Astronomers have seen a stellar remnant widely assumed to spend its time in a restful state of quiet slumber before, during and after receiving matter from its more active partner, acts like an ambulance-chasing survivalist. A cold, old white dwarf some 145 light-years away seems to be devouring the remains of a rocky world as scientists look on — a shocking reminder that planetary systems can remain unruly billions of years after their stars die.
The perpetrator, known as LSPM J0207+3331, is the dense core of a star that was once like our own Sun but has run out of fuel. Roughly Earth-sized, yet about half as massive as the Sun squeezed down to that size, a white dwarf has gravity strong enough that it rapidly rips heavier elements from its relatively thin visible atmosphere. To see those ingredients now is to witness what fresh material is raining from above.
A Dead Star With an Appetite Reveals Rocky Infall
In new work published in The Astrophysical Journal Letters, scientists using the W. M. Keck Observatory have examined the star’s light and detected 13 elements — such as iron, nickel, silicon, magnesium and calcium — present in its otherwise hydrogen-rich atmosphere.
That cocktail does not belong. Metals (astronomical convention for anything heavier than helium) sink quickly in white dwarfs, usually far less than a million years—and often much quicker. Their ongoing accretion is directly suggested by their presence.
The observed elemental ratios suggest that the parent body has been differentiated into layers with a much larger metallic core than compared to ordinary CM carbonaceous chondrites. The team calculates that something about the size of a ~200-kilometer-wide object — on the order of a Vesta-sized or larger fragment — was brought within the star’s gravitational Roche limit, shredded to dust and gas, and funneled down onto the surface. The make-up is broadly Earth-like but with an iron and nickel bias, more reminiscent of iron meteorites than primitive asteroids.
Why This Polluted White Dwarf Messes With the Rules
Dirty white dwarfs are not an oddity: during surveys with NASA and ESA missions between one quarter and one half of all white dwarfs are estimated to be accompanied by rocky debris in their atmosphere, whilst a few percent also host warm dust discs. But most other well-studied cases have atmospheres dominated by helium, in which heavy elements tarry longer and are easier to detect. LSPM J0207+3331 is cool and hydrogen-rich — the most typical class — and yet it is incredibly polluted, stretching the frontier of where astronomers thought that such feasting might be seen.
The age of this star adds to the puzzle. The white dwarf was spawned around three billion years ago, so any planetary shuffling that contributed material to the white dwarf probably occurred long after the system’s initial disorder had settled down. The prime suspect is a gravitational kick from a dark outer giant planet — one large enough to play the role of an orbit-clearing behemoth à la Jupiter — that riled up some discarded asteroid or dwarf-planet relic. That sort of delayed instability can hang around for gigayears as orbits precess and resonances sweep through debris belts.
Planetary Forensics in Starlight Reveal Core-Rich Debris
Keck high-resolution spectroscopy enabled the team to weigh the specific ingredients based on faint, picky indicators found within a star’s spectrum. By comparing those patterns with measurements in labs, and with the composition of meteorites, researchers can reconstruct the geology of exoplanetary fragments — sometimes called “cosmic necrogeology.” The evidence here favors a body that had melted and separated into crust, mantle and an outsized iron core before it died.
Previous infrared observations showed a surrounding dust disk, which is probably contained well within the star’s tidal disruption zone where incoming rocks are torn to pieces by gravitational forces. White-dwarf gravity pulls gas and dust inward, “polluting” the photosphere. Heavy elements can sink quickly to the bottom, bearing a date, stamping the meal as one that was recent and ongoing.
A Preview of the Solar System’s Long-Term Destiny
It is a discovery near to home. It too will subsequently cool to be a white dwarf after the Sun swells into a red giant and ejects its outer shell. Inner planets might not survive the colossal phase, but the outer system — including asteroids, moons and dwarf planets — could. In the billions of years since, gravitational nudges from any remaining giants — Jupiter and Saturn — could sling debris inward to be shredded and gathered up to leave a chemical signature in the solar remnants’ atmosphere.
We have already witnessed various roads to planetary perdition: disintegrating debris in the transit zone of WD 1145+017 — the disintegration event and its aftermath found by Kepler/K2 towards WD 1145+017. A second star, WD J0914+1914, appears to be sucking up gas from an ice-giant-like world. On its journey to becoming a white dwarf, the Zwicky Transient Facility observed one Sun-like star devouring a planet in real time. Along with LSPM J0207+3331, these instances delineate the full menu of end-stage planetary fate.
What Telescopes Will Hunt for Next Around This Star
To test the giant-planet-perturber scenario, astronomers will sift through Gaia’s precision astrometry for subtle stellar wobbles and search for faint, cold companions with the mid-infrared instruments aboard the James Webb Space Telescope.
Further continued spectroscopy at Keck and other large telescopes will monitor how the atmospheric metals evolve with time, to limit the accretion rate and size distribution of infalling fragments.
White dwarfs are not graveyards so much as forensic labs, researchers at institutions such as the Space Telescope Science Institute and the Université de Montréal wrote. This one demonstrates that long after the fire dies, the embers can just keep moving furniture around — and occasionally swallow a planet whole.