Formation of solar systems in star clusters
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Solar System Formation in Star Clusters: Key Processes and Environmental Effects
Birth Environment: Star Clusters and Their Influence
Most stars, including our Sun, are born within star clusters, which are dense groups of stars that can significantly influence the formation and evolution of planetary systems. Evidence suggests that the solar system likely formed in a cluster containing at least 1,000–2,500 stars, with a cluster radius of about 0.75 parsecs. This environment played a crucial role in shaping the solar system’s chemical composition, disk size, and orbital characteristics of its bodies Zwart2018Pfalzner2012Nimmo1986.
Chemical Enrichment: Supernovae and Stellar Winds
The early solar system was enriched with certain short-lived radionuclides, such as ^26Al and ^60Fe. The presence of ^26Al is likely due to material accreted from the wind of a massive Wolf-Rayet star within the same cluster, while ^60Fe was delivered by a nearby supernova. At least two supernovae were necessary: one to provide ^60Fe and another to preserve it in the protoplanetary disk. These enrichment events also contributed to heating the disk, melting dust grains, and encapsulating radionuclides in droplets Zwart2018Zwart2018.
Disk Truncation and Orbital Changes from Stellar Encounters
Close encounters with other stars in the cluster could truncate the protoplanetary disk, limiting its size to about 45 astronomical units (the Kuiper cliff). These encounters also contributed to the observed tilt of the solar system’s disk relative to the Sun’s equator. However, while cluster effects can alter the mean plane of the solar system by about 1°, they are not sufficient to explain the Sun’s full obliquity Zwart2018Batygin2020.
Cluster Type: Leaky Clusters vs. Starburst Clusters
There are two main types of clusters where solar systems can form: starburst clusters and leaky clusters (OB associations). Starburst clusters are very dense, making it unlikely for protoplanetary disks to survive long enough to form solar systems like ours. In contrast, leaky clusters are less dense, allowing disks to survive and planetary systems to develop with high circularity and stability Pfalzner2012Nimmo1986.
Planetary System Stability and Diversity
Most planetary systems in star clusters remain intact, but a subset can be strongly perturbed, especially in the first 50 million years. The survival rate of planets is higher in low-density clusters, with Jupiter often acting as a protective barrier for inner planets. In denser clusters, direct perturbations can disrupt even habitable-zone planets. The diversity of planetary systems observed today is partly due to the variety of cluster environments and the different histories of stellar encounters Dotti2019Cai2017.
Hierarchical and Dynamical Evolution of Clusters
Young clusters often show hierarchical structures, with subgroups merging or dispersing over time. These processes, along with filament dissolution, influence the spatial and kinematic properties of the forming planetary systems. Most clusters in the solar neighborhood dissolve quickly, with over 90% not surviving beyond 10 million years, which affects the long-term evolution of their planetary systems Pang2022Bonatto2011.
Conclusion
The formation of solar systems in star clusters is shaped by a combination of chemical enrichment from massive stars, gravitational encounters with neighboring stars, and the overall cluster environment. The solar system likely formed in a moderately sized, leaky cluster, where it experienced early enrichment and disk truncation but was able to maintain stability and develop its unique structure. These processes help explain both the specific features of our solar system and the diversity seen in exoplanetary systems across the galaxy Zwart2018Zwart2018Pfalzner2012+6 MORE.
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