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Niobium-rich white dwarf points to a planet built from material its own star cast off
Warwick-led astronomers tie niobium at over 1,000 times solar levels in a white dwarf to a candidate planet made from what the star shed as it died. If it holds up, it shows planets can still form after a star has died.
The Scientist · Science desk
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What happened
- The heavy elements surfaced when the team reanalysed Hubble observations from 1999, whose mix of chemicals astronomers could not identify at the time.
- This is the first niobium detected in any white dwarf, and the atmosphere also carries zinc and copper, elements made as stars die.
- NASA's TESS satellite recorded a faint brightness signal from the system that repeats every 4.4 days.
- The team expects the planet's outer atmosphere to be boiling off under the star's radiation, with the gas raining onto the white dwarf to produce its chemistry.
- Similar reborn worlds have been suspected around pulsars, which are much rarer stellar remnants than white dwarfs.
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Why it matters
- constraint If an isolated dying star sheds mass too evenly to build a disk, reborn planets are limited to systems with a companion that pulls the ejecta back, so a common host type does not mean common planets.
- capability Astronomers can now hunt for more reborn worlds by screening other white dwarf spectra for the same carbon and heavy-element signature.
- contradiction The Warwick release says the team discovered the planet, while the paper's own title calls it a candidate and Science News counts it as a first only if confirmed.
Niobium is the clue because of where it is made. It forms in the slow neutron-capture process, or s-process, which runs mostly in certain red giant stars as they shed their atmospheres and die [8]. "It's a chemical signature no ordinary, 'first-generation' planet should carry," said Nicholas Stone of the University of Wisconsin-Madison [2].
The contrast with other polluted white dwarfs is sharp. According to Science News, astronomers have found that between a quarter and half of white dwarfs carry elements such as silicon and iron, probably from rocky planets the star destroyed [16]. HS 0209+0832 has only trace silicon and almost no iron [18]. "We've never observed anything in the universe that's this niobium-rich before," said Jamie Williams, the Warwick doctoral student who is the paper's first author [7] [19].
The team's preferred explanation has two steps. Material the dying star threw off formed a new disk, and a giant planet condensed out of it, rich in the star's own heavy elements [13].
That first step is hard. "A single, isolated star dies and sheds mass in a roughly symmetrical way," Williams said. "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." [23]
The light signal is weaker evidence. It is consistent with a tidally locked, Jupiter-sized gas giant [4] about 6 million kilometres from the white dwarf, roughly 4 percent of the Earth-sun distance [10]. The team found the period too slow to come from the white dwarf's own rotation, so spin is ruled out [9]. Williams did not claim more than that. "We're not really sure, if it's not a planet, what it is," he said. "A planet is the most reasonable explanation." [11] The thing this doesn't tell you is the object's mass. The Jupiter size is what the signal fits [4], and it was not measured directly.
Zifan Lin, a planetary scientist at Washington University in St. Louis who was not involved, said that before this work the process was "more like a hypothesis, more like speculation." [15] "This is the first time we've seen evidence for that process," he said [15]. Theorists predicted second-generation planets more than 15 years ago [6]. I think the chemistry is strong enough to take seriously. The planet is still one candidate, around one star, and it needs confirmation [6].
What to watch
- Independent follow-up confirming that the object orbiting HS 0209+0832 is a planet, the condition both reports attach to the claim.
- Direct evidence of the companion star that Williams says the disk probably needed to form.