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Executive Summary The recent decades of planetary exploration by spacecraft have cen- trally influenced perception and understanding of on 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- portuni~y in the planetary sciencesto extend our exploration outward to discover and study planetary systems that may have formed or are forming around other stars. In this report, the Committee on Planetary and Lunar Exploration (COMPLEX) concludes that a coordinated program of astronomical ob- servation, laboratory research, and theoretical development aimed toward systematically exploring the solar neighborhood for planets and planetary systems in various stages of evolution around other stars, and understanding the dynamics and origin of whatever may be found, would be a techno- logically feasible, scientifically exciting, and potentially richly rewarding extension of the study of bodies within the solar system. It has, in fact, already begun. Discoveries of orbiting bodies with masses far below those of the smallest previously known stars have been announced even though these bodies lie at the edge of detectability and await confirmation. Thus, within just the past few years, a provocative and challenging new opportu- nity has appeared in the planetary sciencesthe opportunity to readdress fundamental scientific and intellectual questions of origin by moving be- yond the limited knowledge afforded by the single example of our own
2 planetary system to a more general understanding based on the existence (or, conceivably, nonexistence) and character of other evolved systems. In the opinion of the committee, the scientific purview of such a pro- gram necessarily includes the antecedents of evolved planetary systems as well. The program, therefore, should encourage and support relevant ob- se~vational, laboratory, and theoretical investigations of precursor matter and of young dusty stellar environments that are likely sites for planet for- mation. Studies of this nature involve a variety of observational techniques within several different areas of astronomical and astrophysical research, and this component of the program is thus intrinsically multidisciplinary. So, too, is the effort to detect and characterize evolved planets, since the instrumental capabilities needed for such observational searches are directly applicable to current endeavors and problems in galactic astronomy, as- trophysics, and cosmology. ~ note one example, an astrometric telescope with the sensitivity recommended in this report would have a substantial impact on determining stellar distances by direct parallax measurements; it could, for instance, yield accurate distances to at least 20 "standard candle" Cepheid variables and thus precisely calibrate the distance scale in the local group of galaxies and improve that of the universe as a whole. The committee concludes, from consideration of the scientific and engineering issues involved and from assessment of existing and developing detection techniques, that the relevant technology is on the bnok of major advances; that it will be technically feasible, on a one- to perhaps three- decade time scale, to make a first assessment of whether extrasolar planets in the Jupiter- to Uranus-mass range are common or very rare; and that resolution of this question is of immense scientific importance. Initiation and support of a formal program designed to answer this question, and to probe all stages of planetary evolution illuminated by the astronomical record, are therefore, in the committee's view, worthy of major scientific effort at this time. In Chapters 5, 6, arid 8 of this report, COMPLEX has set out rec- ommendations for the overall strategic considerations, induding scientific objectives and requirements for the specific measurements that are needed to achieve them and that in the committee's opinion should guide the structure, content, conduct, and time scales of such a program in the near term. In summary, the recommendations to the NASA Office of Space Science and Applzcanons (OSSA) are to: · Initiate, and maintain for at least a decade, systematic observational planet searches that encompass the widest feasible domain of the planeta;ry mass versus semimajor alas discovery space (see Figure s.2 in Chapter s). Specifically,
3 1. Initiate an astrometric observational survey program designed to track the reflex motion of 100 or more stars in the solar neighborhood fr < 10 parsecs) with a design goal for relative astrometnc accuracy of ~ = 10 microarcsec, sentient in a search of adequate duration to detect and track Uranus-mass planets in a solarlike system. ~ Obtain and interpret a record of Doppler shifts in stellar spectral features due to reflex motion, at or above the current measurement accuracy of ~ ~ 10 m s~i for the velocity of the orb Cal reflex' in a survey of the duration and extent specified for the astrometric survey. 3. Until such systematic searches are mounted, maintain ongoing ground-based searches at their present best accuracies, and mvesugate and im- plement improvement of these accuracies if technically and financially feasible. · Augment current observational studies of young stellar systems, and of the physical properties of circumstellar-interstellar dust systems as precursors to and products of planetary systems, on a varied of spatial and spectral resolution scales. Survey a statistically meanu~gfi~l number of stars of varied masses and types to detect such systems. · Continue investigations of lucks between interstellar-circumstellar dust and isotonically "exotic'' grains in solar system materials such as primitive meteorites, interplanetary dust, and comets Important elements and objectives of this effort include collection and curation of rare interplanetary asteroidal- cometary dust particles fIDPsJ; laboratory identification Ad analysis of micron to submicron presolar dust grains preserved in these meteonac and IDP materials; and laboratory simulation and theoretical studies of the astronomical dust cycle, Acceding the Companion and physical and chemical processing of interstellar grains in preplaneta~y and planet-forming environments. · Improve the capability of theoretical models and computer expedients to make specific predictions regarding the observational properties of planetary systems at all stages of their evolu non, and to further develop models to aid in the interpretation of existing data · Encourage the following mulii-disciplinary activates between the re- sponsible divisions at OSSA: participation of planetary scientists uz the design and building of future observatories and facility instruments, and u: the allo- cation of observing time at existing observational facilities; joint support for multidisciplinary scientific initiatives; and joint development of ins~menta- tion for extrasolar observation. Pursue long-range instrumental and strategic initiatives that are con- ceptually applicable and potentially valuable to the investigation of extrasolar planetary materials in later stages of reconnaissance or u' subsequent phases of exploration and intensive study, but that at present are technologically or theoretically too undeveloped to be of immediate utility in implementing the short-term strategy proposed in this report .