First 'phoenix' planet found orbiting dead star

A dead star's chemical remains point to a planet that formed after its star died. If confirmed, it would be the first example of a second-generation world, a type predicted more than 15 years ago.

Scientists report the finding Oct. 5 in Nature Astronomy. They reanalyzed 1999 Hubble Space Telescope observations of a white dwarf called HS 0209+0832. White dwarfs are the dense cores left when low-mass stars like our sun puff up into red giants and blow off their outer layers.

Most white dwarfs have atmospheres made almost entirely of hydrogen and helium. But about a quarter to half of them are “polluted” with elements like silicon and iron — shards of planets the star swallowed as it expanded.

HS 0209+0832 showed something different: trace amounts of silicon, almost no iron, and a huge amount of niobium, a rare metal never before seen in a white dwarf. “We’ve never observed anything in the universe that’s this niobium-rich before,” says astronomer Jamie Williams of the University of Warwick in Coventry, England, a coauthor of the study.

Niobium forms in the slow neutron-capture process, which happens inside red giant stars as they die. Material ejected from the dying star could have coalesced into a new planet rich in such elements. The white dwarf may then have slowly pulled that planet's atmosphere onto itself, leaving a fresh layer of niobium.

The team also found that the white dwarf’s brightness flickers every 4.4 days, based on data from the TESS space telescope. That period is too slow for the white dwarf's own rotation. It could instead come from a gas giant planet orbiting very close to the star — about 4 percent of the Earth-sun distance, or 6 million kilometers. Williams describes it as a silvery version of Jupiter.

“We’re not really sure, if it’s not a planet, what it is,” he says. “A planet is the most reasonable explanation.”

Planetary scientist Zifan Lin of Washington University in St. Louis, who was not part of the study, says this kind of planet formation had been speculation until now. “This is the first time we’ve seen evidence for that process.”

The finding suggests that some planetary systems can get a second life after their stars die. Our own sun will one day become a white dwarf. Whether a second-generation planet could appear here is an open question, Williams says, “but I think it’s definitely possible.”