sophisticated internal models of external phenomena has evolved, it is only a matter of time before all possible ideas inherent in the available sensory perceptions are conceived. It is incorrect to focus on "critical" historical events in cultural evolution, just as it is incorrect to focus on single steps in biological evolution. For example, the importance of what might have happened had the Greeks lost at Marathon has probably been greatly overemphasized. Some developments that subsequently occurred might, to be sure, have been prevented, but others of similar type that did not happen could also have been stimulated. The next stage in the evolution of hominids could be stimulated by our entry into space, including a search for extraterrestrial intelligence. Our past success has been due to our breaking of new ground — to our acceptance of the challenge inherent in exploring new ways of life. This was true even for our distant ancestors who abandoned the relative security of the trees to compete with other carnivores for fleet-footed prey on the open savannah. Hans Lukas Teuber thought the urge to explore and seek new understandings is among our most powerful innate drives. This can only be true because exploration has had high survival value to us. Thus it might be a mistake if, in our present time of environmental, political, and social crises, we turned our back on that great arena we have not explored - the stars. Daniel Kevles, historian from the California Institute of Technology, said he viewed the matter as a religious man. He noted that everybody's assumption seemed to be that a search was worth conducting if it appeared that life were common. However, he thought it would be worthwhile should we not be convinced of this. A null result would be important. The justification of the experiment is that it is a very special test of whether we are alone; this goes beyond the benefits we may gain or the knowledge we could acquire. Only if we can demonstrate that the probability of intelligent life elsewhere is zero, should we not go ahead with the search; this, of course, is impossible, because we are here. Jack Harlan, historian of agriculture from the University of Illinois, thought we must learn what we can about the Universe; it is our destiny. COLLOQUY 3 DETECTION OF OTHER PLANETARY SYSTEMS Prepared by: Jesse L. Greenstein Professor of Astrophysics California Institute of Technology David C. Black SETI Program Office Ames Research Center DETECTION OF OTHER PLANETARY SYSTEMS INTRODUCTION Since we are the only intelligent life we know of, we generally assume, as mentioned in the Introduction, that whatever intelligent species may exist elsewhere also originated on a planet. If the quest for other intelligence is to succeed, the fraction of stars with planetary systems must be reasonably large. What do we know about this fraction? Extraordinarily little, and the little we know is clouded by controversy. A near neighbor (Barnard's star) was supposed to show small perturbations induced by planetary companions. Refinements of astrometric technique have recently shown these perturbations to be in serious question. No other astrometric perturbation was as large; as a consequence we do not know whether any planets, other than our own neighbors, exist in the Galaxy. Is our solar system unique? Is this state of knowledge concerning the frequency of occurrence of planetary systems likely to change in the near future, or must we pursue a search for extraterrestrial intelligence in the absence of basic data? The answer depends on a willingness to invest time, thought, and money in an effort to overcome our ignorance. A first step, an investment of time and thought, is under way. A group of scientists (see Section III-15) under the leadership of Jesse Greenstein, has attempted to define how observations might shed some light on the frequency of low-mass companions to stars. A report to NASA will be based on the two Workshops on Extrasolar Planetary Detection held at Santa Cruz (March 24-25, 1976) and at NASA Ames (May 20-21, 1976), and on a meeting of the astrometric community (U.S. Naval Observatory, May 10-11, 1976). This report will outline technical problems and a coherent program that might produce answers. The time scale for detection is not short, since characteristic indirect methods involve years (planetary revolutions) and direct methods may require an orbiting space telescope. Classical means for detecting low-mass companions to stars involve accurate positional astrometry, useful for nearby stars, or accurate determination of radial velocity changes. Both techniques can be improved with modern technology. Direct detection, at optical or infrared wavelengths, is also subject to orders-of-magnitude technological improvement. A listing of science-related activities associated with detection of other planetary systems is presented in Section III-6. INDIRECT METHODS If a star has a companion, the star will revolve about the center of mass of the system at the angular velocity of its dark companion. The radial component of velocity is observed spectrographically (independent of distance from the observer), the tangential component is observed with respect to an inertial frame, defined by other stars, as a sinusoidal term superposed on the tangential motion of the star (decreasing with distance from the observer). Radial Velocity Techniques What level of accuracy is required to detect the radial velocity effect arising from the orbital motion of a star around the barycenter of a star-planet system? Jupiter's motion around the barycenter of the solar system causes a reflex movement of the Sun of approximately 12 m/sec (with a period of 12 years). The effect of the Earth on the Sun amounts to about 0.09 m/sec. Thus, to detect Jupiter-like planets around other similar stars by radial velocity determinations we need accuracy on the order of 10 m/sec, while 1 m/sec would be desirable. For Earth-like masses, 0.01 m/sec would be none too high an accuracy if it were possible to achieve. How far are we from such accuracy? Typical radial velocity measurements are accurate to about 1000 m/sec, well above the level required. However, an autocorrelation system can already attain a few hundred meters per second on faint stars. One such system is that employed by Griffin (Cambridge) and Gunn (Palomar). This system is photoelectric and was designed to work on faint stars rather than give high accuracy. It regularly gives standard deviations of 250 m/sec on stars as faint as tenth magnitude and in integration times of 5 min. Griffin has expressed the view that this technique can eventually produce accuracies of about 10 m/sec. More complicated wavelength calibrations, impressed on the spectrogram, are planned at Arizona and are yet to be tested. What are the foreseeable limits in terms of accuracy for radial velocity determinations? The general feeling of the Workshop on planetary detection was that a level of accuracy in the neighborhood of 10 m/sec can be obtained if great care is taken. Progress to higher levels of accuracy might be achievable by means of conventional photographic spectroscopy, if certain precautions are met. Such elaborate spectrographs are probably reliable over periods of years, if maintained in a fixed position on a dedicated telescope with an aperture of 1–2 m. Construction time scales for such a radial velocity machine and telescope are short (2-3 years) compared to the detection time (several planetary revolutions, or 2 to 20 years). A fundamental problem is the noise in stellar radial velocities; solar granules have motions comparable to 1000 m/sec, but should average out in integrated light. The Workshop felt that this question should be studied over the next few years, with emphasis on the Sun. We know little about the stability of the solar radial velocity, but magnetographs can study the wave motions and non-radial pulsations, and the work of Hill and others (following Dicke) should determine short-period radial pulsations. The gap in knowledge concerns month-to-year variations in solar diameter, the effect of spottedness, etc. The Workshop felt that a program aimed at determining the stability of the solar velocity is valuable and feasible. Astrometric Techniques Another, and perhaps better known way to discover dark companions to a given nearby star is that of astrometric observation. In order to set the level of accuracy we require, it should be noted that the displacement of the Sun due to its motion around the Jupiter-Sun barycenter corresponds to about 103 arcsec as viewed from a distance of 5 parsecs. The displacement of the Sun due to the Earth is considerably smaller, about 10% arcsec, again as viewed from a distance of |