Probing Planets in Extragalactic Galaxies Using Quasar Microlensing
  Previously, planets have been detected only in the Milky Way galaxy. Here, we
show that quasar microlensing provides a means to probe extragalactic planets
in the lens galaxy, by studying the microlensing properties of emission close
to the event horizon of the supermassive black hole of the background quasar,
using the current generation telescopes. We show that a population of unbound
planets between stars with masses ranging from Moon to Jupiter masses is needed
to explain the frequent Fek line energy shifts observed in the gravitationally
lensed quasar RXJ1131-1231 at a lens redshift of $z=0.295$ or 3.8 billion
light-years away. We constrain the planet mass fraction to be larger than
0.0001 of the halo mass, which is equivalent to 2,000 objects ranging from Moon
to Jupiter mass per main sequence star.
