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8 Recommendations RECOMMENDED PROGRAM In previous reports, COMPLEX and the Space Studies Board have set out strategies for exploration of the inner planets, outer planets, and primitive bodies within our solar system. These explorations, by spacecraft, remote sensing from Earth, and laboratory studies of meteorites, lunar samples, and interplanetary dust, are leading to enormous strides in our knowledge of the physical and chemical state of the solar system, and to a certain extent also of conditions and processes pertaining to its origin and early evolution. But in a broader sense, the depth of our understanding of solar system origin, the mechanisms that produced its regularities of composition and structure, and the degree to which these processes and conditions were unique to our system among the myriad of solar-type stars in the galaxy, have been fundamentally limited by access to only one solar system for detailed observational study. While it is clear from the discussions in Chapter 3 of this report that much theoretical work remains to be done, it Is equally clear that models of solar system origin have matured to the point where they make specific predictions about processes and configurations we would expect to observe in other stellar environments that are evolving toward our type of solar system (e.g., dissipative accretion disks, coplanar planetary orbits, and the like). At the same time, several of the observational techniques discussed 75
76 in Chapter 5 have also matured to a level where the detection and first- order characterization of extrasolar planetary systems appear to be within observational reach with current technology, or with feasible technological advances. This parallel development of the solar system data base, theoretical interpretation, and capabilities for observation beyond the solar system potentially marks the beginning of an era of exploration. Many funda- mental and unanswered questions now can be both meaningfully defined and obse~vadonally addressed, not only concerning just the existence of other planetary systems, but also the extent to which our type of plan- etary configuration is either rare or uniqueor, alternatively, a frequent result of natural evolution by similar processes from common initial stellar conditions. The profound scientific and intellectual significance of even partial answers to such questions is evident, and COMPLEX accordingly recommends that the NASA Office of Space Science and Applications (OSSA) initiate a formalprog~arn of znvesaganon of extrasolar planetary systems and materials as a logical and scientifically necessity extension of the exploration of our own solar system. This new and intrinsically interdisciplinary program must incorporate diverse elements of invesuga~on that have Align ally been camed out within the separate disciplines of planetary science, astrophysics, and astronomy. Its implementation will therefore require scientific and prey gammas coorduzanon across these traditional boundannes. The inherent scientific breadth and importance of this effort extend far beyond the simple "existence theorem" for extrasolar planets to include study of statistical distributions among star classes, of the structure and dynamical behavior of evolved planetary systems, and of ~rcumstellar ma- terials and processes that may precede or postdate planetary formation. Therefore COMPLEX firmer notes that full implementaiion of the search components of this program will necessarily involve eucaminanons of a st~zizs- hCal) significant number of candidate saws. These must be copied out by observations utilizing a variety of techniques and emending over periods of time sufficient to demonstrate, with reasonable probability, the presence or absence of precursor or evolved planetary systems, aru! to characterize such systems if they are found. SCIENTIFIC OBJECTIVES From the discussions in Chapters 5 and 6 of this report, it iS clear that there are multiple approaches and a wide range of observational techniques applicable to the recommended program. It is also evident that the mea- surement capabilities required to achieve an appropriate level of scientific return from the program generally exceed those of existing ground-based astronomical instruments and of orbital telescopes currently in operation
