The impact of Gaia on our understanding of the Vast Polar Structure of
  the Milky Way
  The Milky Way (MW) is surrounded by a Vast Polar Structure (VPOS) of
satellite galaxies, star clusters, and streams. Proper motion measurements for
the brightest MW satellites indicate that they are predominantly co-orbiting
along the VPOS. This is consistent with a dynamically stable structure.
Assuming that all satellites that are aligned with the VPOS also co-orbit along
this structure allows to empirically predict their systemic proper motions.
Testing predictions for individual satellite galaxies at large distances
requires high-accuracy proper motion measurements such as with the Hubble Space
Telescope. However, for nearby MW satellites, Gaia will allow to test these
predictions, in particular for a statistical sample of satellites since proper
motion predictions exist for almost all of them. This will clarify how
rotationally supported the VPOS is. In addition, Gaia will discover Galactic
substructure, in particular stellar streams. The degree of their alignment with
the VPOS might further constrain its stability and nature.
