Astrometric detection of terrestrial planets in the habitable zones of nearby stars with SIM PlanetQuest

SIM PlanetQuest (formerly the Space Interferometry Mission) is a space-borne Michelson interferometer for precision stellar astrometry, with a 9 m baseline, currently slated for launch in 2016. One of the principal science goals is the astrometric detection and orbital characterization of terrestrial planets in the habitable zones of nearby stars. Differential astrometry of the target star against a set of reference stars lying within 1 degrees will allow measurement of the target star's reflex motion with astrometric accuracy of 1 mu as in a single measurement. The purpose of the present paper is to quantitatively assess SIM's capability for detection (as opposed to characterization by orbital determination) of terrestrial planets in the habitable zones of nearby stars. Note that the orbital periods of these planets are generally shorter than the 5 year SIM mission. We formulate a joint periodogram as a tool for planet detection from astrometric data. For adequately sampled orbits (i.e., five or more observations per period over a sampling time span longer than the orbital period), we find that the joint periodogram is more sensitive than the chi(2) test for the null hypothesis. In our analysis 2 of the problem, we use Monte Carlo simulations of orbit detection, together with realistic observing scenarios, actual target and reference star lists, realistic estimates of SIM instrument performance, and plausible distributions of planetary system parameters.