On April 25, Teresa Antoja of the University of Barcelona was one of thousands of astronomers who downloaded and began exploring an exquisite new map of the Milky Way made by the European Space Agency’s Gaia spacecraft. Within a day, she and her colleagues reported the discovery of never-before-seen substructures throughout the galaxy: “shapes such as arches ... snail shells and ridges,” they wrote—each one a clue about the Milky Way’s obscure past.
Antoja’s paper is one of a torrent following the long-awaited second data release from Gaia, which was launched in 2013 and has since charted the positions, brightnesses, and colors of 1.7 billion Milky Way stars, and the velocities of 1.3 billion of those stars. (In September 2016, the Gaia team released its first map with only position and brightness measurements for 1.1 billion stars.) Astronomers, who had previously cataloged just 2.5 million of the brightest stars in the galaxy, are hailing a new era of precision astronomy. These are some of the most important discoveries to come from the Gaia data so far.
A team in France applied their preprepared STREAMFINDER algorithm to the Gaia data and immediately uncovered a rich network of “stellar streams,” or tributaries of stars flowing into and around the Milky Way. “The idea is to trace the streams backward in time along their orbits in order to contemplate the galaxy’s past and its formation history,” said Khyati Malhan of the University of Strasbourg, the lead author of the paper detailing these “galactic archaeology” findings, in an email.