Hubble data from 1999 points to a planet born after its star died

A planet may have formed from the wreckage of a dying star, long after the star’s original planets were born. The evidence sat unread for more than a quarter of a century in the archive of the Hubble Space Telescope, as a set of spectral lines nobody could identify, until a doctoral student matched them to niobium.

That metal should not be there in such amounts. Its presence points to a gas giant about the size of Jupiter that built itself out of the gas a star shed in its final giant phase. Every planet we know, Earth included, formed from the leftovers of a star’s birth. This one would have formed from the leftovers of a death, and it would mean that a star’s planetary story does not have to end when the star does.

The star is HS 0209+0832, a white dwarf roughly 270 light-years from Earth. A white dwarf is the exposed core of a Sun-like star that has burned through its fuel and blown its outer layers into space. A team led by Jamie Williams, a doctoral candidate at the University of Warwick in the United Kingdom, reports in Nature Astronomy that the white dwarf is swallowing material unlike anything in the Solar System.

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“Rather than the white dwarf stage being a kind of epilogue to the story of a star and its planets, this research points to the systems we are familiar with only being the first chapter of a potentially much longer tale, with some new characters showing up,” Williams said in the ESA/Hubble release.

How a 27-year-old spectrum solved its own cold case

When Hubble first pointed at HS 0209+0832, its spectrograph split the star’s ultraviolet light into a rainbow of wavelengths. Atoms in the star’s atmosphere absorb light at specific wavelengths, so each element leaves its own pattern of dark lines. Roughly 100 of those lines matched nothing in the atomic catalogues astronomers used at the time.

Williams went back to the archived data with an updated database of atomic transitions and found that niobium explained many of the mystery lines. “What Hubble is showing us in this white dwarf system is something we haven’t seen before: a high abundance of the element niobium, the signature of which I was unfamiliar with when I first found it in the archival data,” he said.

The team then checked a second, independent record. NASA’s retired FUSE ultraviolet satellite had also observed the star, and its spectra showed the same strong niobium signature. Newer Hubble spectra filled in the rest of the chemistry: the white dwarf’s atmosphere is rich in zinc, copper and niobium, all elements heavier than iron, while silicon and iron, the backbone of rocky planets, are depleted.

The last piece came from NASA’s TESS, a satellite built to find planets by watching stars for small dips in brightness. Over four months it found that the white dwarf brightens and dims by about 0.12 percent on a cycle of 4.399 days. The team reads that rhythm as a planet on a tight orbit about 6 million kilometres from the star, roughly a tenth of Mercury’s distance from the Sun.

Why niobium marks a star’s death, not its birth

Ordinary white dwarfs with “polluted” atmospheres are common. About a quarter to a half of them show metals that should have sunk out of sight within days to millions of years, depending on the star, and the usual explanation is that they are eating shattered asteroids or planets. The chemistry of that rubble has so far closely resembled bodies in our own Solar System: lots of iron, silicon, magnesium and oxygen.

HS 0209+0832 breaks that pattern. Elements heavier than iron are not made by the fusion that powers a star for most of its life. Many of them, niobium among them, are built by the slow neutron-capture process, which runs deep inside Sun-like stars during their last giant phase and is then dredged up to the surface and blown away. “Niobium and other elements heavier than iron are astronomically special because, unlike many common elements, they are not formed in the cores of stars by thermonuclear fusion,” said Nicholas Stone, a theoretical astrophysicist at the University of Wisconsin–Madison and a member of the team.

So the material falling onto the star carries the fingerprint of the star’s own dying gas. The team’s scenario is that part of that ejected gas collected into a new gas giant while the rest dispersed. The white dwarf is still young and very hot, at about 35,000 degrees Celsius, and its radiation is boiling the planet’s atmosphere away. That escaping gas would trail behind the planet like a comet’s tail, settle into a disk and rain down onto the star, which is exactly where Hubble sees it.

