  Using the observed properties of our solar system, in particular the isotopic
compositions of meteorites and the regularity of the planetary orbits, we
constrain the star formation environment of the Sun within the scenario of
(external) radioactive enrichment by a massive star. This calculation yields a
probability distribution for the number of stars in the solar birth aggregate.
The Sun is most likely to have formed within a stellar group containing $N$ =
2000 $\pm$ 1100 members. The {\it a priori} probability of a star forming in
this type of environment is ${\cal P} \approx$ 0.0085, i.e., only about 1 out
of 120 solar systems are expected to form under similar conditions. We discuss
additional implications of this scenario, including possible effects from the
radiation fields provided by the putative cluster environment and dynamical
disruption of the Kuiper Belt. The constraints of this paper place tight
restrictions on the properties of the solar birth aggregate for the scenario of
external enrichment by a massive star; alternately, these tight constraints
slightly favor a self-enrichment scenario for the short-lived radioactive
species.
