  In the search for moons around extrasolar planets, astronomers are confronted
with a stunning observation. Although 3400 of the 4500 exoplanets were
discovered with the transit method and although there are well over 25 times as
many moons than planets known in the Solar System (two of which are larger than
Mercury), no exomoon has been discovered. In the search for exoplanet transits,
stellar light curves are usually phase-folded over a range of trial epochs and
periods. This approach, however, is not applicable in a straightforward manner
to exomoons. Planet-moon transits either have to be modeled in great detail
(including their orbital dynamics, mutual eclipses etc.), which is
computationally expensive, or key simplifications have to be assumed. One such
simplification is to search for moon transits outside of the planetary
transits. The question we address in this report is how much in-transit data of
an exomoon remains uncontaminated by the near-simultaneous transits of its host
planet. We develop an analytical framework and test our results with a
numerical planet-moon transit simulator. For exomoons similar to the Galilean
moons, only a small fraction of their in-transit data is uncontaminated by
planetary transits: 14% for Io, 20% for Europa, 42% for Ganymede, and 73% for
Callisto. The S/N of an out-of-planetary-transit folding technique is reduced
compared to a full transit model to about 38% (Io), 45% (Europa), 65%
(Ganymede), and 85% (Callisto), respectively. For the Earth's Moon, we find an
uncontaminated data fraction of typically just 18% and a resulting S/N
reduction to 42%. These values are astonishingly small. The gain in speed for
any exomoon transit search algorithm that ignores the planetary in-transit data
comes at the heavy price of losing a substantial fraction of what is supposedly
a tiny signal in the first place.
