The Milky Way contains more than one hundred billion stars. Most are in a disc with a dense and bulky center, in the middle of which there is a supermassive black hole, while the rest is distributed in a much larger spherical halo. The stars spin around our galaxy at hundreds of kilometers per second, and their movements contain a large amount of information about their past. The fastest class of stars that can be found there is called hypervelocity stars, which are believed to begin their life near the galactic center and then launch themselves towards the edge of the Milky Way through interactions with the black hole.

In April, ESA’s top surveyor Gaia produced an unprecedented catalog of more than a billion stars, in which astronomers from around the world have worked over the past few months, scrutinizing the properties and movements of stars in our galaxy and beyond with precision never before reached. During this work, a team of astronomers using the latest data set of ESA’s Gaia mission to search for high-speed stars that were ejected from the Milky Way were surprised to find stars heading inward, perhaps from another galaxy.

The scientists, some of the University of Leiden (Holland), were surprised. Out of 1,300 million stars, Gaia measured the positions, the parallaxes, an indicator of their distance, and the 2D movements in the plane of the sky. In seven million of the brightest, he also measured how quickly they approach or move away from us. “Of the seven million Gaia stars with full 3D speed measurements, we found twenty that could travel fast enough to escape the Milky Way,” explains Elena Maria Rossi, one of the authors of the new study.