When our sun has used up its fusion fuel in a few billion years, it will first swell into a red giant and then eject its shells. What remains is the burned-out star core – a white dwarf. This transition also means the end for the inner planets of the solar system: they will be devoured by the red giant or torn apart by the strong tidal forces of the white dwarf. Astronomers have already observed debris from such destroyed planets around several white dwarfs.
A white dwarf with a “suspicious” spectrum
But now astronomers led by Jamie Williams from the University of Warwick have discovered a white dwarf that has a unique companion: a second-generation planet. “Second-generation planets are worlds formed from material ejected when a star dies,” explains Williams. Until now, however, it was unclear whether planets like Phoenix, formed from the ashes of their star, really exist and how exactly they come into being.
The astronomers discovered this “Phoenix Planet” when they examined the white dwarf HS 0209+0832 again with the Hubble Space Telescope and the high-resolution instruments of the Very Large Telescope of the European Southern Observatory in Chile. This young remnant of stars, which has a temperature of almost 35,000 degrees, was already noticed in 1999 because its light spectrum showed an unusually large number of signatures of metals. However, most of them could not be identified at the time. Williams and his team have now made up for that.
In the spectrum of the white dwarf HS 0209+0832, recorded with the Hubble Space Telescope, unusually high amounts of metal stand out. © NASA/ESA, Leah Hustak (STScI)
1000 times more niobium than the sun
With surprising results: “The atmosphere of the white dwarf HS 0209+0832 is highly enriched in elements heavier than iron – including zinc, copper and niobium – but is deficient in the typical rock-forming elements silicon and iron,” report the astronomers. Particularly unusual was the detection of niobium, a metal that has never been detected in a white dwarf before, the team explains. The concentration of this metal was a thousand times higher than in the sun.
Typically, spectral “pollution” in a white dwarf indicates that it is surrounded by debris from its ancient planetary system. This means that residues of rock-forming materials and metals from planetary cores can be detected in the spectrum. But the white dwarf HS 0209+0832 lacks the rock-forming elements; far too many heavy metals are present. This does not match the remains of earlier planets, as Williams and his colleagues report.
Metals (red stars) were detected in the spectrum of the white dwarf HS 0209+0832 compared to Earth (horizontal black line) and other celestial bodies. — © Williams et al. / Nature Astronomy, CC by 4.0
Where do the heavy metals come from?
But where does the white dwarf get all the niobium and other metals from? “This element pattern is an unmistakable sign of the so-called s-process,” explains co-author Nicholas Stone from the University of Wisconsin-Madison. In this process, heavy elements are created by capturing slow neutrons inside red giants. “It is a chemical signature that an ordinary first-generation planet cannot have,” explains Stone.
These heavy elements do not exist in normal white dwarfs because when the red giant collapses, they are thrown out into space along with the stellar envelope. But how do niobium and co get into the atmosphere of the white dwarf HS 0209+0832? According to Williams and his team, there is only one possible explanation: these elements must come from an object that formed from this ejected star shell and that now orbits closely around the white dwarf – a second-generation planet.
A planet from the “ashes” of its star
“The discovery of such a planet around the white dwarf was completely unexpected,” says Williams. The white dwarf HS 0209+0832 is so far the only stellar remnant where such a “phoenix planet” has been found. “This planet has essentially risen from the ashes of the very star it once orbited – that is an incredibly rare phenomenon,” says the astronomer. Normally, the material ejected from the dying star races far out into space instead of forming a disk of material.
But in the case of this white dwarf, a disturbance must have stopped the ejected stellar debris – for example, a nearby partner star. “If such a companion star was present, there are various mechanisms that could lead to the formation of a disk,” report the astronomers. The new planet could then emerge in this disk.
Data from NASA’s TESS space telescope suggests the “Phoenix Planet” is about the size of Jupiter and orbits closely around the white dwarf in just 4.4 days. This close distance explains the unusual spectral signature of HS 0209+0832: The white dwarf’s extreme heat and strong gravitational pull inflate the planet’s gas envelope and strip away its material – including vaporized metals such as nickel, copper and niobium. These collect in the envelope of the white dwarf and thus create the spectral signature.
Gravitational collapse or old core with a new shell?
However, it is still unclear whether this second-generation planet was created completely from the ejected shell remnants of its star or whether its core survived the stellar catastrophe. “In the first case, the planet could have formed close to the white dwarf through direct gravitational collapse in the accretion disk,” explain the astronomers. Such a local collapse can also create massive gas giants around normal young stars.
Alternatively, the “Phoenix Planet” could have formed from the core of an earlier planet that survived the death of its star. Astronomers have already discovered some such planetary cores in the bloated shells of red giants or around white dwarfs. In the case of HS 0209+0832, such a planetary core could have grown into a new gas giant using material from the star’s debris disk.
“This system has birthed a new world from the rubble of the old one. This raises the question of how many such second-generation planets there might still be out there,” says co-author Boris Gänsicke from the University of Warwick. “Perhaps one distant day, even in our own solar system, such a phoenix planet could emerge from the ashes of our sun.”
Source: Jamie Williams (University of Warwick, UK) et al., Nature Astronomy, 2026; doi: 10.1038/s41550-026-02983-7