Double trouble: Gaia reveals (proto)-planetary systems that may
  experience more than one dense star-forming environment
  Planetary systems appear to form contemporaneously around young stars within
young star-forming regions. Within these environments, the chances of survival,
as well as the long-term evolution of these systems, are influenced by factors
such as dynamical interactions with other stars and photoevaporation from
massive stars. These interactions can also cause young stars to be ejected from
their birth regions and become runaways. We present examples of such runaway
stars in the vicinity of the Orion Nebula Cluster (ONC) found in Gaia DR2 data
that have retained their discs during the ejection process. Once set on their
path, these runaways usually do not encounter any other dense regions that
could endanger the survival of their discs or young planetary systems. However,
we show that it is possible for star-disc systems, presumably ejected from one
dense star-forming region, to encounter a second dense region, in our case the
ONC. While the interactions of the ejected star-disc systems in the second
region are unlikely to be the same as in their birth region, a second encounter
will increase the risk to the disc or planetary system from malign external
effects.
