Two planetary bodies sculpt a rare double debris disc around a white dwarf

Astronomers have discovered a white dwarf wrapped in not one but two nested rings of glowing gas, a configuration never seen before around a stellar remnant. The star, catalogued as WD J132800.85–145019.95, or WD J1328–1450 for short, displays emission lines with four distinct peaks instead of the two peaks that every other gaseous debris disc known to date produces. Writing in Nature Astronomy, Nicola Pietro Gentile Fusillo and colleagues report that the quadruple-peaked profiles arise from two separate gaseous rings orbiting the white dwarf at different radii, and they propose that the architecture was carved out by two distinct planetary bodies, ranging in size from a few kilometres to roughly the size of Pluto. The detection of such a stable double disc, the team argues, presents a new challenge for current models of how debris discs form and evolve around dead stars.

Many planetary systems survive the metamorphosis of their host stars into white dwarfs, as demonstrated by the dusty circumstellar discs detected around these stellar remnants and by the chemical fingerprints of planetary debris polluting the atmospheres of the stars themselves. A small fraction of these circumstellar discs also contain gas, revealed through emission lines from various ionic transitions, most commonly the near-infrared calcium II triplet. All 21 previously known gas discs exhibit double-peaked line profiles, the classic signature of a rotating flat disc in which gas on one side of the star approaches the observer while gas on the other side recedes. The mechanism behind the generation of that gas is still debated, although for at least one system it has been linked with the presence of a surviving planetesimal embedded within the dust disc.

WD J1328–1450 was identified as a white dwarf candidate from its Gaia DR2 parallax and colour, and flagged as a potential debris disc host after an infrared excess showed up in its photometry from the VISTA Hemisphere Survey and the Wide-field Infrared Survey Explorer. The team observed the star with X-shooter, an intermediate-resolution echelle spectrograph mounted on the Very Large Telescope at Paranal in Chile. Three spectra were secured on 15 May 2021 with exposure times of 1,800 seconds and 1,750 seconds in the ultraviolet B and visible arms respectively, five more spectra followed on 28 June and 2 July 2021, and two final spectra were obtained on 12 May 2023. The reduced spectra reveal absorption lines from calcium II, magnesium II and silicon II in the stellar atmosphere, alongside emission lines of calcium II, oxygen I, magnesium II and iron II rising from the circumstellar material.

Determining the fundamental parameters of the white dwarf proved unusually difficult, because the many broad and blended emission lines distort the continuum level flanking the helium absorption lines, and the infrared flux from the dusty debris contaminates the photometric fit. The authors therefore adopted an iterative approach, deriving the effective temperature from the spectroscopy and the surface gravity from the photometry and parallax. This converges on an effective temperature of 17,424 ± 567 kelvin and a surface gravity of log g = 7.76 ± 0.19, corresponding to a mass of 0.47 ± 0.09 solar masses and a cooling age of 73 million years, with an uncertainty range of 26 to 49 million years. The photospheric abundances of magnesium, silicon and calcium, measured assuming a bulk Earth composition, allow the team to compute the diffusion timescales of these elements and the downward flux of metals accreted by the star.

The nested double-peaked shape of the emission lines was modelled as two concentric and coplanar circular gaseous discs, each with a surface brightness profile described by a power law in radius. The resulting quadruple-peaked profile depends on the Keplerian velocity at each distance from the white dwarf, the inner and outer edges of both discs, the brightness exponents, and the inclination of the system, where 0 degrees is a face-on disc and 90 degrees is edge-on. Using a Bayesian framework, the team fitted the model to the calcium triplet, whose three emission lines are expected to form under the same physical conditions, sampling the parameter space with an affine-invariant Markov chain Monte Carlo algorithm implemented in the Python package emcee. The fit yields a fully described profile with twelve free parameters, including a blending factor accounting for the partial overlap of the calcium lines at 8,498 and 8,542 angstroms.

The posterior distributions show that the inner gaseous disc extends from roughly 0.14 solar radii to 0.34 solar radii from the centre of the white dwarf, while the outer disc stretches from 0.54 solar radii to 1.93 solar radii. The modelling suggests an inclination above 30 degrees, with a mean value of 77 degrees, and the absence of circumstellar absorption is consistent with this estimate, since a gas disc in thermal equilibrium with the white dwarf radiation would have a relative scale height of only 0.005 to 0.008, requiring an inclination above 89.6 degrees for the line of sight to intersect the disc material. A parallel fit to the oxygen triplet at 7,774 angstroms, with the inclination and systemic velocity fixed to the calcium values, reproduces the inner disc parameters in good agreement, although the outer disc remains less tightly constrained because the double-peaked structures in the central region of that triplet are difficult to disentangle.

