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Failed Stars and Super Planets: A Report Based on the January 1998 Workshop on Substellar-Mass Objects Executive Summary The first direct observation of a bona fide brown dwarf, Gliese 229B in 1995, coupled with the indirect detections of a number of objects of jovian mass or larger, has brought the study of substellar-mass objects (hereafter SMOs) into a new phase of observational investigations. This success, after many years of failed searches and false alarms, is a consequence of the advent of new detectors, refinement of long-standing observing techniques through the use of novel technologies and data-processing schemes, and the persistence of searchers. Observational successes have been mirrored by advances in the theoretical modeling of both the spectra and the structure and evolution of SMOs. Developments in theoretical understanding of SMOs have been enabled by more capable computers, new laboratory data on the properties of materials at high pressure, and the stimulus of discoveries of actual objects. These advances have coincided with, and reinforced, increasing public and NASA interest in the broader issue of how unique our own planetary system is, the likelihood of life elsewhere, and what is required to make discoveries that will answer these questions. Although the intellectual linkage between study of SMOs and the question of the frequency of planetary systems is a firm one, for various programmatic reasons the future investigation of SMOs with both ground-and space-based telescopes has not been well thought through to date. Past reports from the National Research Council's (NRC's) Committee on Planetary and Lunar Exploration (COMPLEX) and NASA advisory groups on the detection and study of extrasolar planets have concentrated primarily on the study of SMOs as the first step toward detecting Earthlike planets around other stars, the ultimate programmatic goal of NASA's new “Origins” initiative. Comparatively less attention has been given to the intrinsic value of studies of SMOs for answering high-priority questions in astronomy and planetary science, including those related to the physics of star and planet formation, the abundance of luminous and dark matter throughout the cosmos, and the basic physics of matter under extreme conditions. The recently demonstrated ability to observe and study SMOs is significant for reasons additional to and unconnected with extrasolar planets. The eventual determination of the abundance of low-luminosity, low-mass objects will place constraints on models of the nature of the dark matter on astrophysical scales ranging from the solar neighborhood through the cosmological both directly (in terms of the contribution of SMOs) and indirectly through their constraints on the stellar initial-mass function. The local SMO contribution is starting to be constrained by sensitive surveys of the Sun's galactic neighborhood to determine directly the abundance of such objects as free-floaters, companions, and cluster members. Attempts to determine the galactic and extragalactic mass contribution of SMOs by detecting their gravitational-lensing effect on background stars have been under way for several years and are beginning to yield constraints. THE WORKSHOP ON SUBSTELLAR-MASS OBJECTS Given the recent successes in discovering SMOs by direct and indirect means, and the shared interest in them by research communities with very different goals and perspectives, the
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Failed Stars and Super Planets: A Report Based on the January 1998 Workshop on Substellar-Mass Objects Space Studies Board organized a workshop to conduct a systematic cross-disciplinary examination of the state of the field. Its purpose was to assess the current state of the field and identify future studies that might contribute to important research goals in star and planet formation, the frequency of planetary systems, the nature of non-luminous matter on scales up through cosmological, the behavior of matter under extreme conditions, and the evolution of atmospheres of objects ranging in mass from planetary through stellar. The state of the field as summarized at the workshop by 21 invited experts is vigorous: substellar-mass objects are now being detected or characterized, on a regular basis, by roughly a half dozen different techniques, both ground- and space-based, with additional approaches nearing the maturity necessary to conduct successful searches. Much of the activity is a result of individual or small-team, principal-investigator-based, projects, rather than large-scale “mission-type” programs, although some of the discoveries have been made with instruments (e.g., the Hubble Space Telescope and Keck telescope) that are the result of large-scale public or private programs. Additional to the availability of large or space-based telescopes are the maturation and ready availability of sensitive detector systems. However, a substantial ingredient in the success of the searches is the invention of novel data-processing schemes (in turn enabled by high-speed and high-capacity computers), calibration techniques (such as the iodine cell utilized in the radial-velocity program), and the autocatalytic growth of observing networks linked by electronic mail and able to confirm transient events (e.g., microlensing networks). Powerful computers also have allowed modeling efforts to move from highly approximate schemes to capabilities more in line with the new data available. In particular, frequency-averaged or “gray” model atmospheres have given way to fully frequency-dependent models, handling tens of millions of spectral lines, essential both for