G=EMC^2 Glazier wrote:
What came first in creating a planet?

Current solar nebula model starts with observed abundances of H and He gas to
dust and volitiles (like water, ammonia, methane). As you shrink such a cloud
under gravity, the dust, a site for the accumulation of volitiles as ices on the
surface, begins to accumulate into larger and larger pieces, using the ices as a
"glue". Static electricity may also play a role. This would, over time, build
up larger and larger solid bodies within the ever spinning cloud of gas, which
itself is spreading out because of the rotation.

The determiner of final outcome is the forming Sun, the heat from which can
drive away volitiles and H and He, leaving just the solid stuff to continue
accumulating in close to the Sun. At greater distances, the temperature in the
solar nebula would have allowed the condensation not only of the solid stuff but
the volitiles too. Here, planet building could lead to larger bodies because
there would then be more stuff to build with.

Eventually, the gravity of these larger bodies would draw in more material,
including the gases. Those bodies close to the Sun could not accumulate as much
as that farther away because it had been driven away by the Sun. Farther out,
this gas material could be included in the planet-building process.

A more complete explanation can be found in most elementary, college-level
astronomy textbooks.