The orbits of the least chemically enriched stars open a window on the formation of our Galaxy when it was still in its infancy .
The common picture is that these low-metallicity stars are distributed as an isotropic , pressure-supported component since these stars were either accreted from the early building blocks of the assembling Milky Way , or were later brought by the accretion of faint dwarf galaxies .
Combining the metallicities and radial velocities from the Pristine and LAMOST surveys and Gaia DR2 parallaxes and proper motions for an unprecedented large and unbiased sample of very metal-poor stars at { [ Fe / H ] } \leq - 2.5 we show that this picture is incomplete .
This sample shows strong statistical evidence ( at the 5.0 \sigma level ) of asymmetry in their kinematics , favouring prograde motion .
Moreover , we find that 31 \% of the stars that currently reside in the disk do not venture outside of the disk plane throughout their orbit .
The discovery of this population implies that a significant fraction of stars with iron abundances { [ Fe / H ] } \leq - 2.5 formed within or concurrently with the Milky Way disk and that the history of the disk was quiet enough to allow them to retain their disk-like orbital properties .