77 or under development. Because of the extreme observational challenge posed by the needed measurements, new astronomical instrumentation de- veloped specifically for study of extrasolar-system materials must include Earth-orbiting telescopes and ancillary analytic equipment. The measure- ment requirements imposed on these instruments are, however, comparable to those desired for many areas of stellar and galactic astronomy. Thus the technological imperatives of the program can best be addressed and implemented by coordination in instrumental design and operation, and by sharing of available resources and facilities, with other related disa- plines. It is important to note, however, that the search programs required for detection and detailed study of extrasolar planetary materials impose important constraints on the commitment and duration of observing time, and could be severely compromised if truncated by overassignment of other scientific tasks to crucial instruments or facilities. It is also possible that additional initiatives may be warranted, based on new and currently unforeseen advances in any of several technologies, which may lead to significant breakthroughs in our ability to achieve the goals stated in Chapter 2. Promising initiatives of this kind should be encouraged and supported through existing instrument~evelopment programs. Finale, the committee emphasizes the necessity of maintaining strong laboratory and theoretical programs, complementary to the observational enterprises, as essential elements of the overall program. Based on the review in Chapter 4 of the current status of research in investigation of extrasolar planetary materials, and the evaluation in Chap- ter 5 of the technological and observational initiatives needed to implement the recommended program, COMPLEX has developed the fo110wingprimmy scientific objectives that in the opinion of the committee are aaamable during the nest decade: 1. To search for evolved ex~asolar planets and planetary systems. This objective requires systematic observational planet searches that encompass the widest feasible domain of the planetary mass versus semimajor axis exploration space displayed in Figure 5.2. Specifically, to a. Initiate and carry forward an astrometric observational survey program, extending for at least a decade and designed to track the reflex motion of 100 or more stars in the solar neighborhood (r < 10 parsecs) with the precision specified in the Measurement Requirements section below; and b. 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, in a survey of the duration and extent specked for the astrometnc survey.
78 2. To charactenze precursors of planetary systems. This objective re- quires continuation and augmentation of present observational studies of known young stellar systems, and of the physical properties of circumstellar dust assemblages as precursors to and products of planetary systems, on a variety of spatial and spectral resolution scales. It also requires a survey of a statistically meaningful number of stars of varied masses and type to determine the frequencies and properties of other systems of this kind. 3. To ch~ractenze precursor planetary materials. This objective re- quires continued and refined investigations of links between interstellar- circumstellar dust and isotonically "exotic" grains in solar system materials such as primitive meteorites, interplanetary, dust, and comets. Important specific objectives of this effort include careful collection in the strato- sphere and on Earth-orbiting facilities and appropriate curation of rare interplanetary asteroidal-cometary dust particles (IDPs); laboratory identi- fication and analysis of presolar dust grains preserved in these meteoritic and IDP materials; and laboratory simulation of the formation and physical and chemical processing of interstellar grains in preplaneta~y and planet- forming environments. 4. To improve the capability of theoretical models and computer ex- periments. This is necessary to make specific predictions regarding the observational properties of planetary systems at all stages of their evolu- tion, and to further develop models to aid in the interpretation of existing data. In particular, it would help encourage development of improved nu- merical techniques such as hydrodynamical computation in two and three dimensions, coupled with radiative transfer and chemical modeling of such systems. MEASUREMENT REQUIREMENTS To types of astronomical observations are specifically encouraged in this repon: searches for evolved planets, and physical studies of the precursors and products of planetary systems. lithe committee has addition- ally emphasized the importance of laboratory analyses of extrasolar matter preseIved in solar system matenals, and of expedients to simulate the physical and chemical processing of dust grains in astronomical environ- ments. COMPLEX outlines below the set of measurement requirements that us its opinion are technolog cop Stainable within the new 10 yr and are needed to achieve the stated scientific objectives of the decada] strategy. 1. For the planet searches, the committee finds that the instrumental performances adopted in Chapter 5 are practical limits in two senses. First, ~ = 10-5 arcsec for astromet~y and ~ = 10 m s~i for Doppler spectroscopy appear to be technological limits imposed by current abilities to design, construct, calibrate, and maintain astronomical instrumentation. Second,
