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[xiii] Stonehenge, Salisbury Plain, England.

Photograph courtesy of Beulah P. Gossett.

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[1] Man's fascination with the heavens is as old as recorded history. Ancient astronomers carefully observed the relative position of stars, as well as the time of year when a particular star or group of stars was visible in the night sky. Two characteristics of the motion of stars were apparent to those early observers. First, the general movement of stars from one night to the next is a slow westward drift, with stars periodically disappearing from view for long intervals of time. Second, there is an overall unison in this motion. Each star moves so as to keep its position fixed relative to its neighboring stars. It is against this background of periodic and uniform motion that the story of planetary detection began.

Early in man's studies of the night sky, it was noticed that five "stars" do not participate in the harmony of motion shared by most celestial objects. The movement of these five planets (from "planetes," Greek for "wandering") baffled man for thousands of years, during which time they played central roles in much of formalized religious and mythological thought.

What is it that made the planets so special'? Unlike other stars, all of the planets drift nightly from west to east relative to the stars. There were also important distinctions among the planets. Two of them appear either as evening or morning "stars," that is, they are either visible shortly after sunset or before sunrise. However, they do not move from the evening to morning sky by passing overhead. Once they appear in the evening sky, they rise higher and higher in the sky on successive nights until they reach a maximum elevation above the horizon. They then begin to recede toward sunset on subsequent evenings until they are lost in its glare. After an interval of many weeks, these two planets reappear just before sunrise and repeat the behavior observed in the evening sky: a gradual rise to a [2] maximum elevation many nights after their appearance in the morning sky, followed by a return to sunrise. The remaining three planets do pass continuously from evening to morning sky, but roughly once a year they appear to move backward in the sky. The extent of this reversed or retrograde motion is different for each planet.

In 1543, Nicolaus Copernicus published his book, "The Revolutions of the Celestial Spheres," providing the explanation of the apparent motion of the five nomads. Copernicus stated that the Earth is not the center of the universe, rather that it and the five wanderers are all planets revolving around the Sun. Figure 1 (a) shows how Copernicus' model explains the motion of the two evening-morning stars. Note that the planet nearest the Sun is visible in the evening sky at position 1, moves to a maximum separation from the Sun at position 2, is visible near sunset at position 3, and is seen in the morning sky before sunrise at position 4. The explanation of the nearly annual retrograde motion of the other three planets is shown in figure 1 (b). Over most of Earth's orbit, the outer planet appears to move eastward in the sky relative to the distant stars. However, as Earth overtakes the outer planet (positions a and b in the figure), the latter appears to move backward (from a' to b') in relation to the distant stars. The extent of retrograde motion is largest for the planet closest to Earth's orbit. Copernicus' "discovery" of our planetary system forever altered man's concept of himself and his relationship to the universe as a whole.

The solar system family of planets at the time of Kepler and Galileo consisted of Mercury, Venus, Earth, Mars, Jupiter, and Saturn. The next addition to the family occurred in 1791 when the planet Uranus was discovered by Sir William Herschel. Uranus was carefully observed by Herschel and others for many years, at which time it became apparent that something was wrong. The planet was not following its predicted orbit. Leverrier of France and Adams of Britain analyzed the planet's motion and independently arrived at the conclusion that the perturbation in Uranus' orbit was due to another, more distant planet. Their calculations and predictions were generally ignored by their fellow astronomers. However, the planet Neptune was discovered in 1846 by Galle of the Berlin Observatory. Galle used Leverrier's work and found the planet within 1° of the predicted position. The discovery of Neptune is extremely important because it was the first planet detected by combining the science of....

(a) Apparent motion of one of the "evening-morning stars.

{b) Apparent motion of planets with orbits larger than Earth 's.

Figure 1. Schematic representation of the relative motions of the planets.

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[4] ....astronomy and the discipline of mathematics. The last planetary member of the solar system to be discovered was Pluto. The existence of Pluto was postulated in 1914 by Galliot and Lowell to explain perturbations in the orbits of Neptune and Uranus. It was discovered in 1930 by Tombaugh working at the Lowell Observatory.

Scientific and technological developments in the late nineteenth and early twentieth centuries made it possible to measure the separation between two photographically recorded star images with accuracies of a few thousandths of a millimeter. As a consequence of this accuracy, it became possible to measure stellar angular separations as small as a few hundredths of an arcsecond (an arcsecond is an angle equal to 1/3600 of a degree or 1/206,265 of a radian).

The first unseen object to be detected outside the solar system was not a planet, but a new type of star. Astronomers had long puzzled over small variations in the motion of the star Sirius. Instead of moving along a straight line, Sirius wobbled from one side to the other of its predicted course (fig. 2). F. W. Bessel calculated that the....

