When you look up at the night sky, the stars appear fixed — but things are not as they appear. In fact, every star in our galaxy is moving. While it’s easy for astronomers to measure whether a star is moving toward or away from us, it’s much harder to measure a star’s motion in the plane of the sky, or side to side. This is because the stars are so very distant, it takes years for even the most minute change to become visible. It’s why the constellations have appeared essentially the same over time; but given enough time, they will eventually warp and change as the motion of the stars that make them up becomes apparent.

This “sideways” motion, called proper motion, has only ever been measured for stars in the Milky Way — until now. Recently, a group of astronomers combined data from the Hubble Space Telescope and the European Gaia mission to measure the proper motions of several stars in the Sculptor dwarf galaxy, a small, nearby satellite of the Milky Way. Their work, published yesterday in Nature Astronomy, now presents a possible challenge to the standard models of dark matter haloes believed to surround galaxies such as our own.

The Gaia mission measures the positions of stars very precisely. While most of these stars are in our Milky Way, its targets do include some stars in nearby galaxies, such as the Sculptor dwarf. The Hubble Space Telescope has also observed some of these same stars, measuring their positions 12 years ago. Davide Massari of the University of Groningen and colleagues at the Kapteyn Astronomical Institute were able to combine the Gaia and Hubble data — no easy feat, as the two measure position differently — to find that 15 stars could be accurately tracked between the two epochs.