Abstract
We used Keck adaptive optics observations to identify the first planet
discovered by microlensing to lie in or near the habitable zone, i.e., at
projected separation $r_\perp=1.10.1\,$AU from its $M_L=0.86\pm
0.06\,M_ødot$ host, being the highest microlensing mass definitely identified.
The planet has a mass $m_p = 4.80.3\,M_Jup$, and could in principle
have habitable moons. This is also the first planet to be identified as being
in the Galactic bulge with good confidence: $D_L=7.70.44$ kpc. The
planet/host masses and distance were previously not known, but only estimated
using Bayesian priors based on a Galactic model (Yee et al. 2012). These
estimates had suggested that the planet might be a super-Jupiter orbiting an M
dwarf, a very rare class of planets. We obtained high-resolution $JHK$ images
using Keck adaptive optics to detect the lens and so test this hypothesis. We
clearly detect light from a G dwarf at the position of the event, and exclude
all interpretations other than that this is the lens with high confidence
(95%), using a new astrometric technique. The calibrated magnitude of the
planet host star is $H_L=19.160.13$. We infer the following probabilities
for the three possible orbital configurations of the gas giant planet: 53% to
be in the habitable zone, 35% to be near the habitable zone, and 12% to be
beyond the snow line, depending on the atmospherical conditions.
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