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1 Introduction The recent decades of planetary exploration by spacecraft have cen- trally influenced perception and understanding of our solar system. But fundamental questions remain, questions of origin and uniqueness that cannot be answered with confidence as long as we are limited by access to just this one example of a solar system. This report addresses a new op- portunity in the planetary sciences to extend our exploration outward to discover and study planetary systems that may have formed or are forming around other stars. The intellectual desire to look beyond the confines of our own system for answers to these unknowns is of course not new, but what has emerged only recently is the chance to actually find them. The questions to be asked about other planetary systems have been posed and sharpened against a broad and growing base of local knowledge just when the technological ability to detect other systems is poised on the brink of actual discovery. Until the lams there was no real evidence for the existence of small condensed orbiting bodies, over than tiny stars, ~ other stellar systems. But in just the past few years a number of viable candidates have been found, some perhaps as small as Jupiter. We are truly at the edge of a scientific and philosophic revolution. Investigation of the existence and nature of extrasolar planets, and of the matter from which they form and evolve, is unquestionably an endeavor of rich scientific interest and consequence to planetary science, astronomy, and astrophysics. It also involves the profound philosophic question of the relation of Earth and humanity to the rest of the universe. Histones of s
6 scienc~indeed histories of human intellectual development during and after the Renaissanceemphasize four major earlier revolutions, each of which displaced humanity ever further from a seemingly central and unique position of place and perception. Two of these were purely astronomical. First, the Copernican revolution overturned the Aristotelian "common sense" view that Earth occupies the center of the universe. Much later, in the 192Os, what might be called the "Shapley revolution," after one of the principal astronomers involved, showed that the solar system was not at the center of our galaxy, nor was the d~sk-shaped galaxy a unique system including all known stars and "spiral nebulae" as had been thought. Rather, the solar system was out toward the edge, in one arm of our spiral galaxy. And so-called spiral nebulae had been misidentified. They were not spiral nebulae at all, but distant galaxies, and our own galaxy was just one of maIly. Between these two revolutions in our perception of cosmic place came two more, one biological and the other physical. Charles Darwin showed that we were not caretakers of creation, as had been imagined in earlier Western thought, but rather one of many interdependent, evolving species in a long history of species that had emerged, changed, and died. And with the advent of relativity and quantum mechanics from Albert Einstein, Max Planck, Niels Bohr, and Werner Heisenberg, notions of a privileged human observer, direct perception of physical law, and a classically deterministic universe disappeared forever. Each of these revolutions had the effect of displacing humankind from its assumed anthropocentric position. In this context one dramatic question remains. Is it possible that Earth, the habitable conditions on Earth, and indeed the life that has evolved to fit those conditions constitute rare accidents? Modern theorists have proposed answers, but we will never really know, in a scientific sense, until we have surveyed a statistically valid sample of star systems with enough sensitivity to determine whether they have Jupiter-size, Uranus-size, and ultimately even Earth-size planetary bodies near them. Barring interception of signals from intelligent life elsewhere by radio listening searches, only such a survey can address this question. Either a positive or negative answer would have profound conse- quences. The search might reveal that planetary bodies are common stellar companions, existing near 10 percent, 50 percent, or even 90 percent of all stars. Then we would find ourselves in the midst of a fundamental extension of earlier revolutions, moving us still further from an anthropocentric view in demonstrating that our home system of planets and perhaps life itself are not unique or special creations. Scientifically, such a result would put much theoretical work on a firmer footing by clarifying origins and allowing statistical studies. A negative result no detections after a sensitive search
