A team of astronomers led by the University of Warwick in England has discovered a rare second-generation planet orbiting the ashes of the dead star from which it was born, a smouldering white dwarf.White dwarfs are the collapsed dead cores left behind when stars run out of fuel. They are known to pull in nearby material from planets, which can be detected in their atmosphere. This chemical signature can tell astronomers details about the planets that are in orbit, and the signal is typically dominated by rock-forming elements like silicon and iron.While similar ‘reborn’ worlds have been suspected to exist around pulsars, the latest discovery shows that a second generation of planets can form around a white dwarf.First author Jamie Williams, PhD student in the Department of Physics, University of Warwick said: “Second-generation planets are worlds that form out of the material a star casts off as it dies. 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.”Published in Nature Astronomy, the study, funded by the European Research Council (ERC), focuses on white dwarf HS 0209+0832, whose atmosphere contains an unusual chemical signature. It shows unusually heavy elements, including zinc, copper and niobium found at levels over 1,000 times higher than the Sun’s. These are all elements created during the death of a star and notably the first-time niobium has been found in a white dwarf.Data from Nasa’s TESS satellite provided further evidence. Researchers detected a faint, regular brightness signal directly from the planet that was repeating every 4.4 days, which is consistent with a Jupiter-sized gas giant tidally locked in a tight orbit. At such a close distance from the white dwarf, the planet’s outer atmosphere is expected to be boiling away under the intense radiation, with that escaping material raining down onto the white dwarf’s surface and producing the unusual chemical signature.Prof Boris Gänsicke, Department of Physics, University of Warwick, and ERC grantee said, “What’s remarkable about the planet around HS 0209+0832 is that this isn’t a planet from somewhere else, or a survivor from the system’s birth, it looks like it was built from the very material its own star cast off as it died. In a sense, this system has given birth to a new world using the foundations of the old one.”If confirmed, HS 0209+0832 would be the first white dwarf found to host a second-generation planet, which opens a new way to search for similarly reborn worlds — by looking for this same carbon and heavy-element signature in the light of other dead stars.Dr Nicholas Stone, Department of Astronomy, University of Wisconsin-Madison said, “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. It’s a chemical signature no ordinary, ‘first-generation’ planet should carry, which told us that this new planet was something different.”The team believes the most likely explanation is that the white dwarf, HS 0209+0832, is feeding off a newly formed, second-generation giant planet — one that condensed out of a new disc of material formed during the star’s death.Because that disc was made from the star’s own expelled material, it would naturally be rich in the unusual heavy elements now showing up in the white dwarf’s atmosphere.
