An Earth-mass Planet in a 1 au Orbit around an Ultracool Dwarf

We combine Spitzer and ground-based Korea Microlensing Telescope Network microlensing observations to identify and precisely measure an Earth-mass (1.43^(+0.45)_(-0.32) M⊕) planet OGLE-2016-BLG-1195Lb at 1.16^(+0.16)_(-0.13) au orbiting a 0.078^(+0.016)_(-0.012) M⊙ ultracool dwarf. This is the lowest-mass microlensing planet to date. At 3.91^(+0.42)_(-0.46) kpc, it is the third consecutive case among the Spitzer "Galactic distribution" planets toward the Galactic bulge that lies in the Galactic disk as opposed to the bulge itself, hinting at a skewed distribution of planets. Together with previous microlensing discoveries, the seven Earth-size planets orbiting the ultracool dwarf TRAPPIST-1, and the detection of disks around young brown dwarfs, OGLE-2016-BLG-1195Lb suggests that such planets might be common around ultracool dwarfs. It therefore sheds light on the formation of both ultracool dwarfs and planetary systems at the limit of low-mass protoplanetary disks.