We report the gravitational microlensing discovery of a sub-Saturn mass planet , MOA-2009-BLG-319Lb , orbiting a K or M-dwarf star in the inner Galactic disk or Galactic bulge .
The high cadence observations of the MOA-II survey discovered this microlensing event and enabled its identification as a high magnification event approximately 24 hours prior to peak magnification .
As a result , the planetary signal at the peak of this light curve was observed by 20 different telescopes , which is the largest number of telescopes to contribute to a planetary discovery to date .
The microlensing model for this event indicates a planet-star mass ratio of q = ( 3.95 \pm 0.02 ) \times 10 ^ { -4 } and a separation of d = 0.97537 \pm 0.00007 in units of the Einstein radius .
A Bayesian analysis based on the measured Einstein radius crossing time , t _ { E } , and angular Einstein radius , \theta _ { E } , along with a standard Galactic model indicates a host star mass of M _ { L } = 0.38 _ { -0.18 } ^ { +0.34 } M _ { \odot } and a planet mass of M _ { p } = 50 _ { -24 } ^ { +44 } M _ { \oplus } , which is half the mass of Saturn .
This analysis also yields a planet-star three-dimensional separation of a = 2.4 _ { -0.6 } ^ { +1.2 } AU and a distance to the planetary system of D _ { L } = 6.1 _ { -1.2 } ^ { +1.1 } kpc .
This separation is \sim 2 times the distance of the snow line , a separation similar to most of the other planets discovered by microlensing .