Authors:Ryu et alAbstract:Most planetary events discovered up to date by the planetary caustic of close-separation planets have low-mass ratios. In next-generation microlensing experiments with a wider field of view and a higher cadence, it is possible to obtain densely covered planetary signals induced by the planetary caustic of close-separation planets without missing events. Therefore, the planetary caustic perturbation of close-separation planets would be the more important channel to detect low-mass exoplanets in the next generation of microlensing surveys. In this paper, we investigate the theoretical properties and detection conditions for the planetary caustic perturbation of close-separation planets. To find the properties of the planetary caustic perturbation, we construct deviation maps by subtracting the single-lensing magnification of the lens star from the planetary lensing magnification for various lensing parameters. We find that each deviation area of the positive and negative perturbations disappears at the same normalized source radius according to a given deviation threshold regardless of mass ratio but disappears at a different normalized source radius according to the separation. We also estimate the upper limit of the normalized source radius to detect the planetary caustic perturbation. We find simple relations between the upper limit of the normalized source radius and the lensing parameters. From the relations, we obtain an analytic condition for the theoretical detection limit of the planet, which shows that we can sufficiently discover a planet with a sub-Earth-mass for typical microlensing events. Therefore, we conclude that our planet-detection condition of can be used as an important criteria for maximal planet detections, considering the source type and the photometric accuracy and expect that a number of low-mass planets will be added from the next-generation microlensing experiments.