  Many aspects of planet formation are controlled by the amount of gas
remaining in the natal protoplanetary disk (PPDs). Infrared observations show
that PPDs undergo a transition stage at several Myr, during which gas densities
are reduced. Our Solar System would have experienced such a stage. However,
there is currently no data that provides insight into this crucial time in our
PPD's evolution. We show that the Isheyevo meteorite contains the first
definitive evidence for a transition disk stage in our Solar System. Isheyevo
belongs to a class of metal-rich meteorites whose components have been dated at
almost 5 Myr after the first solids in the Solar System, and exhibits unique
sedimentary layers that imply formation through gentle sedimentation. We show
that such layering can occur via gentle sweep-up of material found in the
impact plume resulting from the collision of two planetesimals. Such sweep-up
requires gas densities consistent with observed transition disks. As such,
Isheyevo presents the first evidence of our own transition disk and provides
new constraints on the evolution of our solar nebula.
