Extraterrestrial Civilizations (16 page)

The trouble is, though, that we can’t see whether any stars have planets in attendance. Even at the distance of the nearest star (Alpha Centauri, which is 4.3 light-years from us) there would be no way of actually seeing even a large planet the size of Jupiter or greater. Such a planet would be too small to see by the reflected light of its star. Even if a telescope were invented that could make out that dim flicker of reflected light, the nearness of the much greater light of its star would utterly drown it out.

We must give up hope of direct sighting then, at least for now, and resort to indirect means.

Consider our own Sun, which is a star that certainly has a planetary system. The remarkable thing about the Sun is that it rotates so slowly on its axis that 98 percent of the angular momentum of the system resides in the insignificant mass of its planets.

If angular momentum passed from the Sun to its planets when those planets were formed (by any mechanism), then it is reasonable to suppose that angular momentum might pass from any star to its planets. If, then, a star has a planetary system, we would expect it to spin on its axis relatively slowly; if it does not, we would expect it to spin relatively rapidly.

But how does one go about measuring the rate at which a star spins when even in our best telescopes it appears as only a point of light?

Actually, there is much that can be deduced from starlight even if the star itself is but a point of light. Starlight is a mixture of light of all wavelengths. The light can be spread out in order of wavelength from the short waves of violet light to the long waves of red light, and the result is a “spectrum.” The instrument by which the spectrum is produced is the “spectroscope.”

The spectrum was first demonstrated in the case of sunlight by Isaac Newton in 1665. In 1814, the German physicist Joseph von Fraunhofer (1787–1826) showed that the Solar spectrum was crossed by numerous dark lines, which, it was eventually realized, represented missing wavelengths. They were wavelengths of light that were absorbed by atoms in the Sun’s atmosphere before they could reach the Earth.

In 1859, the German physicist Gustav Robert Kirchhoff (1824–1887) showed that the dark lines in the spectrum were “fingerprints” of the various elements, since the atoms of each element emitted or absorbed particular wavelengths that the atoms of no other element emitted or absorbed. Not only could spectroscopy be used to analyze minerals on Earth, but it could be used to analyze the chemical makeup of the Sun.

Meanwhile, the art of spectroscopy had been refined to the point where the light of stars, though much dimmer than the light of the Sun, could also be spread out into spectra.

From the dark lines in the stellar spectra much could be worked out. If, for instance, the dark lines in the spectrum of a particular star were slightly displaced toward the red end, then the star would be receding from us at a speed that could be calculated from the extent of the displacement. If the dark lines were displaced toward the violet end of the spectrum, the star would be approaching us.

The significance of this “red shift” or “violet shift” was quite evident from work that had been done on sound waves in 1842 by the Austrian physicist Christian Johann Doppler (1803–1853) and then applied to light waves in 1848 by the French physicist Armand Hippolyte Louis Fizeau (1819–1896).

Suppose, now, that a star is rotating and that it is so situated in space that neither of its poles is facing us, but that each pole is located at or near the sides of the star as we view it. In that case, at one side of the star between the poles the surface is coming toward us, and on the opposite side it is receding from us. The light from one side causes the dark lines to shift slightly toward the violet, the light from the other causes them to shift slightly toward the red. The dark lines, shifting perforce in both directions, grow wider than normal. The more rapidly the star rotates, the wider the dark lines in the spectrum.

This was first suggested in 1877 by the English astronomer William de Wiveleslie Abney (1843–1920); and the first actual discovery
of broad lines produced by rotation came in 1909 through the work of the American astronomer Frank Schlesinger (1871–1943). It was only in the mid-1920s, however, that studies on the rotation of stars began to be common and the Russian-American astronomer Otto Struve (1897–1963) was particularly active here.

It was indeed found that some stars do rotate slowly. A spot on the Sun’s equator travels only about 2 kilometers (1¼ miles) per second as the Sun makes its slow rotation on its axis, and many stars rotate with that equatorial speed or not very much more. On the other hand, some stars whirl so rapidly on their axis as to attain equatorial speeds of anywhere from 250 to 500 kilometers (165 to 330 miles) per second.

It is tempting to assume that the slow-rotators have planets and have lost angular momentum to them, while the fast-rotators do not have planets and have retained all, or almost all, their original angular momentum.

That is not all that can be learned in this way, however. When stellar spectra were first studied, it was clear that while some had spectra resembling that of the Sun, others did not. In fact, stellar spectra differed from each other widely and, as early as 1867, Secchi (the astronomer who had anticipated Schiaparelli’s discovery of the Martian canals) suggested that the spectra be divided into classes.

This was done, and eventually the various attempts to label the classes ended in the spectra being listed as O, B, A, F, G, K, and M, with O representing the most massive, the hottest, and the most luminous stars known; B was next, A next, and so on down to M, which included the least massive, the coolest, and the dimmest stars. Our Sun is of spectral class G and is thus intermediate in the list.

