The next phase in this evolution is to use space-based facilities to undertake measurements that cannot be done from the surface of Earth. The Space Interferometry Mission (SIM) and the Kepler photometric mission, both in the planning stages, represent efforts in this direction. This evolutionary path will lead to a steady increase in the extent of the SMO parameter space accessible for study. The confidence gained with the exciting results from each step helps to justify the greater complexity and cost of the next step.

This evolutionary process must be carefully fostered and nurtured. A balanced program that fully explores all of parameter space is essential. In particular, to advance SMO studies in the near term and to optimize the development of space-based facilities NASA must balance its major space expenditures with adequate funding for the early steps involving ground-based techniques and flight demonstrations. For example, a significant amount of effort needs to be delivered to ground-based, transit-photometry efforts to provide a dramatic demonstration of the viability of this technique. The detection of a new 51 Pegasi-like system by photometry would make a compelling case for future space-based photometry missions. Similarly, the vigorous exploitation of the capabilities of current ground-based interferometers will help researchers to improve the designs of much larger and more expensive future ground-and space-based interferometers.

Extensive astrometric surveys with facilities such as NPOI (Navy Prototype Optical Interferometer) PTI (Palomar Testbed Interferometer), and CHARA (Center for High Angular Resolution Astronomy) and others will also help train the next generation of scientists who will then design and fully utilize the much more expensive space missions such as SIM and Planet Finder.

Spectroscopic Studies of Nearby SMOs

The great difficulty in detecting SMOs is the result of their low luminosity. It is now clear, however, that the so-called stellar mass function (i.e., the number of objects in a given mass range) does not continue to rise through the bottom of the main sequence to the SMO domain. Rather, the mass function is flat or gently declining through the transition to SMOs, with perhaps a rise below 10 MJ. Thus, SMOs are at best as numerous as stars, and perhaps only half so abundant. As a result, SMOs cannot contribute more than 10% of the mass of the galactic neighborhood. Thus SMOs will always be elusive candidates for discovery and characterization regardless of the increased capability of telescopes and instruments, because they do not stand out in sheer numbers against the background of the low-mass stellar population. The diverse techniques of detection described in Chapter 1 will continue to provide cohorts of candidates whose true nature will be understood through more detailed observations with different tools.

The technique of choice for studying individual SMOs will continue to be spectra, the bulk of which will be collected from the ground. Continued improvement in optical and infrared detectors, the greater availability of large telescopes, and progress in developing adaptive-optics techniques (to separate the light of close companions from that of their parent star) are all required to probe the spectra of SMO candidates discovered to date and anticipated in the coming years.

These technologies, developed on ground-based systems, have direct application to the goal of acquiring spectra from 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. This will have the additional



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