Last updated on December 21, 2013 at 4:59 EST  A New Take On
Planetary Accretion  February 29, 2012  The prevailing model for
planetary accretion, also called fractal assembly, and dating
back as far as the 18th century, assumes that the Solar System´s
planets grew as small grains colliding chaotically, coalescing
into bigger ones, colliding yet more until they formed
planetesimals. The planetesimals then collided until they formed
planets as varied as the Earth and Jupiter.  The model assumes
that this occurred in an extremely hot (as high as 1,600 degrees
Celsius) environment for the inner Solar System, fostered by a
dusty, two-dimensional disk post-dating the Sun.  The basic
modern model, developed by Russian astronomer Victor Safronov,
and further developed by planetary scientist George Wetherill,
is called the Solar Nebular Disk Model and was made available in
English in the early 1970s. It has remained essentially the same
over the past 40 years.  But not everyone is convinced the model
is correct. How