On Planetary Companions to the MACHO-98-BLG-35 Microlens Star

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Abstract
We present observations of microlensing event MACHO-98-BLG-35 which reached a peak magnification factor of almost 80. These observations by the Microlensing Planet Search (MPS) and the MOA Collaborations place strong constraints on the possible planetary system of the lens star and show intriguing evidence of a low mass planet with a mass fraction 4*10^{-5} < \epsilon < 2*10^{-4}. A giant planet with \epsilon = 10^{-3} is excluded from 95% of the region between 0.4 and 2.5 R_E from the lens star, where R_E is the Einstein ring radius of the lens. This exclusion region is more extensive than the generic "lensing zone" which is 0.6 - 1.6 R_E. For smaller mass planets, we can exclude 57% of the "lensing zone" for \epsilon = 10^{-4} and 14% of the lensing zone for \epsilon = 10^{-5}. The mass fraction \epsilon = 10^{-5} corresponds to an Earth mass planet for a lensing star of mass ~0.3 M_{sun}. A number of similar events will provide statistically significant constraints on the prevalence of Earth mass planets. In order to put our limits in more familiar terms, we have compared our results to those expected for a Solar System clone averaging over possible lens system distances and orientations. We find that such a system is ruled out at the 90% confidence level. A copy of the Solar System with Jupiter replaced by a second Saturn mass planet can be ruled out at 70% confidence. Our low mass planetary signal is significant at the 4.5\sigma confidence level, and if this planetary interpretation is correct, then the MACHO-98-BLG-35 lens system constitutes the first detection of a low mass planet orbiting an ordinary star without gas giant planets.

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