Hubble Solves Merger Mystery From Milky Way’s Early Years - NASA Science (.gov)

Diligent sleuthing by astronomers has broken open a cold case in the data archive of NASA’s Hubble Space Telescope. In a study published Monday in Nature Astronomy, researchers report uncovering a surprising chemical clue that indicates the white dwarf star HS 0209+0832 may host a second-generation planet. A white dwarf is the remnant core of a low-mass star that has burned through all its nuclear fuel and lost its outer envelope of gas and dust to space. A second-generation planet is a world that forms around the stellar remnant from its cast-off material.

A new supercomputer simulation traces how thousands of small galaxies eventually became the Milky Way.