  One of the primary surprises of exoplanet detections has been the discovery
of compact planetary systems, whereby numerous planets reside within ~0.5 AU of
the host star. Many of these kinds of systems have been discovered in recent
years, indicating that they are fairly common orbital architecture. Of
particular interest are those systems for which the host star is low-mass, thus
potentially enabling one or more of the planets to lie within the Habitable
Zone of the host star. One of the contributors to the habitability of the Earth
is the presence of a substantial moon whose tidal effects can stabilize axial
tilt variations and increase the rate of tidal pool formation. Here we explore
the constraints on the presence of moons for planets in compact systems based
on Hill radii and Roche limit considerations. We apply these constraints to the
TRAPPIST-1 system and demonstrate that most of the planets are very likely to
be worlds without moons.
