core accretion model is probably the most
accepted model of planetary formation.
According to this theory, the planets
formed near their current orbits from material
found there. Material near the center of the
disk would be too hot for light elements to
condense, and solar wind would also sweep the
lighter elements out to greater distances, so
the outer planets should be lower in density
than the inner planets. As indicated in the
question, this is not the case in our solar
system; Neptune and Uranus are more dense than
Saturn. One of the more-accepted explanations
is that of Neptune and Uranus
formed in roughly the same region as Jupiter and
Saturn, but then migrated out as a result of
Planetary migration is counter to the accretion
model, but the discovery of planets which could
not have formed in their current locations, such
Hot Jupiters, indicates that movement of gas
giants is not implausible and may even be common.
In addition, their are many theories for the
formation of planets, with none being universally
The answer is that density is not the way
that the planets arranged themselves.
Exactly how the planets did arrange themselves is
still an area of active research. We now know of
many, many Jupiter-sized and larger gas giants
located much closer to their suns than even the
orbit of Mercury, let alone Earth. Clearly, the
configuration of our solar system with rocky
planets inward, gas giants in the middle, and icy
Kuiper objects like Pluto and Eris outward is not
the only way that nature can do it. We still
don't know just how (un-)unique our own solar
system is, though.
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