As stellar spectra were more and more closely studied, each spectral class could be divided into ten subclasses: B0, B1 … B9; A0, A1 … A9; and so on. Our Sun is of spectral class G2.

The American astronomer Christian Thomas Elvey (1899–), working with Struve, found by 1931 that the more massive a star, the more liable it was to be a fast-rotator. The stars of spectral classes O, B, and A, together with the larger F-stars, from F0 to F2, were very likely to be fast-rotators.

The stars of spectral classes F2-F9, G, K, and M were virtually all slow-rotators.

Half the spectral classes, then, are fast-rotators and half are slow-rotators, but that doesn’t translate into an equal division of stars. The smaller stars are more numerous than the larger ones, so that there are more stars, by far, that are spectral class G or smaller than are spectral class F or larger. In fact, only 7 percent of all the stars are included in spectral classes 0 to F2.

In other words, there are not more than 7 percent of the stars that are fast-rotators and fully 93 percent of the stars that are slow-rotators. This would make it seem that at least 93 percent of the stars have planetary systems.

In fact, we might not even be truly able to eliminate the 7 percent of the fast-rotators. They happen to include the particularly massive stars, which are likely to have a much higher total angular momentum to begin with than smaller stars would have. They might have enough angular momentum left to spin rapidly even after they had lost some to their planets.

Or—the loss of angular momentum to the planets may take time and as we shall see, the really massive stars are all young stars. It may be that they haven’t yet had time to transfer the angular momentum.

From the data on stellar rotation, then, it seems fair to conclude that at least 93 percent—and possibly 100 percent—of stars have planetary systems.

So far, so good, but we must admit that stars may be fast-rotators or slow-rotators for reasons that have nothing to do with planets. Some stars may simply form from clouds that have more angular momentum to begin with—or less.

Can we therefore look for other types of evidence?

We can, if we stop to consider that when two bodies attract each other gravitationally, the attraction is two-way. The Sun attracts Jupiter, but Jupiter also attracts the Sun.

If two bodies, attracting each other gravitationally, were exactly equal in mass, neither would rotate about the other, properly speaking. Contributing equally to the gravitational interaction, they would
each circle around a point exactly midway between the two. This point around which they would circle is the “center of gravity.”

If the two bodies were unequal in mass, the more massive body would be less affected by the attraction and would move less. If the more massive body is twice the mass of the less massive, the center of gravity would be twice as close to the center of the more massive body as to the center of the less massive body. Suppose we consider the Moon and the Earth. The Moon is usually considered as revolving about the Earth, but it doesn’t revolve about the Earth’s center. Both it and the Earth revolve about a center of gravity that always lies between Earth’s center and the Moon’s center.

As it happens, the Earth is 81 times as massive as the Moon, so the center of gravity has to be 81 times as close to the center of the Earth as to the center of the Moon. The center of gravity of the Earth-Moon system is 4,750 kilometers (2,950 miles) from the Earth’s center. It is 348,750 kilometers (239,000 miles), 81 times as far, from the Moon’s center.

The center of gravity of the Earth-Moon system is so close to the Earth’s center that it is 1,600 kilometers (1,000 miles) under the Earth’s surface. Under the circumstances, it is certainly reasonable to consider the Moon as revolving about the Earth; it is, after all, revolving about a point inside the Earth.

The center of the Earth also moves in a small circle about that center of gravity once every 27⅓ days. If the Moon weren’t there, the Earth would move around the Sun in a smooth path. Because of the presence of the Moon, the Earth makes a small wave 27⅓ days long in its path about the Sun—twelve and a fraction of these waves through each complete turn. The wobble of the Earth’s could, in theory, be measured from out in space, and from it the presence of the Moon and perhaps its distance and size could be worked out even if, for some reason, it could not be directly seen.

This is true of Jupiter and the Sun, too. The Sun is 1,050 times as massive as Jupiter, so the center of gravity of the Sun-Jupiter system should be 1,050 times as close to the Sun’s center as it is to Jupiter’s center. Knowing the distance between the two centers, it turns out that the center of gravity is 740,000 kilometers (460,000 miles) from the center of the Sun. This means that the center of gravity is 45,000 kilometers (28,000 miles)
outside
the Sun’s surface.

The center of the Sun circles this center of gravity every 12 years.
The Sun, in its smooth progress about the center of the Galaxy, wobbles slightly, moving first to one side of its path, then to the other.

If only the Sun and Jupiter existed, an observer from a post in space, from which it was too far to see Jupiter directly, might deduce the presence of Jupiter from the Sun’s wobble.

Actually, the Sun also possesses three other large planets: Saturn, Uranus, and Neptune, each of which has a center of gravity with the Sun, though not one that is ever as far from the Sun’s center as Jupiter’s. This makes the Sun’s wobble a rather complicated one and that much harder to interpret.

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–).