Planetesimals Are Circularized Into Planetary Orbits Because Of Friction And Viscosity In The Gas And Dust In The Gravity Settled Midplane Of The Protoplanetary Disk.

Collisions And Gravitational Interactions Can Uncircularize Orbits.

Elliptical Orbits Are More Common Because Circularization Depends On The Planetesimals' Starting Positions, Directions And Speeds.

A particle falling from above or below the plane of rotation only has to overcome pressure, whereas one falling along the plane of rotation must resist both pressure and centrifugal force.









Planet Formation: Disk Formation and Evolution