Koshimoto et al. (2014) present the detection of a massive gas giant planet from a gravitational microlensing event identified as OGLE-2008-BLG-355. This planet is estimated to have ~4.6 times the mass of Jupiter and it orbits a star whose mass is estimated to be ~0.37 times the mass of the Sun. The planet is at a projected separation of roughly 1.7 AU from its host star. Such a planetary system poses a challenge to the core-accretion model of planet formation which predicts that such a massive planet should only form around a more massive host star. Nevertheless, the mass of the host star is only weakly constrained, and it can range anywhere from 0.20 to 0.67 times the mass of the Sun. If the core-accretion model of planet formation stands true for this planetary system, future observations should show that the mass of the host star is closer to the upper part of the estimated range.
Similar to OGLE-2008-BLG-355, two previously gravitational microlensing events, OGLE-2003-BLG-235 and MOA-2011-BLG-293, involve planets much more massive than Jupiter in orbit around host stars that initially appeared too low in mass. However, subsequent observations found that the host stars of both planetary systems have masses closer to the upper limit of their initial predicted mass ranges, and they are therefore massive enough to support the formation of such massive planets around them via the core-accretion model of planet formation.
Reference:
Koshimoto et al. (2014), “OGLE-2008-BLG-355Lb: A Massive Planet around A Late type Star”, arXiv:1403.7005 [astro-ph.EP]
