The Planes of Satellite Galaxies Problem, Suggested Solutions, and Open
  Questions
  Satellite galaxies of the Milky Way and of the Andromeda galaxy have been
found to preferentially align in significantly flattened planes of satellite
galaxies, and available velocity measurements are indicative of a preference of
satellites in those structures to co-orbit. There is increasing evidence that
such kinematically correlated satellite planes are also present around more
distant hosts. Detailed comparisons show that similarly anisotropic phase-space
distributions of sub-halos are exceedingly rare in cosmological simulations
based on the $\Lambda$CDM paradigm. Analogs to the observed systems have
frequencies of $\leq 0.5$ per cent in such simulations. In contrast to other
small-scale problems, the satellite planes issue is not strongly affected by
baryonic processes because the distribution of sub-halos on scales of hundreds
of kpc is dominated by gravitational effects. This makes the satellite planes
one of the most serious small-scale problem for $\Lambda$CDM. This review
summarizes the observational evidence for planes of satellite galaxies in the
Local Group and beyond, and provides an overview of how they compare to
cosmological simulations. It also discusses scenarios which aim at explaining
the coherence of satellite positions and orbits, and why they all are currently
unable to satisfactorily resolve the issue.