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[5] ....perturbations in the motion of Sirius were due to an unseen companion of considerable mass. Although Bessel was able to predict the position of the companion, it remained undiscovered until 1864. Later studies show that the companion's mass is approximately equal to the mass of the Sun.

The perturbation in Sirius' motion is only about 4 arcsec. If the companion had the mass of Jupiter rather than that of the Sun, the perturbation would have been proportionately smaller. Such a small perturbation is at the limit of the measurement capability of present telescopes. However, there are a number of stars less massive than Sirius, a few of which are also closer to the solar system. Peter van de Kamp of the Sproul Observatory, realizing that perturbations in the motion of these nearby, less massive stars might be detectable, initiated the first systematic search for other planetary systems.

The first results with the Sproul Observatory's 24-in. telescope were presented in 1943 by K. A. Strand. The results indicated that one of the two small stars in the binary star system, known as 61 Cygni, had a dark, or unseen, companion. The 61 Cygni study was soon followed by other tentative discoveries of planetary companions to small stars with such strange-sounding names as BD + 43° 4305, Epsilon Eridani, Lalande 21185, and Barnard's star (see table 1). But there were bothersome similarities in these results. The perturbations were always at the limit of the error of the studies. Also, the orbital periods of the proposed planets discovered with the Sproul telescope were all very nearly multiples of 8 years. The 61 Cygni study is an exception; it is only partially based on data from the Sproul telescope. It is also worrisome that the eccentricities....

[6] ....of the calculated planetary orbits are all large, averaging above 0.5. By comparison, the eccentricities of planetary orbits in the solar system are generally below 0.1 (Mercury and Pluto being the only exceptions).

Perhaps the best known study of this kind is that of Barnard's star. This is the only case in which the proposed dark companion has a mass comparable to that of a planet in the solar system (refs. 1, 2). A more recent study (ref. 3) has shown that the perturbation evidenced in the Sproul plates does not appear on plates taken with the 20-in. telescope of the Van Vleck Observatory, nor does it appear on plates taken with the 30-in. Thaw telescope at the Allegheny Observatory. Hershey (ref. 4) showed that there are epochs of "discontinuity" in the Sproul data which coincide with times when the lens of the telescope was adjusted. In light of demonstrations that the suspected perturbations were instrumental in origin, it may be said that there is no unequivocal evidence for the existence of planets outside the solar system.

With one exception, the telescopes presently used in astrometry, that branch of astronomy concerned with the position and motion of the stars, are about 60 years old, and only two were designed for high-precision astrometric observations. The rest have been converted to photographic use by addition of a colored filter in conjunction with yellow-sensitive photographic emulsions. None of the existing instruments are designed to meet the requirements of ultrahigh precision and long-term stability necessary for successful detection of very small perturbations.

An important aspect of a search for other planetary systems concerns the extent of the search: Is it to include a handful of nearby stars in hope of discovery, or is it to include a sufficiently large number of stars to provide a basis for meaningful statistics on the existence of other planetary systems? Discovery of another planetary system would be a significant event, but the fundamental questions are "What is the frequency of occurrence of planetary systems?" and "Do all types of stars have planetary companions'?"

[7] Present telescopes may succeed in discovering a planet revolving around a nearby star, but they will leave the fundamental questions unanswered. A new instrument (or instruments) is needed. Technology has advanced considerably since the last large refracting telescope was made, and it is reasonable to suppose that, through application of modern technology, a telescope could be built that would be far more accurate than existing instruments. Project Orion is a first step toward the goal of developing such a telescope, perhaps more importantly a first step toward a comprehensive program to search for extrasolar planets.

The classic technique for searching for extrasolar planets is that of precise astrometric studies. Because the field of astrometry has not enjoyed an infusion of modern technology to the extent that its sister subdisciplines within astronomy have, it was decided that the major emphasis of Project Orion would be to develop a design concept for a ground-based astrometric telescope that could, in principle, significantly increase the potential accuracy of astrometric observations. In the spring of 1976, two workshops were held to examine the state of current techniques and instrumentation in the context of detecting extrasolar planets (unpublished report to the Office of Space Science of the NASA, 1976). Among the techniques that seemed potentially capable of detecting extrasolar planets, two emerged as feasible alternatives to existing techniques. Both involved detection of light from extrasolar planets, in contrast to astrometric studies in which the presence of a planet is inferred from its effect on an observed star. A portion of Project Orion involved studies of these two techniques whereby extrasolar planetary radiation in one case intrinsic thermal radiation, in another case, reflected visual radiation - might be detected.

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