7 among a large sample of stars would be equally or perhaps even more profound, both intellectually and scientifically. It would certainly lead to basic reassessments in our thinldug about solar system onion, the possibil- ity of life elsewhere, and the general processes governing the evolution of stellar systems. PREMISES OF THIS REPORT In 1975, 1978, and 1980, the Committee on Planetary and Lunar Ex- ploration (COMPLEX) of the Space Science Board (SSB, now the Space Studies Board) published three reports, which taken together encompass our entire planetary system. The first, "The Outer Planets," is included in the Space Science Board's Report on Space Science 1975 (National Academy of Sciences, 1976~. Strategy for E~ploranon of the Inner Plan- ets: 1977-1987 and Strategy for the E~lorahon of Primitive Solar-System Bodies Asteroids, Comets, and Meteoroids: 198~1990 were published by the National Academy of Sciences in 1978 and 1980, respectively. The strategies for exploration of the outer solar system and of the inner planets were subsequently updated by two reports: A Strategy for Exploration of the Outer Planets: 198~1996 (National Academy Press, 1986) and 1990 Update to Strategy for Exploration of the Inner Planets (National Academy Press, 1990~. These five reports are the committee's principal advisory documents on scientific exploration of the solar system. In its 1980 report, COMPLEX set forth general goals for investigation of primitive bodies within the solar system. The committee then looked briefly outward to a broader context in which exploration of our planetary system is exploration of just 1 of some 10~i stars in the galaxy. That short discussion emphasized the relationships that must exist between the history of our own solar system and the origin and evolution of other stellar systems. In highlighting the essential role that extrasolar observations will eventually play in our understanding of solar system history, this section of the 1980 report foreshadowed the current interest and activity in the detection and study of extrasolar planetary material, and it stands as a direct antecedent to the present report Remarkable observational advances during the past few years have pointed to the presence of solid matter at vanolls stages of development around other stars. The whole-sky survey from the Infrared Astronomical Satellite (IRAS) provided compelling evidence for the presence ir1 nearby smr-forming regions of dusty matter surrounding a significant fraction of young (t < 10 million yr) solar-type, pre-main-sequence stars. The infrared spectra of such stars suggest that these materials occur in disklike circumstellar structures of dimensions comparable to the solar system and masses comparable to a "minimum mass" solar nebula. The IRAS also
8 found spatially resolved dust shells (or rings) around several older, main- sequence stars, among them Vega, Beta Pictoris, and Fomalhaut. In the particular case of Beta Pictoris, a disklike structure has actually been detected. Extensive investigations of this system indicate that the dust grains are larger than typical interstellar grains, unplying that they may have experienced growth processes similar to those that operated in the early solar system. The detection of star-orbiting bodies with masses significantly below those of the known lowest-mass stars Is now technically feasible, by direct imaging, spectroscopy, or orbital reflex effects, and reports of the existence of such objects are appearing at an increasing rate. Continuing discoveries of circumstellar structures and objects relevant to this study can be expected to lead to rapid and fundamental changes in our understanding of this field and consequently to parallel modification of the specific search strategy proposed in this report. The committee elects, however, that the general approach and techniques discussed here will continue to be appropriate. In light of recent observations and the potential for future discoveries, the SSB requested in 1985 that COMPLEX extend its consideration of exploration strategies to planetary systems outside the solar system. The committee was charged to assess both the significance of detection and study of extrasolar planetary material to our understanding of the origin and evolution of our own solar system, and the status and prospects for development of techniques that could be applied to investigation of objects far beyond direct access by spacecraft. The base of information for the recommendations of this report was developed during 1984 to 1987 in COMPLEX deliberations combined with presentations by scientific and technical experts from outside the commit- tee. Of particular importance were extensive briefings by participants in a series of workshops, sponsored by the National Aeronautics and Space Administration (NASA) from 1976 to 1984, on the scientific rationale for and feasibility of astrometric, Doppler spectrometric (radial velocity), photometric, and direct-=aging techniques for detection and study of ex- trasolar planetary material. After evaluating the information from these and other sources, COMPLEX has formulated the present report, which is in response to the charge from the Space Studies Board: 1. Address the scientific rationale and goals for investigations of other planetary Stems and extrasolar planetary material in me context of current knowledge and the framing of scientific questions concerning the origin and evolution of planetary systems in general, and of our solar system in particular; 2. Examine the stams of theoretical understanding of star and planet
