Context :

Aims : We report the discovery of a planet with a high planet-to-star mass ratio in the microlensing event MOA-2009-BLG-387 , which exhibited pronounced deviations over a 12-day interval , one of the longest for any planetary event .
The host is an M dwarf , with a mass in the range 0.07 M _ { \odot } < M _ { host } < 0.49 M _ { \odot } at 90 % confidence .
The planet-star mass ratio q = 0.0132 \pm 0.003 has been measured extremely well , so at the best-estimated host mass , the planet mass is m _ { p } = 2.6 Jupiter masses for the median host mass , M = 0.19 M _ { \odot } .

Methods : The host mass is determined from two “ higher order ” microlensing parameters .
One of these , the angular Einstein radius \theta _ { E } = 0.31 \pm 0.03 mas has been accurately measured , but the other ( the microlens parallax \pi _ { E } , which is due to the Earth ’ s orbital motion ) is highly degenerate with the orbital motion of the planet .
We statistically resolve the degeneracy between Earth and planet orbital effects by imposing priors from a Galactic model that specifies the positions and velocities of lenses and sources and a Kepler model of orbits .

Results : The 90 % confidence intervals for the distance , semi-major axis , and period of the planet are 3.5 { kpc } < D _ { L } < 7.9 { kpc } , 1.1 { AU } < a < 2.7 { AU } , and 3.8 { yr } < P < 7.6 { yr } , respectively .

Conclusions :