79 at these levels of sensitivity both techniques will record signals originating in stellar stochastic variab~litr turbulence, star spots, and the like that will introduce systematic noise effects interfering with interpretation of the data as they relate to stellar reflex motion. (It should be noted, however, that while such signals may be unwelcome "noise" for detection of reflex effects, they will contn~ute sensitively to knowledge about these kinds of stellar variability and in this sense will be valuable.) In the absence of better understanding of the relevant aspects of stellar activator, more sensitive measurements than those recommended below are thus not currently useful for planetary searches by these techniques. Therefore the committee recommends, for astrometric and radial velocity planet searches: a. That the design goal for relative astrometric accuracy be ~ = 10 microarcsec. If our solar system could be observed for the required times from 10 parsecs with this accuracy, the ellipse amplitude for Jupiter would be 50~, and Uranus and Neptune would generate reflex motions of amplitude & and 16~, respectively. It should be clearly understood, however, that a 10-yr astrometric observing program, while Lacking Jupiter for nearly a complete cycle, would follow Neptune and Uranus for only ~ to 10 percent of their orbital periods and would thus require longer periods of observation for unambiguous detection and period determination. b. That Doppler spectroscopic observations be camed out at a sensi- tivil~ of 10 m s~ ~ or better for the velocity of the orbital reflex Such accuracy would detect a Jupiter-mass object within the orbital radius of Venus around a solar-mass star, or at the orbital radius of Jupiter around a 0.1 Me star. Inclusion of Doppler measurements in the planet search strategy opens a unique region of the discovery space diagram, one sensitive lo relatively low-mass planets in close orbits (see Figure 5.23. c. That both the asymmetry and Doppler spectroscopy search pros grams be maintained connnuous~fior at led a decade. An extended period Of observation is of the utmost importance to a successful outcome. 2. For comprehensive physical studies of precursor systems and planetary products, COMPLEX fins that observations from Tracy to radio waYelengd~s are required on a Yanety of spatial and special scales rangyngirom moderate to very high resolution For example, searches for protostellar candidates may initially proceed at moderate spatial resolution (1 to 5 arcsec), but studies of the structure and dynamics of evolving protostars will require the highest spatial resolution we can muster (<0.1 arcsec). While many spectroscopic observations of dust in precursor systems can be conducted at spectral resolutions of A/~) ~ 102, dynamical and kinematic studies of col- lapsing objects and bipolar outflows, analyses of chemical abundances, and determinations of excitation conditions require very high spectral resolution (~//~A ~ 104 to 106~. Toinvestigatelinksbetween~terstellar-c~cumstellar
80 and solar system dust Mains, the committee urges refinement of both obser- vautional and analytical z~echniqu es. Infrared observations of novae and pre- main-sequence systems with subarcsecond spatial resolution and spectral resolutions of A/~A > 102 are needed to provide information on circum- stellar dust condensation and evaporation sequences that can be directly compared with condensation models and relevant solar system materials. Ultraviolet and infrared observations of comets require improvement to an extent suiliment to provide meaningful comparisons of the spectral signa- tures of cometary and extrasolar dust: at a minimum, this should include a sensitive search for the 2175-A feature, high-resolution spectroscopy of silicate features, and searches for all major infrared signatures seen In interstellar dust. 3. For laboratory studies of dust, COMPLEX recommends continued support and anugynenz~ation of current efforts to refine laboratory techniques and instrumentahon to the point where micron to submicron e~rasolar grains preserved in primitive sold system materials can be identified, isolated, and analyzed For micron-sized grains, chemical and isotopic requirements are determination of elemental compositions for major elements, including the low atomic number elements carbon and nitrogen; measurement of elements heamer than silicon at the 100-ppm level; isotopic analyses of carbon, hydrogen, oxygen, and nitrogen, when they are major elements, to an accuracy of 1 percent; and similar or better isotopic accuracies for several heavier elements, including magnesium, silicon, and calcium. Laboratory ultraviolet, optical, and infrared studies of such grains are also needed, with resolution and sensitive comparable to those obtained for observation of dust in astronomical environments. Me committee Firmer urges that active encouragement be given to laboratory simulation and ~eoreacal studies of the astronomical dust cycle, including the condensation and chemical nature of dust grains in c~rcumstellar environments; the formation, evolution, and chemical and physical properties of my-organic grain mantles; and grain and mantle processing in the interstellar medium and during infall into accretion disks and incorporation into subplaneta~y objects. STRATEGY AND IMPLEMENTATION The initial decadal science objectives of the recommended program, set out earlier in this chapter, specifically emphasize the new development or augmentation of observational, laboratory, and theoretical capabilities, and subsequent survey modes of astronomical search and study. This emphasis clearly defines a reconnaissance phase of exploration strategy. Requirements for measurement accuracy are necessarily severe because of the extreme observational and analytic challenges posed by the nature of the targets. In previously recommended strategies for the study of
