Two rings of debris around a dead star hint at what planetary systems leave behind

Nicola Gentille Fusillo, an astronomer at the Università degli Studi di Trieste in Italy, led the international team that found two rings of debris around the white dwarf J1328−1450 using the X-Shooter instrument on the European Southern Observatory’s Very Large Telescope in Chile’s Atacama Desert. The star lies 1,600 light-years from Earth.

J1328−1450 is the dense, burnt-out core left after a star similar to the Sun ran out of fuel and expelled its outer layers. The team found gas arranged in two concentric rings, with a clear gap between them.

Two planetesimals appear to have survived the star’s death and now likely maintain the separate gaseous discs. As they orbit the white dwarf, they interact with dust grains and release gas. The inner object is estimated to be the size of a typical asteroid, a few kilometres in diameter. The outer one could be as large as Pluto, or even the Moon.

The inner ring runs from about 100,000 to 240,000 kilometres from the star. The outer ring extends from roughly 375,000 kilometres to more than 1.3 million kilometres. Calcium, oxygen, magnesium and iron are present in the disc. The inner ring is rich in magnesium gas, while the outer ring is largely stripped of it.

Measurements with X-Shooter showed gas moving around the white dwarf at two distinct speeds. The system also produced four peaks in its spectral signature. All 21 comparable systems known to date have produced a signal with two peaks, whereas the four peaks at J1328−1450 point to two gas rings divided by an empty gap.

Observations repeated between 2021 and 2023 showed that the structure remained stable for more than two years. The study, published in Nature Astronomy, presents the first example of a complex system with a double debris disc and potentially two surviving planetesimals.

Felipe Lagos-Vilches, an astronomer at the University of Warwick and the Universidad Técnica Federico Santa María in Chile, contributed to the analysis of the system. Boris Gänsicke, a professor at Warwick, led the team that discovered the first gaseous debris disc around a white dwarf twenty years ago.

“We have known for some time that planets may survive beyond the transformation of their central star, but this is the first example of a complex system with a double debris disc and potentially two surviving planetesimals,” Fusillo said.

Our Sun will become a white dwarf in billions of years. Before that stage, a red giant star swells dramatically and can swallow planets whole. The newly observed system offers a view of the material that may remain around a star after that transformation.