  We report the discovery of a massive planet OGLE-2008-BLG-355Lb. The light
curve analysis indicates a planet:host mass ratio of q = 0.0118 +/- 0.0006 at a
separation of 0.877 +/- 0.010 Einstein radii. We do not measure a significant
microlensing parallax signal and do not have high angular resolution images
that could detect the planetary host star. Therefore, we do not have a direct
measurement of the host star mass. A Bayesian analysis, assuming that all host
stars have equal probability to host a planet with the measured mass ratio
implies a host star mass of M_h = 0.37_{-0.17}^{+0.30} M_Sun and a companion of
mass M_P = 4.6^{+3.7}_{-2.2} M_Jup, at a projected separation of r_proj =
1.70^{+0.29}_{-0.30} AU. The implied distance to the planetary system is D_L =
6.8 +/- 1.1 kpc. A planetary system with the properties preferred by the
Bayesian analysis would be a challenge to the core-accretion model of planet
formation, as the core-accretion model predicts that massive planets are far
more likely to form around more massive host stars. This core accretion model
prediction is not consistent with our Bayesian prior of an equal probability of
host stars of all masses to host a planet with the measured mass ratio. So, if
the core accretion model prediction is right, we should expect that follow-up
high angular resolution observations will detect a host star with a mass in the
upper part of the range allowed by the Bayesian analysis. That is, the host
would probably be a K or G dwarf.

Earth and Planetary Astrophysics