9 formation, the state of relevant observations, the interplay between ob- servat~onal and theoretical studies, and the relevance to related areas of astronomical, astrophysical, and exobiological research of a program to detect and study other planetary systems; 3. Assess the stems of current research in this and related areas, the ev- ~dence from classical and newly developed techniques, and the significance of recent discoveries; 4. Consider and evaluate the observational capabiliiiesexisting, under development, and conceptualneeded for the measurements required to meet the science objectives; and 5. Propose a strategy for the investigation of other planetary systems and extrasolar planetary materials for the decade 1990 to 2000. In developing this strategy and recommendations for its implemen- tation, COMPLEX has adopted the position, also taken in its previous strategy reports, that long-term science objectives are most flexibly and en- duringly defined in ways that separate strategic approaches from the specific means chosen by NASA to implement them. The committee wishes, how- ever, to reiterate here a point emphasized in its 1978 report, that this or any other strategy of scientific exploration is meaningful only to the extent that it is actually carried out. Implementation requires operational planning, funding, and execution. Execution of the strategy depends on missions; in the present context this committee broadly defines "missions" to include development, deployment, and support of earth-based and earth-orbital instruments and facilities. OVERVIEW AND PURVIEW From direct observations, we see remarkable structural and chemical regularities within our solar system: low eccentricities and inclinations of planetary orbits, and coherent radial trends in composition. Similar regularities are displayed, in miniature, within the major satellite systems of the giant planets Jupiter and Saturn. These observations suggest that the coherent structure of at least this system was not an accident of random events. In the hierarchy of the planetary sciences, the next level of inquiry asks if our system is one of a broad set with similar charactenstics, or if formation of our planetary system around the Sun was a result of extraordinary processes. The implications of this question are intellectually profound and scientifically fundamental to our understanding of solar system genesis. It is difficult to address without data on We frequency of occurrence, structure, and dynamics of planetaIg systems around other stars. No matter what the answer, we will learn much about the origin and early evolution of the solar system from investigation of extrasolar planetary and preplanetary systems,
11 reassessment in 10 years. Many c~rcumstellar bodies should have periods of revolution that are less than a decade, and for them even relatively brief observational searches may be expected to provide unambiguous informa- tion. Others, such as Uranus and Neptune in our tom system, have longer periods, and more extended observations will be necessary. For this reason and others, the committee has stressed the importance of the duration as well as the accuracy of the astronomical searches in developing measure- ment requirements. But the disadvantages of attempting to formulate a detailed and responsible strategy with open-ended duration In this rapidly evolving area, based as it would necessarily be on doubtful assumptions concerning future technological developments and potential breakthroughs in observational instrumentation and techniques, were in the committee's opinion compelling. The decadal time frame was therefore chosen to trig- ger a review of the progress and prospects of the program at a time when much of the recommended observational technology should be in place, and the data base applicable to various aspects of the investigation more firmly developed. COMPLEX emphasizes that there is no implication in this choice of strategic time frame that initiation, evolution, and te~ination of the research effort would all be accomplished within 10 years. Many of the various research areas discussed in this report, in the context of a specific program for investigation of extrasolar planetary ma- terials, are of necessity intrinsically interdisciplinary in nature and have broad observational and theoretical connections with other fields of sci- entific endeavor. Comprehensive investigation of precursor materials, for example, extends deeply and quantitatively into astronomical' astrophys- ical, and chemical studies of molecular clouds and of cloud evolution toward condensation at their cores. In this report, COMPLEX has fo- cused its attention primarily on observation and study of post-collapse or post~ondensation development, rather than on more general aspects of molecular cloud systems. Even so, essential elements of the investigation recommended here interweave in significant ways with this and other as- sociated fields of scientific pursuit, and in the committee's opinion, new NASA-wide institutional structures involving the Solar System Exploration, Astrophysics, and eventually Life Sciences divisions will be required to fully implement these cross-disciplinary connections. In this context, it is important to note specifically the purview of this study and of the exploration strategy derived from it. The committee includes considerations and recommendations pertaining to the following: · Searches for extrasolar-system planets, preplanetary precursor sys- tems, and other orbiting materials or objects of less than stellar mass; · Determination of the characteristics of any planetary systems that may be discovered;
