OGLE-2018-BLG-1269Lb: A Jovian Planet with a Bright I = 16 Host

Abstract

We report the discovery of a planet in the microlensing event OGLE-2018-BLG-1269 with a planet–host mass ratio q ~ 6 × 10⁻⁴, i.e., 0.6 times smaller than the Jupiter/Sun mass ratio. Combined with the Gaia parallax and proper motion, a strong one-dimensional constraint on the microlens parallax vector allows us to significantly reduce the uncertainties of lens physical parameters. A Bayesian analysis that ignores any information about light from the host yields that the planet is a cold giant M₂ = (0.69)_(-0.22)^(+0.44) M_J orbiting a Sun-like star M₁ = 1.13_(-0.35)^(+0.72) M_⊙ at a distance of D_L = 2.56_(-0.62)^(+0.92) kpc. The projected planet–host separation is a_⟂ = 4.61_(-1.17)^(+1.70) au. Using Gaia astrometry, we show that the blended light lies ≾ 12 mas from the host and therefore must be either the host star or a stellar companion to the host. An isochrone analysis favors the former possibility at >99.6%. The host is therefore a subgiant. For host metallicities in the range of 0.0 ⩽ [Fe/H] ⩽ +0.3, the host and planet masses are then in the range of 1.16 ⩽ M_₁/M_⊙ ⩽ 1.38 and 0.74 ⩽ M_₂/M_J⩽ 0.89, respectively. Low host metallicities are excluded. The brightness and proximity of the lens make the event a strong candidate for spectroscopic follow-up both to test the microlensing solution and to further characterize the system.

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