Shape of protoplanets

[petrushka.googol]

When the first planets formed they all aggregated into similar shapes (approx spherical masses).?

Why? Even though speed of rotation differs greatly the net result was, and is always, approximately spherical. Also the earth is not a perfect sphere but an oblate spheroid. Why?

 

Please advise.

 

Thanks in advance.

[swansont]

To a good approximation the planets are spheres, and this is because gravity is a radial force. Without rotation, the equipotential surface is a sphere. With rotation, the equipotential surface gains the equatorial bulge and polar flattening.

 

On the earth, the bulge and flattening are each ~ 20 km, as compared to a radius of almost 6400 km. That, of course, would vary with the rotation speed.

[J.C.MacSwell]

In the absence of any shear forces, the shape will tend to spherical, or bulged more or less depending on rotation. This holds for any planet large enough where the shear forces overwhelm the strength of the material it is made of, though it is less so closer to the surface where the forces are reduced.

Edited January 24, 2014 by J.C.MacSwell