12 · Measurement of system elements in ways that facilitate comparisons with planets or dust in other systems; · Characterization (spatial, spectral, and temporal) of young stellar systems that may be in the process of forming planetary systems; and · Study of the physical and chemical nature of materials that primitive stellar systems may compose. The committee excludes or discusses only briefly and makes no de- tailed recommendations concerning the following areas of scientific or technological study: · The general history of molecular clouds prior to condensation into protostellar systems; · The capabilities of specific current and proposed astronomical fa- cilities to probe the dynamics of molecular clouds, and the measurement capabilities required to enable such studies in the future; · The cosmic history of the biogenic elements; and · Programs focused principally on the search for extraterrestrial in- telligence (SETI). The following section of this introductory chapter defines the tech- nical nomenclature adopted by the committee for the preparation of this report. Chapter 2 sets out the fundamental scientific goals of a program to detect and study extrasolar preplanetary materials, planets, and planetary systems. Chapter 3 summarizes current theoretical understanding of stellar and planetary formation, and Chapter 4 the present state of our obser- vational knowledge of the precursors, processes, and products of possible planet-forming astronomical environments. Chapter 5 addresses methods and requirements for detecting and tracking planets, including currently feasible technological capabilities to mount and sustain credible searches for planets around neighboring stars. Specific measurement requirements are defined and developed, with emphasis on astrometric and Doppler spectroscopic instruments and techniques for detecting stellar reflex mo- tion, which accounts for one component of the search strategy. Chapter 6 deals with ongoing and future physical studies of extrasolar materials that are possible precursors of evolving and evolved planetary systems, and their scientific and programmatic relevance to the strategy recommended in this report. Connections between this search for planetary and preplaneta~y objects and other fields of astronomical and astrophysical research, and the influence on some of these disciplines of the new observational capabili- ties intrinsic to planet-searching instruments, are discussed in Chapter 7. In Chapter 8 COMPLEX presents its recommended scientific objectives, measurement requirements, and overall decadal strategy for investigation of extrasolar planetary materials.
13 NOMENCLATURE Planetary matter around other stars may be expected to span a vast range of mass and size. The search for such material is not limited to possible "Jupiters" or "Earths." Planetary materials can be thought of not only as any solid matter accreted to planetary size, but also, in earlier evolutionary stages, as dispersed dust grains condensing from the gas phase of a circumstellar accretion disk or infalling from precursor cloud environments. The following review of phenomena encountered at different masses, starting with small material, discusses each mass interval as it relates to the question of extrasolar planetary material. The terminology is that chosen by the committee to simplify the discussion, to allow framing of useful research questions, and at the same time to be as congruent as possible with existing literature. The nomenclature is summarized In Table 1.1 and is schematically illustrated in Figure 1.1. According to current theory and observation, as a star forms it is surrounded by a cloud of gas and dust. One such cloud was the solar nebula, in which our own planetary system foxed. Current observations and models indicate that interstellar dust grains were preserved in much of this cloud, and cooling produced condensation of even more grains. Such dust-rich clouds are inferred to exist around many solar-type pre- main-sequence stars, from observation of excess infrared radiation emitted by heated circumstellar grains. Particles of mass smaller than about 1 fig (d < 10-2 cm) are conveniently called dust. An important research question is whether observed circumstellar dust is currently actively aggregating into planetary bodies, undergoing removal from a system without accretion, or being produced by erosion of already-formed, larger asteroidlike or cometlike bodies. Theory suggests that gravitational settling of dust grains and aggre- gates toward the equatorial plane of a forming solar system, together with collisions, leads to the gravitational clumping or mechanical accretion (by mutual impact) of these grains into asteroid-sized objects. Ultimately, as is thought to have been the case in our solar system, accretion may produce planets at least as large as the terrestrial planets, and perhaps as large as the cores of the giant planets (around 10 to 20 Meg. A physical basis for subdividing the nomenclature comes from noting that the strength of bodies smaller than a few hundred kilometers in diameter is typically greater than the internal gravitational stresses, so that they can maintain nonspherical shapes. Bodies larger than ~10-2 cm but smaller than a few hundred kilometers are too big to be called dust, but they are smaller and usually more irregular in shape than are objects typically considered planets. Star- orbiting bodies in this mass range are called subplaneta~y objects. In our solar system, asteroids and comet nuclei are examples, but in this report
14 Ct ~ ·C} Q) ~ C) CO a, C) o - CC ~S _ _ a ~ ~ E O §, ~ , ~ ~ ~ ~ £- ~ ~ ~ c) - - ~ ~ ~ ~ 00 = .= ° ~ E O E ~ U'~ Z° ~S E Y 3 ~ E I_ a C C E E 8 E Y,_ Y .e e e ~ =Y E E 8 ._ --y a e a E .Y 8 CY <: TE E Y 18i oel ul ~e 8 c u, ° ~ 0 X ~ °. X ~ X X ~ ° ~ c~ ~ _ ~ ~ ~ _ c, o c' as ~ _ ~ ~ _t D .~ ~ D,$ tt?
