Title: The nature of the giant exomoon candidate Kepler-1625 b-i,
Abstract: The recent announcement of a Neptune-sized exomoon candidate around the
transiting Jupiter-sized object Kepler-1625 b could indicate the presence of a
hitherto unknown kind of gas giant moons, if confirmed. Three transits have
been observed, allowing radius estimates of both objects. Here we investigate
possible mass regimes of the transiting system that could produce the observed
signatures and study them in the context of moon formation in the solar system,
i.e. via impacts, capture, or in-situ accretion. The radius of Kepler-1625 b
suggests it could be anything from a gas giant planet somewhat more massive
than Saturn (0.4 M_Jup) to a brown dwarf (BD) (up to 75 M_Jup) or even a
very-low-mass star (VLMS) (112 M_Jup ~ 0.11 M_sun). The proposed companion
would certainly have a planetary mass. Possible extreme scenarios range from a
highly inflated Earth-mass gas satellite to an atmosphere-free water-rock
companion of about 180 M_Ear. Furthermore, the planet-moon dynamics during the
transits suggest a total system mass of 17.6_{-12.6}^{+19.2} M_Jup. A
Neptune-mass exomoon around a giant planet or low-mass BD would not be
compatible with the common mass scaling relation of the solar system moons
about gas giants. The case of a mini-Neptune around a high-mass BD or a VLMS,
however, would be located in a similar region of the satellite-to-host mass
ratio diagram as Proxima b, the TRAPPIST-1 system, and LHS 1140 b. The capture
of a Neptune-mass object around a 10 M_Jup planet during a close binary
encounter is possible in principle. The ejected object, however, would have had
to be a super-Earth object, raising further questions of how such a system
could have formed. In summary, this exomoon candidate is barely compatible with
established moon formation theories. If it can be validated as orbiting a
super-Jovian planet, then it would pose an exquisite riddle for formation
theorists to solve.