Theories on how the Milky Way formed are set to be rewritten following discoveries about the behavior of some of its oldest stars.

An investigation into the orbits of the Galaxy’s metal-poor stars—assumed to be among the most ancient in existence—has found that some of them travel in previously unpredicted patterns.

“Metal-poor stars—containing less than one-thousandth the amount of iron found in the Sun—are some of the rarest objects in the galaxy,” said Professor Gary Da Costa from Australia’s ARC Center of Excellence in All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and the Australian National University.