What mechanism within the gas-and-dust disk tends to circularize planetary orbits?

Answer

Friction and drag causing damping of highly eccentric paths

The nebular hypothesis predicts that the orbital characteristics of the planets—prograde, co-planar, and nearly circular—are inherent results of their formation process within the protoplanetary disk. Specifically regarding the shape of the orbits (eccentricity), accretion within a gaseous and dusty environment provides a mechanism for smoothing out irregular paths. As the small planetesimals interact gravitationally with the surrounding nebula, they experience drag and friction. This persistent, gentle interaction acts to damp down any highly elongated or eccentric orbital paths, gradually pulling them toward more stable, nearly perfect circular shapes. This contrasts sharply with chaotic encounter theories, which would favor highly elliptical orbits.

What mechanism within the gas-and-dust disk tends to circularize planetary orbits?

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