“When Jamie asked me about niobium in relation to this study I was truly gobsmacked, as that element had not been reported in any other white dwarf analysed to date. Once we realised it was there, everything fell into place,” said Boris Gaensicke, a Warwick astronomer and co-author.

How it compares with other planets around dead stars

Only a handful of planets are known around white dwarfs. The best known, WD 1856+534 b, is a Jupiter-sized world that TESS caught crossing in front of its white dwarf every 1.4 days. Like Earth, it is assumed to be a survivor: a first-generation planet that formed with its star and somehow ended up close to the remnant.

The very first exoplanets ever confirmed circle a pulsar, the collapsed core left by a supernova, and one leading explanation is that they too formed from debris after the explosion. But nothing in a pulsar’s signal reveals what those planets are made of. At HS 0209+0832 the argument rests on chemistry, which is why the team describes this as the first candidate with the composition expected of a planet built from a dying star’s ejecta.

The comparison also looks forward. In several billion years the Sun will swell into a red giant, lose a large share of its mass and settle as a white dwarf. If second-generation planets can form in that shed gas, the Solar System’s white-dwarf future could include worlds that do not exist yet.

What the study does not settle

The planet has not been seen directly. It is inferred from two lines of evidence: the strange chemistry of the gas falling on the star and the 4.4-day brightness cycle. The paper calls it a candidate throughout, and the TESS signal has more than one reading. It could be the glow of a planet’s hot day side rotating in and out of view, or a cometary tail of escaping gas passing in front of the star, as the authors themselves note.

The composition argument is strong, but it rests on a single system. No model yet explains in detail how a gas giant gathers itself out of a dying star’s outflow, how often that happens, or how long such a planet survives the white dwarf’s radiation. Williams thinks this one probably will, because the star will cool and stop stripping it. That is a projection, not an observation.

Other explanations for the metals have to be ruled out with more data. The authors argue that the helium in the atmosphere and the near absence of rock-forming elements point to a planet’s escaping atmosphere rather than a shredded asteroid, but a second system with the same signature would carry far more weight than any single measurement.

Common questions about second-generation planets

What is a second-generation planet?

It is a planet that forms around a stellar remnant, such as a white dwarf, out of material the star ejected late in its life. Earth and the other Solar System planets are first-generation planets, made from the gas and dust left over when the Sun formed.

What is a white dwarf?

A white dwarf is the hot, dense core left behind when a star like the Sun runs out of fuel and sheds its outer layers. It packs roughly the mass of a star into a ball about the size of Earth and slowly cools over billions of years.

How can Hubble find a planet it cannot see?

Hubble measured the chemical elements falling onto the white dwarf, and those elements matched the gas a dying star releases rather than ordinary rock. A separate satellite, TESS, detected a regular brightness cycle of 4.4 days that the team attributes to a planet on a close orbit.

Will the Sun form new planets after it dies?

Nobody knows yet. The Sun will become a white dwarf in several billion years and shed a large part of its mass on the way, so the raw material will exist. This discovery suggests that planets can form from such material, but one candidate cannot say how often it happens.

Why is niobium important?

Niobium is heavier than iron, and stars cannot make it through the fusion that powers most of their lives. Much of it is produced during a star’s final giant phase, so finding a lot of it in the gas falling onto a white dwarf points to material that came from a dying star rather than from ordinary planets.

What comes next

Hubble first observed the star in 1999, and the new analysis was published in Nature Astronomy on 5 October 2026, together with an ESA/Hubble release. Williams plans to use Hubble over the next several years to look for more white dwarfs with the same chemistry, which would show whether HS 0209+0832 is a one-off or the first of a population. Further observations of its 4.4-day cycle at other wavelengths could also tell a glowing planet apart from a passing tail of gas.

Reference: Williams et al., “Discovery of a second-generation planet candidate accreting onto a white dwarf,” Nature Astronomy, 2026. DOI: 10.1038/s41550-026-02983-7

Tags: astronomy, hubble, exoplanets, white dwarf, HS 0209+0832, TESS

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