15 16 . 1 .5 ~5 - ~ .12 a: .06 .03 8 4 t <,, 2 all ~ 1 C) =.25 J ~ .12 _ .03 _ .01 SCHEMATIC MASS DI~I~ON · :~ OF KNOWN STARS A-::;- : · ~5 Q 9- J3 ~ SUB~ET=Y..- of it. DUST SUB- .STELLAR OBJECTS ,: - ~xxx ~ . 0.001 0 01 o, ( SOLAR MASSES ) . .~ _o , , lo-6 loss 10 MASS 3 ~ , . . . . 10-4 0.~1 0.01 0.1 ( JUPITER MASSES ) , lo 100 1000 1 10 100 FIGURE 1.1 Me various classes of target objects in the search for extrasolar planetary materials and systems, arranged schematically on a diagram of mass versus diameter. Dots in upper right give empirical data on main-sequence stars of solar composition (Popper , Anne Rev. Ashore Astroph.ys. 18, 1153. Below the mass cutoff for nuclear reactions at about 80 MJUp,ter, theoretical models at minimum radius for solar composition are shown by plus and multiplication symbols (respectively from D'Antona , As. 1 320, 633; and Burroughs, Hubbard, and Lunine , Atrophy J: 345, 939. The decline from a maximum near 2 MJUpter to a minimum near 80 MJUp~ter is due to compression as more mass is added. This is the regime of "substellar'' objects in this report's nomenclature. Empirical data for the "planetary" regime are shown by planetary symbols; two large satellites are included. Leo other regimes, of "subplanetary" objects and "dust," fall on scale to the lower left. Chart above the upper figure boundary denotes the on ved mass distnbution of stars, with a peak around 0.25 M<3 and a cutoff (observational) near 0.1 M<~> . the committee generally avoids applying these terms to other star systems to avoid connotations of composition or origin. Extrasolar subplaneta~y objects will not be individually observable in any direct way, although it is possible that their presence could be inferred. For example, it has been suggested that inner portions of the Beta Pictoris system that are seemingly empty of dust may be zones where dust has aggregated into larger bodies in the subplaIletary-mass range; if so, the resulting configuration might be similar to our asteroid belt. The term "planet" carries the connotation of a larger, relatively spher- ical body in a mass range at least as great as that from Mercury to Jupiter.
