The Milky Way's warped shape probably is due to a long-running collision with a smaller galaxy. But which galaxy this might be is a mystery.
This insight comes courtesy of the Gaia spacecraft, a European Space Agency mission designed to map the movement of up to a billion stars with high precision.
"We measured the speed of the warp by comparing the data with our models based on the obtained velocity, the warp would complete one rotation around the center of the Milky Way in 600 to 700 million years," lead author Eloisa Poggio, a Ph.D. student at the Turin Astrophysical Observatory in Italy, said in a statement from the European Space Agency. (By comparison, the sun takes about 220 million years to orbit the Milky Way.)
Which galaxy caused this collision remains a mystery. One possible candidate is the dwarf galaxy Sagittarius, which scientists say likely did move through the Milky Way disc a few times in the past. Moreover, current models show that Sagittarius is being absorbed into the Milky Way. But more study will be required to support this possibility.