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Planetary systems in the foreground of the Galactic Bulge form a class of (planetary) binary lens systems that can be detected through photometric microlensing experiments. An Earth mass planet orbiting a lensing star can perturb the would-be single lens microlensing light curve due to the lensing star, briefly but spectacularly. Gravitational microlensing is the only known ground-based method to probe earth mass planets orbiting around main sequence stars. (Earth mass objects orbiting a neutron star were discovered in 1992 by Wolszczan and Frail.)

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The Galactic Bulge (Dec: -30°) can be best observed from Southern Hemisphere observatories in South America, Australia, and South Africa (Lat: -30°) or even in Antarctica. Planetary binary lensing is rare and the planetary signals are fairly brief -- with the lower mass planets generating rarer and the shorter planetary signals. Thus, "microlensing planet patroling" requires continuous coverage of microlensing alert events, and the three continents are more or less optimally distributed for this purpose. The South Pole offers the possibility of continuous monitoring during the Southern winter (when weather permits). (The top of this map has the same longitude as this web server, 273.8°.)