  The extreme outer Galaxy (EOG), the region with a Galactic radius of more
than 18 kpc, is known to have very low metallicity, about one-tenth that of the
solar neighborhood. We obtained deep near-infrared (NIR) images of two very
young ($\sim$0.5 Myr) star-forming clusters that are one of the most distant
embedded clusters in the EOG. We find that in both clusters the fraction of
stars with NIR excess, which originates from the circumstellar dust disk at
radii of $\leq$0.1 AU, is significantly lower than those in the solar
neighborhood. Our results suggest that most stars forming in the
low-metallicity environment experience disk dispersal at an earlier stage ($<$1
Myr) than those forming in the solar metallicity environment (as much as
$\sim$5--6 Myr). Such rapid disk dispersal may make the formation of planets
difficult, and the shorter disk lifetime with lower metallicity could
contribute to the strong metallicity dependence of the well-known
"planet-metallicity correlation", which states the probability of a star
hosting a planet increases steeply with stellar metallicity. The reason for the
rapid disk dispersal could be increase of the mass accretion rate and/or the
effective far-ultraviolet photoevaporation due to the low extinction; however,
another unknown mechanism for the EOG environment could be contributing
significantly.
