  We combine $Spitzer$ and ground-based KMTNet microlensing observations to
identify and precisely measure an Earth-mass ($1.43^{+0.45}_{-0.32} M_\oplus$)
planet OGLE-2016-BLG-1195Lb at $1.16^{+0.16}_{-0.13}$ AU orbiting a
$0.078^{+0.016}_{-0.012} M_\odot$ 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.
