Astronomers have identified what may be the first planet of a second generation — a world formed from material expelled by a dying star and now orbiting the stellar remnant it came from. The candidate planet circles the white dwarf HS 0209+0832, a young, hot stellar core with a temperature of about 35,000 degrees and an atmosphere dominated by hydrogen. The discovery, published this week in Nature Astronomy, suggests that planets can be reborn from the ashes of their own star.
The white dwarf's spectrum, first captured in 1999 by the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope, revealed carbon, aluminum, silicon, calcium, titanium, nickel, and zinc at levels more than a thousand times higher than those found in the Sun. It also marked the first time niobium has been observed in a white dwarf. According to Nicholas Stone, an associate professor at the University of Wisconsin-Madison and a co-author of the study, this pattern is an unmistakable signal of the s-process — a nuclear reaction that builds heavy elements inside dying stars during their red giant phase. «This chemical signature is something no normal first-generation planet should show, and it told us this new planet was something different,» Stone said.
Second-generation planets are exceptionally rare. Similar reborn worlds had been hypothesized around pulsars, but this is the first time a planet-like body has been suspected around a white dwarf. «Finding a planet like this around a white dwarf was completely unexpected,» said Jamie Williams, a doctoral student at the University of Warwick and the study's lead author. «It's a bit like finding a planet reborn from the ashes of the same star it once orbited.»
The planet does not reveal itself directly. Instead, it betrays its presence through the white dwarf, which is pulling in material from its surroundings. The most likely explanation, according to the authors, is that HS 0209+0832 is accreting matter from a giant second-generation planet that condensed from a new disk of material formed during the star's death. Because that disk was made of the star's own expelled material, it would naturally be rich in the unusual heavy elements now detected in the white dwarf's atmosphere.
Forming a protoplanetary disk in such conditions is difficult, which is why these planets are so rare. An isolated star dies and expels mass in a roughly symmetrical way, dispersing it into interstellar space. To form the disk needed for planet birth, HS 0209+0832 likely required a companion star to steer the expelled material back into orbit rather than let it escape. Supporting this interpretation, NASA's Tess telescope detected a faint periodic brightness signal coming directly from the planet, repeating every 4.4 days and consistent with a Jupiter-sized gas giant in a tight, synchronously rotating orbit.
At such a close distance from the white dwarf, the planet's outer atmosphere is expected to be evaporating under intense radiation, with the escaping material falling onto the star'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, suggesting a new way to search for other reborn worlds: by looking for the same carbon and heavy-element fingerprint in the light of other dead stars.
The finding also raises a question that cannot be answered for now: whether the Sun, after becoming a red giant and then a white dwarf, might create the conditions for a second generation of planets born on the foundations of our solar system and the new chemical elements that will form.