Why couldn the brown dwarf have been born next to the original star?
The white dwarf and brown dwarf are currently separated by a distance of 2/3rds of our Sun’s radius. Since the white dwarf is descended from a star like our Sun, then there’s no way the brown dwarf could have been born there (otherwise it would have been born inside the Sun-like star, which is impossible). In fact, we think it was most likely born at a separation more like that of the Earth and Sun. Then, when the star became a red giant, its expanding atmosphere overtook and engulfed the brown dwarf, dragging it in. A red giant’s atmosphere is much more tenuous than the Sun’s (same mass, much bigger volume, hence less dense). So the brown dwarf could survive inside it.
