Cloud of gas approaching Milky Way likely originated there

New Scientist : Smith’s Cloud is a hydrogen gas cloud that is 11 000 light-years long, 2500 light-years across, and 40 000 light-years from Earth. It was discovered in 1960 by Gail Smith, who detected radio waves emitted by the hydrogen atoms. In 2008 further observations revealed that it was moving toward the Milky Way. Now Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland, and his colleagues have found evidence that suggests the cloud probably originated in the Milky Way. Using the Hubble Space Telescope, they examined the light passing through the cloud to determine what traces of heavier elements are present there. Sulfur levels indicate that the cloud has been exposed to stars, so it could not be an isolated remnant of the early universe. Based on the cloud’s trajectory, simulations suggest it was ejected 70 million years ago, about 15 000 light-years farther out from the galactic center than Earth. However, what could have caused the ejection is unclear because Smith’s Cloud is much more massive than other clouds pushed by supernovae, the main force that can impact a cloud’s trajectory.