The phoenix planet: Astronomers find a world reborn from its star’s ashes

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 has 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, a Ph.D. student in the Department of Physics at the 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.”

Heavy elements reveal an unusual origin

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 atmospheres. 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 this is the first time niobium has been found in a white dwarf.

Dr. Nicholas Stone, of the Department of Astronomy at the 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.”

A companion may have shaped the disk

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 disk of material formed during the star’s death. Because that disk 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.

Williams added, “Forming the protoplanetary disk 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 disk 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.”

TESS detects a 4.4-day signal

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.

Boris Gänsicke, a professor in the Department of Physics at the University of Warwick, 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?”

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