Level 1 — Absolute Beginner

A star does not live for ever. When a star like our Sun gets very old, it becomes small and hot. We call this kind of dead star a white dwarf.

Astronomers looked at one white dwarf with a very big telescope in Chile. They found two rings of broken rock and gas around it. Two rings together is new. Nobody has seen this before.

The scientists think two large rocks make the rings. The smaller rock is only a few kilometres wide. The bigger rock may be as large as the Moon.

Our Sun will also die one day, in billions of years. Then it will look like this star. So this star shows us a possible future for our own home.

a star

A huge ball of hot gas in space that gives light.

the Sun

The star that gives light and heat to the Earth.

old

Having existed for a long time.

a telescope

An instrument used to look at distant objects in space.

a ring

A circle of material around something.

rock

The hard material that mountains and stones are made of.

the Moon

The large round object that moves around the Earth.

a future

The time that will come after now.

Level 2 — Elementary

Astronomers using the Very Large Telescope in Chile have found something nobody expected around a dead star. The star is a white dwarf called WD J1328-1450, and it is surrounded not by one ring of shattered material but by two, separated by a clear gap.

That matters because 21 similar gas discs around white dwarfs have been found before, and every one of them produced a signal with two peaks. This star produced a signal with four peaks, which is what told the team there were two separate rings rather than one.

The researchers believe each ring is being fed by a surviving object. The inner object may be only a few kilometres across, roughly the size of an asteroid. The outer one could be far larger, somewhere between the dwarf planet Ceres and bodies the size of Pluto or our own Moon. Neither object has actually been seen.

The work was published in the journal Nature Astronomy on 26 October. Felipe Lagos-Vilches of the University of Warwick said the system "offers a remarkable glimpse of what the future of planetary systems, including systems like our own, might look like". In several billion years, our Sun will become a white dwarf too.

an astronomer

A scientist who studies stars, planets and space.

a white dwarf

The small, hot, dense remains of a star like the Sun.

shattered

Broken into many small pieces.

a gap

An empty space between two things.

a peak

The highest point of a curve or a signal.

an asteroid

A small rocky object orbiting the Sun.

a dwarf planet

A round object orbiting the Sun that is smaller than a planet.

a glimpse

A quick or partial view of something.

Level 3 — Intermediate

A team using the X-shooter instrument on the European Southern Observatory's Very Large Telescope in Chile has identified the first double debris disc ever seen around a dead star. The object, the white dwarf WD J1328-1450, carries two concentric rings of gas: an inner ring extending from 0.14 to 0.34 solar radii and an outer ring from 0.54 to 1.93 solar radii, with a gap between them.

The discovery turns on a subtle spectroscopic detail. All 21 gas discs previously detected around white dwarfs produced double-peaked emission lines, the signature of a single rotating ring. This star produced quadruple-peaked lines, which is extremely difficult to explain with one disc and straightforward to explain with two. The star itself is unremarkable by comparison: an effective temperature of 17,424 kelvin and a mass of about 0.47 times that of the Sun.

Chemistry sharpens the picture. Calcium, magnesium and silicon show up in the star's atmosphere, and calcium, oxygen, magnesium and iron in the gas, which means the white dwarf is actively swallowing the debris of destroyed planetary material. Strikingly, the inner ring is rich in magnesium while the outer is depleted of it, and the magnesium-to-silicon ratio of 0.2 sits far below the 0.85 to 1.2 typical of other debris-accreting white dwarfs. The authors infer two surviving bodies shepherding the rings, the inner one perhaps a few kilometres wide, the outer potentially as large as Pluto or the Moon.

The researchers are careful about what they have not shown. Neither body has been observed; both are inferred from the geometry of the gas. The magnesium depletion rests on a single line that may be blended with emission. The team considered alternatives, including a single self-shadowing disc and an unresolved pair of white dwarfs, and rejected both as unsupported or improbably fine-tuned. Lead author Nicola Pietro Gentile Fusillo of the University of Trieste called it "the first example of a complex system with a double debris disc and potentially two surviving planetesimals".

a debris disc

A flat ring of broken rock, dust or gas orbiting a star.

concentric

Sharing the same centre, one inside the other.

spectroscopic

Relating to the analysis of light split into its separate colours.

an emission line

A bright line in a spectrum produced by a specific chemical element.

a solar radius

A unit of length equal to the radius of the Sun.

to accrete

To gradually gather material onto a surface by gravity.

depleted

Reduced in quantity; largely used up or missing.

a planetesimal

A small solid body from which planets form.

Level 4 — Advanced

Astronomers have for the first time resolved a double debris disc around a stellar remnant, a configuration that current theory did not anticipate and does not comfortably accommodate. Writing in Nature Astronomy on 26 October, Nicola Pietro Gentile Fusillo of the University of Trieste and INAF, with colleagues at Warwick and elsewhere, report that the white dwarf WD J1328-1450 is encircled by two concentric gaseous rings: an inner annulus spanning 0.14 to 0.34 solar radii and an outer one from 0.54 to 1.93 solar radii, divided by a persistent gap. The observations were made with the X-shooter spectrograph on the European Southern Observatory's Very Large Telescope at Paranal.

The evidence is spectroscopic rather than photographic, and its force lies in an anomaly. Each of the 21 gas discs previously catalogued around white dwarfs yielded double-peaked emission lines, the canonical signature of a single Keplerian ring viewed edge-on. This system yields quadruple-peaked profiles, which a single structure cannot easily reproduce. The host is otherwise ordinary, with an effective temperature of 17,424 kelvin, a surface gravity of log g 7.76 and a mass of 0.47 solar masses, and it carries a fractional infrared excess of 2.7 percent indicating accompanying dust.

Composition supplies the forensic detail. Calcium, magnesium and silicon pollute the stellar atmosphere while calcium, oxygen, magnesium and iron appear in the circumstellar gas, confirming the ongoing ingestion of disrupted planetary material. The inner annulus is magnesium-rich and the outer magnesium-poor, and the system's magnesium-to-silicon ratio of 0.2 lies conspicuously outside the 0.85 to 1.2 range typical of polluted white dwarfs. From the sharpness of the disc edges the authors derive dynamical limits on the sculpting bodies: for iron-like densities, a maximum diameter of roughly 346 kilometres at the inner boundary and about 2,340 kilometres at the outer, rising toward 3,500 kilometres for a moderately strong rocky composition, which would place the outer object anywhere between Ceres and a body the size of Pluto or the Moon.

What distinguishes the paper is the candour of its caveats. Neither planetesimal has been detected; both are reconstructions from gas geometry, and the authors concede that the complexity of the underlying emission lines introduces systematic uncertainties that are difficult to quantify, and that their preferred solution is not demonstrably unique. The magnesium depletion hangs on one potentially blended line. Rival architectures, a single self-shadowing disc and an unresolved double degenerate, were weighed and discarded as theoretically unsupported and implausibly fine-tuned respectively. Felipe Lagos-Vilches of Warwick emphasised that "the two gaseous rings are remarkably stable and remain separated by a gap", and framed the wider significance plainly: the system "offers a remarkable glimpse of what the future of planetary systems, including systems like our own, might look like". Replication on other polluted white dwarfs is the obvious next test.