Detection probability of a low-mass planet for triple lens events: implication of properties of binary-lens superposition

In view of the assumption that any planetary system is likely to be composed of more than one planet, and that a multiple planet system with a large-mass planet has a greater chance of detailed follow-up observations, the multiple planet system may be an efficient way to search for sub-Jovian planets. We study the central region of the magnification pattern for the triple lens system composed of a star, a Jovian mass planet and a low-mass planet to answer the question of if the low-mass planet can be detected in high-magnification events. We compare the magnification pattern of the triple lens system with that of a best-fitted binary system composed of a star and a Jovian mass planet, and check the probability of detecting the low-mass secondary planet whose signature will be superposed on that of the primary Jovian mass planet. Detection probabilities of the low-mass planet in the triple lens system are quite similar to the probability of detecting such a low-mass planet in a binary system with a star and only a low-mass planet, which shows that the signature of a low-mass planet can be effectively detected even when it is concurrent with the signature of the more massive planet, implying that the binary superposition approximation works over a relatively broad range of planet mass ratio and separations, and the inaccuracies thereof do not significantly affect the detection probability of the lower-mass secondary planet. Since the signature of the Jovian mass planet will be larger and lasting longer, thereby warranting more intensive follow-up observations, the actual detection rate of the low-mass planet in a triple system with a Jovian mass can be significantly higher than that in a binary system with a low-mass planet only. We conclude that it may be worthwhile to develop an efficient algorithm to search for 'super-Earth' planets in the paradigm of the triple lens model for high-magnification microlensing events.