16 No universally accepted definition is available. Therefore for this dis- cussion, a planet is defined as a star-orbiting object (of no presupposed, particular mode of origin) with a mass greater than that required for the development of a near-spherical shape (roughly the mass of the asteroid Vesta) but smaller than the upper limit in mass discussed below. Planets are thus generally larger than the asteroids of our own solar system, although Ceres would qualifier in this nomenclature as a very small planet. The upper limit on planet mass was chosen as follows. If sufficient mass is added to a body, normal crystalline or liquid matter becomes compressed and electron degeneracy becomes dominant. As seen in Figure 1.1, the radius of the body actually begins to decline with increasing mass at about this point. The precise position of turnover in the mass-radius diagram depends on composition and rotation and Is thus somewhat ill- defined. However, it occurs near 2 MJUpiter for a range of pure hydrogen and solar-composition models. This transition, at about 2 M,, is thus chosen as the physical basis for defining the upper limit for planets and the lower limit for substellar objects. This limit also corresponds well with common conceptions of what a planet is. The boundary between substellar and stellar states is similarly (and, in this case, traditionally) defined by a physical transition. At about 80 MJUPI=, hydrogen-burning nuclear reactions in the interior of the body can supply the entire radiated energy. Objects more massive than this are defined as stars. Bodies with a mass between 2 and 80 M~Up'`er have sometimes been called "brown dwarfs," because they contract for cosmically long times and radiate in the infrared; however, the radiation is for the most part gravitational in origin, not nuclear. Traditionally, no lower mass limit has been indicated for the term brown dwarf, and considerable controversy has been engendered as to whether certain objects should be called brown dwarfs or planets. Because the term brown dwarf has been controversial as well as vague, it is not used in this report. Instead, a substellar object Is defined as having a mass between 2 and 80 MJUp~e~, and a star as having a mass greater than 80 Mixup'=. The purpose of this terminology is not to create arbitrary categories, but rather to employ a language that simplifies discussion, is relatively congruent with common usage, and does not connote specific modes of origin. Definitions are based entirely on a mass spectrum, divided (except at the dust-subplanetaIy boundary) by physical phenomena. Subdivisions involving composition or other properties could be added, but Thble 1.1 is adequate for the present discussion. The terms substellar and subplanet~y are parallel and avoid the preconceptions associated with terms such as "brown dwarf," "asteroid," or "comet." Implications of origin are likewise avoided by defining an object as a planet solely on the basis of mass range.
17 Several other widely used special termsplanetesimal, protoplariet, prm toss=, and pre-maut-sequence star have not Men director included in this set of definitions, but are listed in Table 1.1 according to the masses these objects are usually considered to have. They appear in this report in con- texts where their usage is conventional and their meanings clear. The term planetesimal, connoting young or preplanetary bodies with a potential for further aggregation, is used in a general way to designate objects in the subplanetary-mass or low-planetary-mass range, usually in reference to a system where mere is a potential for collis~onal or gravitational interaction leading to evolution of the system as a whole. Proloplane! refers to an object of planetary E ISS at an early stage of development, a body with a substantial fraction of its final mass sewing as a nucleus for accumulation of more material. Similarly, a protostar is an object of stellar mass in its earliest evolutionary stage, during which hydrodynamic accretion onto an equilibrium core occurs. A pre-main-sequence star refers to a later phase when the object is entirely in hydrostatic equilibnum, but has too low an internal temperature for generation of appreciable nuclear energy. In general, important scientific considerations and questions, partic- ularly as they relate to stars, substellar objects, and planets, are com- municated readily and clearly with this terminology. One can frame the complicated question of whether planets form by different mechanisms, such as accretion, gravitational collapse, or capture, and the observational question of whether substellar objects or planets in any given system are in nearly coplanar and circular orbits. If a star has only one substellar or planetary companion, does that body orbit in or near the equatorial plane of the star? Positive answers to the last question might imply origin in disk- shaped, dissipative precursor systems. Negative answers might suggest, for example, capture of companions in dense, primordial, star-forming clusters or a high degree of asymmetry in the collapse process. In terms of this nomenclature, one can summarize the results, as of mid-1990, of observational searches for evolved extrasolar planetary systems by stating that very-low-mass stars have already been found around some stars, and several discoveries of substellar objects have been reported- including one (HD114762b, with a mass perhaps as low as ~10 M - tern that at this writing appears particularly convincing. Another body in the substellar-mass range, the ~2~MJUpi~er companion to the eclipsing millisec- ond pulsar 1957+20, has been reliably detected but is not included here since it was originally a stellar object that has been reduced to its present mass by pro~nmib to the pulsar. Announced detections of planets have either not been confirmed by subsequent observation or are beset with such uncertainties that discovery cannot yet be claimed.