  Direct imaging surveys have found that long-period super-Jupiters are rare.
By contrast, recent modeling of the widespread gaps in protoplanetary disks
revealed by ALMA suggests an abundant population of smaller Neptune to
Jupiter-mass planets at large separations. The thermal emission from such
lower-mass planets is negligible at optical and near-infrared wavelengths,
leaving only their weak signals in reflected light. Planets do not scatter
enough light at these large orbital distances, but there is a natural way to
enhance their reflecting area. Each of the four giant planets in our solar
system hosts swarms of dozens of irregular satellites, gravitationally captured
planetesimals that fill their host planets' spheres of gravitational influence.
What we see of them today are the leftovers of an intense collisional
evolution. At early times, they would have generated bright circumplanetary
debris disks. We investigate the properties and detectability of such irregular
satellite disks (ISDs) following models for their collisional evolution from
Kennedy and Wyatt 2011. We find that ISD brightnesses peak in the $10-100$ AU
range probed by ALMA, and can render planets detectable over a wide range of
parameters with upcoming high-contrast instrumentation. We argue that future
instruments with wide fields of view could simultaneously characterize the
atmospheres of known close-in planets, and reveal the population of long-period
$\sim$ Neptune-Jupiter mass exoplanets inaccessible to other detection methods.
This provides a complementary and compelling science case that would elucidate
the early lives of planetary systems.
