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

We present observations of the microlensing event MACHO 98-BLG-35, which reached a peak mag- ni—cation factor of almost 80. These observations by the Microlensing Planet Search (MPS) and MOA collaborations place strong constraints on the possible planetary system of the lens star and show intriguing evidence for a low-mass planet with a mass fraction 4 ) 10~5 " v " 2 ) 10~4. A giant planet with v \ 10~3 is excluded from 95% of the region between 0.4 and 2.5 from the lens star, where is R E R E the Einstein ring radius of the lens. This exclusion region is more extensive than the generic ii lensing zone,ˇˇ which is 0.6¨1.6 For smaller mass planets, we can exclude 57% of the ii lensing zone ˇˇ for R E. v \ 10~4 and 14% of the lensing zone for v \ 10~5. The mass fraction v \ 10~5 corresponds to an Earth-mass planet for a lensing star of mass D0.3 A number of similar events will provide sta- M _ . tistically signi—cant 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 —nd that such a system is ruled out at the 90% con—dence level. A copy of the solar system with Jupiter replaced by a second Saturn-mass planet can be ruled out at 70% con—dence. Our low-mass planetary signal (few Earth masses to Neptune mass) is sig- ni—cant at the 4.5 p con—dence level. If this planetary interpretation is correct, the MACHO 98-BLG-35 lens system constitutes the —rst detection of a low-mass planet orbiting an ordinary star without gas giant planets.20 Subject headings: gravitational lensingplanetary systemsstars: low-mass, brown dwarfs