On the Detection of Exomoons Transiting Isolated Planetary-Mass Objects
  All-sky imaging surveys have identified several dozen isolated planetary-mass
objects (IPMOs), far away from any star. Here, we examine the prospects for
detecting transiting moons around these objects. We expect transiting moons to
be common, occurring around 10-15% of IPMOs, given that close-orbiting moons
have a high geometric transit probability and are expected to be a common
outcome of giant planet formation. IPMOs offer an advantage over other directly
imaged planets in that high-contrast imaging is not necessary to detect the
photometric transit signal. For at least 30 (>50%) of the currently known
IPMOs, observations of a single transit with the James Webb Space Telescope
would have low enough forecasted noise levels to allow for the detection of an
Io-like or Titan-like moon. Intrinsic variability of the IPMOs will be an
obstacle. Using archival time-series photometry of IPMOs with the Spitzer Space
Telescope as a proof-of-concept, we found evidence for a fading event of 2MASS
J1119-1137 AB that might have been caused by intrinsic variability, but is also
consistent with a single transit of a habitable-zone 1.7$R_\oplus$ exomoon.
Although the interpretation of this particular event is inconclusive, the
characteristics of the data and the candidate signal suggest that Earth-sized
habitable-zone exomoons around IPMOs are detectable with existing
instrumentation.