The team proposes that the two rings were sculpted by two distinct planetary bodies orbiting within the debris, with sizes ranging from a few kilometres to the scale of Pluto. Supporting evidence for differentiated composition comes from an elegant excitation argument. The oxygen I line at 8,446 angstroms requires an excitation energy of 10.74 electronvolts, whereas the magnesium I line at 8,806 angstroms needs only 5.93 electronvolts, roughly half. Because oxygen emission is clearly detected from both discs, the excitation conditions in the outer disc must be more than sufficient to produce magnesium I emission if magnesium were present at comparable abundances. Scaling the observed inner-disc magnesium flux by the factor 1.22, derived from the calcium profiles showing that the outer disc produces on average 1.22 times more flux than the inner disc, predicts an expected peak flux of 2.37 × 10⁻¹⁷ erg per second per square centimetre per angstrom. Instead, the team measures only continuum emission averaging (1.58 ± 0.13) × 10⁻¹⁷ erg per second per square centimetre per angstrom, meaning the predicted peaks would stand about six sigma above the continuum. The complete absence of detectable magnesium I emission from the outer disc therefore provides model-independent evidence that magnesium is significantly depleted there relative to the inner disc.

Characterising the circumstellar dust proved equally intricate. A standard flat, geometrically thin and optically thick disc model can reproduce the infrared excess, but only with a face-on inclination of 0 degrees, which, assuming the dust is coplanar with the gas, would not produce observable multi-peaked emission lines. Fixing the flat model to the gas-derived geometry, with an inner edge at 0.14 solar radii and an outer edge at 1.93 solar radii corresponding to temperatures of 1,546 and 216 kelvin, underestimates the observed infrared flux. The team therefore turned to the radiative transfer code MCFOST, implementing a disc with a finite vertical structure, a scale height at the inner edge equal to the white dwarf radius of 0.015 solar radii, and a moderate flaring exponent of 1.2. Assuming amorphous carbonaceous dust optical constants, grain sizes from 0.5 to 5 micrometres following a distribution proportional to a to the power of minus 3.5, a total dust mass of 5 × 10¹⁸ grams, edges at 0.25 and 2.0 solar radii, and an inclination of 70 degrees, the model reaches a maximum temperature of about 2,000 kelvin at the inner edge, consistent with the sublimation temperature expected for amorphous carbon. The authors caution that the problem remains highly degenerate and that this solution is not unique.

As a final check, the infrared excess was modelled as a single blackbody at the distance of the white dwarf, yielding a temperature of 1,314 ± 34 kelvin and an area of 2.81 × 10²¹ square centimetres, roughly 20 per cent of the area of the Sun or about 20 times that of Jupiter. The presence of such a large but undetected body is clearly unrealistic, confirming that the thermal infrared flux mainly originates from dust. The fractional infrared brightness of the debris disc, calculated as the ratio of the integrated blackbody flux to the white dwarf flux, comes out at 2.7 per cent, one of the largest values among all known white dwarfs with dusty debris. With the X-shooter spectra publicly available through the European Southern Observatory archive, WD J1328–1450 now stands as a natural laboratory for testing how planetary bodies survive the death of their host stars and continue to shape their surroundings, forcing theorists to reckon with debris architectures far richer than the single flat discs their models were built to explain.

Subject of Research: A double gaseous debris disc around the white dwarf WD J1328–1450, sculpted by two planetary bodies

Article Title: A double debris disc around a white dwarf sculpted by two planetary bodies

Article References: Gentile Fusillo, N. P., Lagos-Vilches, F., Gänsicke, B. T., Manser, C. J., & Koester, D. (2026). A double debris disc around a white dwarf sculpted by two planetary bodies. Nature Astronomy. https://doi.org/10.1038/s41550-026-03004-3

Image Credits: AI Generated

DOI: 10.1038/s41550-026-03004-3

Keywords: white dwarf, debris disc, gaseous disc, planetary bodies, emission lines, calcium triplet, X-shooter, Very Large Telescope, infrared excess, circumstellar dust, planetary system survival, MCFOST