About half of the objects observed are expected to be multiple star systems. Doppler spectroscopy observations have already shown the presence of planets orbiting individual stars in multiple star systems (Cochran et al., 1997). Further, protoplanetary disks exist in binary systems such as HR4796A (Jayawardhana et al 1998, and Koerner, et al., 1998). Numerical integrations have shown that there is a range of orbital radii (between about 1/3 and 3.5 times the stellar separation) for which stable orbits are not possible (Wiegert and Holman, 1997; Holman and Wiegert, 1999). Based on observations (Heacox and Gathright 1994, figure below), about 23% will not have stable orbits between 0.4 and 2 AU. We expect to be able to determine the range of binary separations for which planetary orbits do exist.

	Distribution of Binary Star Separations

From the target list of approximately 150,000 dwarf stars, the number of each spectral type for which a planet of a given size can be detected is shown below (assuming a single near-grazing 6.5 hour transit with an SNR=4).

A model of the Galaxy for the selected FOV was developed using the luminosity function of Wielen, Jahreiss and Kruger (1983) to obtain the stellar distribution. The galactic model is the same used by Bahcall and Soneira (Bahcall, 1986). It defines the number of stars per pc³ per magnitude. This model was normalized to the star density in the FOV from the USNO-A1.0 data base (Monet,1996) to provide the number of stars per magnitude interval, spectral type and luminosity class. The model was cross-checked with the spectral distribution of all stars with |b|<10º in the catalog of Positions and Proper Motions (Röser and Bastion, 1988); against the distribution of dwarfs and giants of the Bahcall and Soneira model; and against the number of M-dwarfs in the Catalog of Nearby Stars (Gliese and Jahreiss, 1991).

Of the 223,000 stars in the FOV with mv<14, an estimated 61% or 136,000 are dwarfs. In the first year of operation about 25% of these are identified and excluded as being too young, rotating too fast, or too variable to be useful, resulting in 100,000 usable target stars.

Based on the model of stellar distribution and dependence of detectable planet size on stellar type and brightness, the number and type of stars monitored as a function of planet size is shown in the figure below.

	Number of Dwarf Stars for Which Planets Can Be Detected.

The solid lines show the number of dwarf stars of each spectral type for which a planet of a given radius can be detected at >8 sigma. The conservative numbers are based on 4 near-grazing transits with a 1 year period and stars with m_v<14.

The symbols along each solid line indicate the approximate apparent magnitude of the stars contributing to the integral number of stars.

The dashed lines show a significant increase in the number of stars (a factor of 2 at R=1.0 R_e) when assuming 4 near-central transits with a 1-year period. An even greater increase is realized for 8 near-grazing transits with a 0.5-year period.

We define Earth-size to be between 0.5 and 2.0 Earth masses (0.8 R_e to 1.25 R_e) and large terrestrial planets to be between 2 to 10 Earth masses (1.25 R_e to 2.0 R_e). Planets less than about 0.5 M_e that reside in or near the HZ are likely to lose their life-supporting atmospheres because of their low gravity and lack of plate tectonics.

Planets of more than about 10 M_e (R>2.0 R_e) are likely to attract a hydrogen-helium atmosphere and become gas giants like Jupiter and Saturn.