Dead star's double debris disc hints at surviving worlds

Astronomers have found a double debris disc around the white dwarf WD J1328-1450, a pattern never seen before around a dead star. The gas around the remnant moves at distinct speeds, revealing twin rings separated by a clear gap, according to the European Southern Observatory. The discovery, reported in a report on the announcement, was made with the X-shooter instrument on the Very Large Telescope in Chile. It gives astronomers a rare look at a planetary system after the death of the star at its centre.

A white dwarf is the dense core left behind when a star similar to the Sun runs out of fuel and sheds its outer layers. That transformation can engulf nearby planets and break apart rocky bodies that survive the expansion. The wreckage can then settle into a disc of gas and dust around the remnant. Until this discovery, every known gaseous disc around such a star had shown the signature of a single flat rotating disc. The new system breaks that pattern, and the break is what makes the finding important for models of how planetary systems end.

The research was led by Nicola Pietro Gentile Fusillo of the University of Trieste and the Astronomical Observatory of Trieste, with colleagues at the University of Warwick and the Technical University Federico Santa Maria in Chile. Their paper was published in the journal Nature Astronomy, and a link to the paper is available through the journal. The team also drew on earlier observations that had flagged the remnant as a debris disc host because of excess infrared light, a sign of warm dust in orbit around the star.

How astronomers spotted the twin rings

The clue came from the shape of light emitted by gas in the disc. Gas on one side of a rotating disc moves toward an observer, while gas on the opposite side moves away, so a single disc normally produces a pair of peaks in the spectrum. In this system, the emission lines showed four distinct peaks. The team modelled the signal as a pair of concentric circular discs, each with its own inner and outer edge, and found that the model matched the observed lines. The inner ring stretches from about 0.14 solar radii to about 0.34 solar radii from the centre of the remnant, while the outer ring stretches from about 0.54 solar radii to about 1.93 solar radii.

The rings also differ in composition, which may be the most telling detail of all. Calcium, oxygen, magnesium and iron were detected in the disc material, a mix similar to rocky planetary material found in the Solar System. The inner ring is rich in magnesium gas, while the outer ring is largely stripped of magnesium, according to the researchers. That difference suggests the material feeding each ring came from bodies with different compositions. An alternative explanation, the team noted, is that the rings came from different parts of a single body, comparable to the crust and the core of a planet.

Stability is another striking feature. Co-author Felipe Lagos-Vilches, of the University of Warwick and the Technical University Federico Santa Maria, described the rings as "remarkably stable" and still separated by a gap. The lead author described the system as the "first example of a complex system" with a pair of surviving planetesimals. The most likely explanation, according to the team, is that a pair of planetesimals survived the death of the star and now maintains the rings. As each body orbits through dust in the disc, collisions release gas, producing the luminous trails that astronomers can measure.

What the pattern may mean for planetary survival

The sizes estimated for the surviving bodies span a wide range. The body in the inner ring is thought to be the size of a typical asteroid, a few kilometres across. The body in the outer ring could be far larger, reaching the scale of Pluto or even the Moon. Those estimates are limits rather than direct measurements, because the bodies themselves have not been imaged. The evidence for them comes from the structure and composition of the rings, and from the way gas is released as the bodies move through dust.

The finding also matters because it challenges a picture built from earlier discs. All 21 previously known gaseous discs around white dwarfs showed the signature of a single flat disc, so models of debris discs were built around that shape. A stable pair of rings, with different compositions and a gap between them, requires a more complex explanation, and the authors argue that surviving planetesimals provide one. The remnant itself is relatively young for a white dwarf, with a cooling age of about 73 million years, which means the system offers a view of planetary wreckage not long after the star died, in astronomical terms.

For the Solar System, the system offers a distant preview rather than a prediction about any particular planet. The Sun will eventually become a white dwarf in a process that may destroy Earth, according to the European Southern Observatory. Lagos-Vilches said the system offers a "remarkable glimpse" of what the future of planetary systems might look like. The work also shows that death does not necessarily end the story of a planetary system. Bodies can survive, debris can settle into new structures, and the chemistry of destroyed worlds can still be read in the light of the star left behind.

Related coverage on genznewz looks at how Earth gained water as it grew and at signals from ancient supernovae. A detailed summary of the analysis sets out the spectral modelling behind the twin-ring interpretation.