University of Warwick astronomers have discovered the first example of a second-generation planet orbiting a white dwarf, a world that appears to have formed from the remains of the dead star it now orbits.
A team of astronomers led by the University of Warwick and funded by the European Research Council (ERC), have identified the first second-generation planet ever found around a white dwarf. While similar 'reborn' worlds have been suspected to exist around pulsars, this discovery shows that a second generation of planets can form around this much more common stellar remnant.
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."
White dwarfs (the collapsed cores left behind when stars run out of fuel) 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.
Published in Nature Astronomy, the study focuses on the white dwarf HS 0209+0832, whose atmosphere contains an unusual chemical signature. It showed unusually heavy elements including zinc, copper, and, most notably, 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.
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 believe 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.
Jamie Williams added: "Forming the protoplanetary disc in this situation is not easy and helps explain why these planets are so rare. A single, isolated star dies and sheds mass in a roughly symmetrical way. To form a disc of material necessary to birth a planet, HS 0209+0832 likely required a companion star that pulled the ejected material back into orbit, rather than letting it escape."
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.
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.
Professor Boris Gänsicke, Department of Physics, University of Warwick, and ERC grantee concluded: "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.
"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."
