  The recent announcement of a Neptune-sized exomoon candidate orbiting the
Jupiter-sized object Kepler-1625b has forced us to rethink our assumptions
regarding both exomoons and their host exoplanets. In this paper I describe
calculations of the habitable zone for Earthlike exomoons in orbit of
Kepler-1625b under a variety of assumptions. I find that the candidate exomoon,
Kepler-1625b-i, does not currently reside within the exomoon habitable zone,
but may have done so when Kepler-1625 occupied the main sequence. If it were to
possess its own moon (a "moon-moon") that was Earthlike, this could potentially
have been a habitable world. If other exomoons orbit Kepler-1625b, then there
are a range of possible semimajor axes/eccentricities that would permit a
habitable surface during the main sequence phase, while remaining dynamically
stable under the perturbations of Kepler-1625b-i. This is however contingent on
effective atmospheric CO$_2$ regulation.