synthesizing spectra to compare with data and for properly characterizing atmospheric energy balance. The discovery of a cohort of Jupiter-mass planets in close orbits around their parent stars has stimulated more elaborate hydrodynamical models of SMO formation, again enabled by high-speed computers. Because SMO interiors are under high pressure and (except for the most massive or youngest objects) moderately degenerate, the behavior of matter under extreme conditions is a crucial issue in understanding the formation and evolution of these bodies. Theoretical and experimental advances in high-pressure physics have led to improved characterization of the physical properties of SMOs. FINDINGS As a result of the presentations and discussions at the workshop and subsequent deliberations, the Steering Group for the Workshop on Substellar-Mass Objects formulated a number of findings about the current state of research related to SMOs. These findings are organized under headings related to the five questions posed in NASA 's request for an examination of pertinent issues (see preface). Status of Current Research Activities The study of SMOs is currently in a state of high vigor after several decades of false starts and frustrations. The key to the new successes lies in technological advancements in ground-based telescopes buttressed by results from key spacecraft programs and theoretical studies of growing power and fidelity. The challenge for NASA and other funding agencies is to foster these programs in such a way that they contribute to NASA's ultimate
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Failed Stars and Super Planets: A Report Based on the January 1998 Workshop on Substellar-Mass Objects programmatic goal of discovering terrestrial planets in orbit around other stars, but without encouraging a premature narrowing of focus toward a single, high-cost technique or mission. The Most Compelling Issues for Near-Term Study Investigations of SMOs are still in their infancy. The number of known brown dwarfs and extrasolar giant planets is still relatively small and provides an insufficient basis for drawing definitive conclusions about the range of properties exhibited by these objects. Detailed information on the properties of most SMOs is still lacking. Thus the most compelling issues to be addressed in the near term are the following: Devising detection strategies to increase the population of known SMOs beyond the several hundred expected from the Deep Infrared Survey of the Southern Sky (DENIS) and the 2-Micron All-Sky Survey (2MASS) and, thus, increase the extent of SMO parameter space accessible for study; and Performing spectroscopic and other diagnostic studies to characterize individual, nearby SMOs. In the first of these, the ground-based radial-velocity surveys that have moved to the new generation of large-aperture telescopes, coupled with the continued development of astrometry, will increase the sample size of SMOs by an order of magnitude. The next phase is to use space-based facilities to undertake measurements not possible from Earth. The Space Interferometry Mission (SIM) and the Kepler photometric mission both represent efforts in this direction. A balanced program that fully explores as much as possible of the the development of space-based facilities, NASA must balance its relevant parameter space is essential. In particular, to advance SMO studies in the near term and to optimize major space expenditures with adequate funding for the early steps involving ground-based techniques and flight demonstrations. For characterizing individual objects, the technique of choice will continue to be spectroscopic studies. The technologies needed to probe the spectra of SMO candidates have direct application to the goal of acquiring the spectra of terrestrial planets around nearby stars using space-based telescopes. However, the continued advancement of SMO studies requires that NASA encourage a range of approaches that will have broad scientific benefit for the detection and characterization of SMOs. Success in this effort will have the additional benefit of providing the potential for alternative and unexpected solutions to the problem of characterizing extrasolar terrestrial planets. Close-in orbit companions such as 51 Pegasi B may not be amenable to spectroscopic study in the foreseeable future. Multiple techniques, including astrometry and detection of broadband reflected light, will need to be applied to understand the nature of these objects. Contributions to Broader Scientific Goals SMOs are a bridge between stars and planets, in that the physics of their atmospheres and interiors represents a genuine transition from stellar physics to planetary physics. Moreover, the ability to model the structure, evolution, and appearance of SMOs represents an important test of basic physics in a little-explored range of parameter space. In addition to greatly
