  The Sun likely formed as part of a group of stars. A close stellar flyby by
one of the solar siblings is probably responsible for the sharp outer edge in
the solar system`s mass distribution. The frequency of such close flybys can be
used to determine the likely type of birth environment of the solar system.
Young stellar groups develop very quickly, expanding significantly within just
a few Myr. Here we model this strong dynamical development of young stellar
groups and determine the resulting close flyby history. We find that solar
system equivalents are predominantly produced in areas with stellar densities
in the range 5 $\times$ 10$^4$ pc$^{-3}<$ n$_{local} <$ 2 $\times$
10$^5$pc$^{-3}$. Remarkably we find that only two very distinct types of
stellar groups can be considered as serious contestants as the cradle of the
Sun -- high-mass, extended associations ($M_c >$ 20 000 $M_{sun}$) and
intermediate-mass mass, compact clusters ($M_c <$ 3000 $M_{sun}$). Current day
counterparts would be the association NGC 2244 and the M44 cluster,
respectively. In these two types of stellar groups, close flybys take place at
a sufficiently high rate, while not being too destructive either. A final
decision between these two remaining options will require incorporation of
constraints from cosmo-chemical studies.
