An international team led by astronomers from the University of Trieste and the University of Warwick has discovered a double ring of planetary debris around a dead star. The discovery may foreshadow what happens to our own Solar System.
The study, published in Nature Astronomy, focuses on WD J1328−1450, a white dwarf star about 1,600 light years from Earth. A white dwarf is the dense, burnt-out core left behind when a star like the Sun runs out of fuel.
As stars like the Sun approach the end of their lives, they swell and shed their outer layers, engulfing nearby planets. Rocky bodies that survive can break apart and form a disc of debris around the stellar remnant. Until now, every gas disc found around a white dwarf has been a single, flat, rotating structure.
Observations with the X-shooter instrument on the European Southern Observatory's Very Large Telescope in Chile showed that WD J1328−1450 breaks this pattern. The 21 comparable systems known to date all produce a signal with two peaks. This one produces four, the signature of two concentric rings of gas separated by an empty gap.
The inner ring extends from about 100,000 to 240,000 kilometres from the star, while the outer ring stretches from roughly 375,000 kilometres to more than 1.3 million kilometres. Repeated observations between 2021 and 2023 show the structure stayed stable for more than two years.
Dr Felipe Lagos-Vilches, Department of Physics, University of Warwick, said: "The structure we see in this system is particularly intriguing because the two gaseous rings are remarkably stable and remain separated by a gap. It hints that dead stars can have disc structures far more complex than previously realised."
The gas contains calcium, oxygen, magnesium and iron, a mix similar to the rocky material found in our own Solar System. The two rings differ sharply, however. The inner ring is rich in magnesium, while the outer ring has far less of it. This suggests the debris comes either from two planetary bodies that formed under different conditions, or from different layers of a single body.
The team proposes that two surviving planetary bodies, each orbiting within its own ring, keep the structure intact. By interacting with surrounding dust, they would release the gas that is observed. Their sizes cannot be measured directly, but models suggest the inner body is no larger than a typical asteroid, a few kilometres across, while the outer body could be as large as Pluto or even the Moon.
Lead author Nicola Pietro Gentile Fusillo, University of Trieste and INAF-Astronomical Observatory of Trieste, said: "We cannot see the two planetary bodies directly, but we can infer their likely presence from the shape and composition of the gas. It is like reconstructing the architecture of a destroyed planetary system from the traces left by its final pieces. For the first time, we can see that such a complex configuration can survive even around a dead star."
Professor Boris Gänsicke, Department of Physics, University of Warwick, said: "When our team at Warwick discovered the first gaseous debris disc around a white dwarf twenty years ago, we couldn't have foreseen how much these systems would tell us about the fate of planetary systems. Every gaseous disc known until now has shown the same signature, so a white dwarf with two separate discs is a real surprise. I can't wait to see more detailed observations to learn which planetary bodies may have shaped this unusual system."
The finding challenges current ideas about how debris discs form and evolve, and about the origin of gas around white dwarfs. Our Sun will become a white dwarf in billions of years, a process that may destroy Earth.