81 our own planetary system, initial reconnaissance has aimed for rapid first- order characterization, not necessanly at high levels of precision It has been followed relatively quickly by a transition to more lengthy stages of detailed exploration and intensive study, implemented by specific spacecraft missions, designed to address well~efined questions, to known individual targets. For investigation of extrasolar planetary materials the duration of the reconnaissance phase is more difficult to assess, although it is likely to be long for planet searches in particular. We may anticipate that it will result, on several different time scales, in the definition of targe~in space, in the laboratory, and in theoretical development for more detailed exploration and intensive study. It is important, during this reconnaissance, to develop increasingly sensitive and sophisticated instruments and techniques for followers observational and laboratory studies. This would help prepare technologically to exploit these targets once they are identified. In this come=, COMPLEX offers the following recommendations and advice on exploration strategy and prog~ammo~zc implementation to the Office Of Space Science and Applications (OSSA): 1. The committee recommends development, fabrication, installation, and long-term support of an astrometac observanonal facility that meets the astrome~c measurement requirements specify above u' Measurement Requirements. 2. The committee recommends that planetary search prog~arns utilizing astrometry and Doppler spectroscopy at their current state-of-the-art sensitiv- itries, be established and supported for a minimum observational period of 10 yr following their inifimion. The committee further recommends that u: the interim, ongoing g~ound-based searches be continued at their present best accuracies, and that the potential for unprovement of these accuracies be investigated and implemented if technically and financially feasible. 3. The committee recognizes the profound importance of understand- ing the mechanisms of star formation in the galaxy for theories of the formation and evolution of our own planetary system. 1b develop such knowledge requires comprehensive investigation of possible precursors and products of extrasolar planetary systems. This in turn requires a broad range of coordinated observational, laboratory, and theoretical studies as enumerated above in the Scientific Objectives and Measurement Require- ments sections of this chapter. The committee therefore recommends that current studies in these areas be continued and ~gynented and that new studies be initiated as necessary to meet the scientific objectives of this strategy. 4. The technical challenges intrinsic to detailed astronomical obser- vation of evolving preplanetaIy systems and their precursor materials are
82 severe. They mandate use of the most sensitive existing observational fa- c~lities, improvement of current instrumentation, and development of more advanced telescopic imaging and spectroscopic detection systems. Utiliza- tion of the next generation of such observational system~notably HST, SIRTF, SOFIA, and large ground-based optical and radio telescopes and ar- rays (and other facilities under development or planned by NASA, NSF, and other U.S. agencies, and by other nations) will be critical for meeting the scientific objectives of the recommended program in this area. COMPLEX therefore encourages the following multidisciplinary acavmes between the re- sponsible divisions at OSSA: participation of planetary scientists in the design and building affuture observ~ones and facility instruments and in the alloca- tion of obse~g time at erasing observational facilities; joint support for mulii- discipli~ary scientific initiatives; and joint development of instrumentanon for e.=rasolar observation. In consideration of the highly interdisciplinary nature of the recommended program and the anticipated costs of implementing it, thecommitteefilrtheradvisesthatrelevantfilrldingagencies,includ~gseparate programmatic divisions withut NASA, should joint) support the cross-cumng technological developments and multidisciplinary scientific initiatives required for these extrasolar observations and studies 5. The committee recommends pursuit of lon~range instrumental and strategic initiatives that are conceptually applicable and poteniia11y valuable to the investigation of e~rasolar planetary materials u' later stages of reconnais- sance or in subsequent phases of eacploranon 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