Astronomers Find What Looks Like the First Planet Reborn From a White Dwarf's Ashes

Astronomers have discovered what appears to be the first second-generation planet found around a white dwarf, a world that seems to have formed from the remains of the dead star it now orbits. A team led by the University of Warwick reports the finding in Nature Astronomy.

Similar reborn worlds have been suspected around pulsars, but this result shows that a second generation of planets can form around the far more common stellar remnant. "Second-generation planets are worlds that form out of the material a star casts off as it dies," said first author Jamie Williams, a Ph.D. student in Warwick's Department of Physics. "They're incredibly rare, and finding one around a white dwarf was completely unexpected. It's a bit like finding a planet that has risen from the ashes of the very star it once orbited."

White dwarfs are the collapsed cores left behind when stars run out of fuel. They pull in material from nearby planets, and that material shows up in their atmospheres. The chemical signature usually tells astronomers about the planets in orbit and is typically dominated by rock-forming elements such as silicon and iron.

The white dwarf in this study, HS 0209+0832, looked different. Its atmosphere contains unusually heavy elements, including zinc, copper and, most notably, niobium at levels more than 1,000 times higher than in the sun. It is the first time niobium has been found in a white dwarf. All of these elements are created during the death of a star. "This pattern of elements is a telltale sign of the 's-process,' a nuclear reaction that builds heavy elements inside dying stars during their bloated red giant phase," said Nicholas Stone of the University of Wisconsin-Madison. "It's a chemical signature no ordinary, 'first-generation' planet should carry, which told us that this new planet was something different."

The team's leading explanation is that the white dwarf is feeding on a newly formed giant planet that condensed out of a new disk of material created during the star's death. Because that disk was made from the star's own ejected material, it would naturally be rich in the heavy elements now showing up in the white dwarf's atmosphere.

Making such a disk is hard, which helps explain why these planets are so rare. "A single, isolated star dies and sheds mass in a roughly symmetrical way," Williams said. The team thinks HS 0209+0832 likely needed a companion star that pulled the ejected material back into orbit instead of letting it escape.

Data from NASA's TESS satellite supplied further evidence. Researchers detected a faint, regular brightness signal from the planet that repeats every 4.4 days, consistent with a Jupiter-sized gas giant locked tightly to the star. At that distance the planet's outer atmosphere is expected to be boiling away under intense radiation, and the escaping gas rains down onto the white dwarf's surface and produces the odd chemistry.

The result is described as a candidate, and it still needs confirmation. If it holds up, it opens a new way to hunt for reborn worlds, by looking for the same carbon and heavy-element signature in the light of other dead stars. Boris Gänsicke, a professor in Warwick's Department of Physics, said the system "has given birth to a new world using the foundations of the old one." He added: "Finding this one example raises the question of how many more might be out there—and might our own solar system host a second-generation planet formed from the ashes of our sun?"