least one of them, then, in order not to force ourselves to accept a highly unlikely coincidence, we must consider that planetary systems are very common, perhaps even universal.
Attempts to determine the presence or absence of such wobbles in the motions of stars were conducted at Swarthmore College under the guidance of the Dutch-American astronomer Peter Van de Kamp (1901–).
The Danish-American astronomer Kaj Aage Gunnar Strand (1907–), working under Van de Kamp, detected a tiny wobble in the motion of one of the stars of the 61 Cygni two-star system, and deduced the presence of a companion body circling it, one that was much too small in mass to be a star. It was massive enough to be a large planet, however, one that was eight times as massive as Jupiter. This discovery was announced in 1943.
Since then a similar wobble was discovered in connection with Barnard’s star, a small star only 6 light-years away. In its case, the wobble may indicate the presence of two planets, one as massive as Jupiter, orbiting in 11.5 years, and one as massive as Saturn, orbiting in 20 to 25 years. Other nearby stars, such as Ross 614 and Lalande 21185 have also shown wobbles that seem to indicate the presence of large planets.
In short, we have discovered not one but half a dozen small, nearby stars that may have large planets. Under the circumstances (and it must be admitted that the observations are so close to the limit of what can be seen that