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 = { \langle N \rangle } \approx 2000 \pm 1100 members .
The 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 .