The concept of
the solar-system as a timeline, or sequence, and of the outer planets
as the gas shrouds gathered around the rock cores of former, contracted
Earths, suggests to me that these small, rocky and icy bodies of
the Kuiper Belt and The Oort Cloud are actually the fragmented remains
of the rock cores of Earths that went before the previous six.
The remains of the previous six are still visible to us, but the
remains of the rest now lie outside our field of vision, except
when their elliptical orbits bring them close to the sun and they
become briefly visible as comets.
These are the Invisible Earths.