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Failed Stars and Super Planets: A Report Based on the January 1998 Workshop on Substellar-Mass Objects improving current understanding of the general theory of star and planet formation, studies of SMOs are likely to contribute to broader scientific goals in areas such as those represented by the following three compelling examples: Modeling the atmospheres and interiors of SMOs; Testing models of the formation of SMOs; and Understanding the stability and evolution of multiplanet systems. To advance these areas requires that: Laboratory and associated computational efforts be undertaken to construct accurate spectral-line lists; Parallel, vector, or superscalar processors become widely available so that theoretical models can take full advantage of current understanding of the physics of SMOs; Larger telescopes and more sensitive detectors be brought to bear on characterizing brown dwarfs; Experimental and theoretical studies of the behavior of materials at high pressure continue to be supported and to increase in capability; and A broad range of observational strategies for detection (i.e., astrometric, photometric, radial-velocity, and microlensing studies) and characterization (e.g., spectroscopic studies over a broad range of the electromagnetic spectrum) of SMOs be undertaken to ensure that the sample space of these objects is large enough to generalize the frequency and mechanisms of formation. Opportunities for Interdisciplinary Research The study of SMOs is necessarily interdisciplinary in nature, and progress in this field will require planetary scientists and astronomers to communicate and collaborate with each other as well as with colleagues from across a broad range of disciplines in the physical sciences. The Contribution of Studies of SMOs to Achieving Long-Term Scientific Priorities Studies of SMOs have direct relevance to a number of long-standing scientific goals and priorities, their most obvious role being to provide a testing ground for honing the instrumentation and observational techniques necessary to detect extrasolar terrestrial planets. Another key area is the contribution of SMO studies to constraining the identification of missing mass in the universe. Although it appears that SMOs do not constitute the bulk of the matter in the universe, they represent unique probes of galactic structure. Observations of microlensing events have particular promise for probing the mass function of brown dwarfs and understanding the composition of the galactic halo. With appropriate developments, microlensing could provide a shortcut to the detection of extrasolar terrestrial planets. To ensure continued progress in this area, NASA and other agencies should foster coordination and collaboration among various search programs to enable ongoing discoveries and to follow up on possible candidate events. Because microlensing groups have different primary goals, the various agencies supporting primary and follow-up microlensing observations should work together to minimize potential disruptions caused by differences in their prime goals.
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Failed Stars and Super Planets: A Report Based on the January 1998 Workshop on Substellar-Mass Objects Measurement of higher-order microlensing events is required to determine the sources of lens effects in some cases. The reflex motion resulting from Earth's orbit around the Sun, for example, causes the trajectory of the background star relative to the lensing object to deviate from a straight line. This parallax effect induces asymmetries in the light curve of microlensing events which should be of order 1% if the lenses lie in the halo, but negligibly small if the lenses are in the Large Magellanic Cloud. Thus, a search for parallax asymmetries as an adjunct to the microlensing program will yield additional important information on the nature of the objects creating the lensing events. CONCLUDING REMARKS The ultimate programmatic goal of NASA's Origins program—discovering another Earth—is a laudable one upon which no specific recommendation is laid. In addressing this goal, however, NASA should take the following actions: Continually assess the new information that studies of SMOs are providing on the formation, frequency, and characteristics of planetary systems, and invest judiciously in developing observational and theoretical techniques that will foster new discoveries. This investment should be in addition to the funding NASA is already providing for technological development of future large projects such as the Space Interferometry Mission and the Terrestrial Planet Finder. The funding must be flexible and peer-reviewed in recognition of the nature of the activities, which are distributed, principal-investigator-based projects to observe and model SMOs by using a variety of different approaches. The small-scale nature of these activities suggests that existing procedures (e.g., periodic peer review of proposals and resulting publications) will be adequate to identify and prioritize the approaches and techniques deserving of additional investment. Invest with care in select ground-based facilities, instrument, and computational programs that will significantly broaden the near-term opportunity for innovation in the identification and characterization of SMOs. Addressing the broader issue of the appropriate balance of support for ground-based programs among NASA, the National Science Foundation, and other appropriate agencies is beyond the scope of this report. This important topic is best addressed by the decadal survey committee in the context of the findings of the study on the federal funding of astronomical research currently being conducted by the NRC's Committee on Astronomy and Astrophysics. Consult with other agencies (e.g., the National Science Foundation) to avoid duplication and to open a broader set of opportunities for research and discovery through cooperative or collaborative funding. In sum, SMO research is at the heart of trying to understand the matter content of the universe, the ubiquity and properties of planetary systems, and the relationship (in both genesis and physical properties) between stars and objects not massive enough to ever become stars. By studying SMOs we extend our understanding of the cosmos from the ubiquitous macroscale of stars through to the planets and, hence, ever closer to the human realm.
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