White dwarf tides leave kilometer-size asteroid fragments instead of dust

PSI Senior Scientist Jordan Steckloff presented research Oct. 29, 2026, at 11:45 a.m. PDT at an American Astronomical Society Division for Planetary Sciences press conference, tentatively as the fourth presenter, showing that white dwarf tides leave asteroid fragments roughly 0.1 to 1 kilometer across rather than dust.

The work appears in a paper titled Size Limits on Tidal Debris around White Dwarfs: The Kilometer-size Barrier, published in The Astrophysical Journal Letters. The paper looks at how well white dwarf tides can shatter asteroids after calculations include the strength holding the rocks together. Steckloff said Van der Waals attractions between grains give comets and asteroids a strength at least 10 to 100 times weaker than freshly fallen snow, meaning nothing is truly strengthless. “Nothing is truly strengthless,” Steckloff said.

Typical rubble-pile internal strength is approximately 10 to 1000 pascals, about one million times weaker than solid rock. For a white dwarf with about 0.6 solar masses, strengths of 10 pascals and 1 kilopascal produce fragments of roughly 100 meters and 1,000 meters across, respectively. The kilometer-size barrier represents the characteristic sizes of tidal fragments, and most debris mass is contained in fragments of that size. Fragments in that strength range top out at about 0.1 to 1 kilometer across.

Earlier models assumed white dwarf debris disks form when tidal forces fragment rubble-pile minor planets with zero internal strength. White dwarfs are dense, Earth-sized remnants of Sun-like stars, packing roughly solar mass into a small volume with gravity strong enough to destroy nearby objects. Such dead stars commonly sit amid rings of planetary wreckage, including dust from asteroids torn apart by stellar gravity. White dwarfs were long believed to have no stellar companions, then astronomers found metal-polluted surfaces that they attributed to rocky debris raining down.

Observations showed some white dwarfs surrounded by debris disks, and later found rocky bodies such as small planets, moons, or asteroids in those disks. WD 1145+017 appears to have several small rocky bodies in orbit that are being pulled apart, with debris falling onto the white dwarf’s surface. The fragments are too large to be strongly affected by Poynting-Robertson drag, so they must experience collisions or rotational breakup before becoming the fine dust observed around white dwarfs. Poynting–Robertson drag moves dust inward only if grains are small enough to feel the drag but not so large that radiation pressure drives them away, so fragmentation alone cannot make the finest white-dwarf dust.

After disruption, asteroid fragments remain on practically the same orbits, undergoing only tiny velocity changes of about one part per million to one part per 100 million. Because asteroid fragments remain on nearly identical orbits after breaking apart, the debris may stay more concentrated around white dwarfs than earlier models predicted. Velocity changes during disruption are only tiny fractions, so fragments do not spread widely and could maintain dense debris structures around dead stars.

Comet Shoemaker-Levy 9 passed near Jupiter in 1992 and Jupiter’s gravity tore it into fragments roughly 100 to 1000 meters across, which later collided with the planet. Similar fragmentation made families of comets passing close to the Sun. The report adds that comparable tidal breaking has created groups of comets traveling near the Sun. Co-author Kat Volk said learning how debris forms and evolves around white dwarfs is important for putting observations into context and understanding the end stages of a star’s life.

The authors call the progression from an intact minor planet to dust accreted onto a white dwarf an outstanding challenge, and their Letter constrains the first step of that process. NASA 2XRP grant 80NSSC20K0267 funded the work, and co-authors include PSI Senior Scientist Kathryn Volk and Dimitri Veras of the University of Warwick. The session was available by livestream or to registered attendees who could ask questions.

The team modeled rubble piles made of water ice, iron, and material densities matching the lunar highlands, Vesta, and Earth. Itokawa, Ryugu, and Bennu are the three asteroids scientists have sampled, and all three are rubble piles held together by cohesive strength rather than being strengthless. The force holding rubble-pile asteroids together is the Van der Waals force, a weak short-range interaction that attracts and repulses atoms and molecules. These weak but real forces hold rubble-pile asteroids together and prevent them from shattering into arbitrarily small pieces under white dwarf tides.

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