Asteroids torn apart near white dwarfs may leave most of their mass in fragments about 100 meters to 1 kilometer across.A physical model shows that weak attractions between grains can halt repeated tidal breakup before the debris becomes dust.Collisions or rotational breakup must further process these fragments, adding an intermediate step to models of stellar debris disks.<\/p>\n
An asteroid can survive its own destruction in surprisingly large pieces. Near a white dwarf, gravity may tear the original body apart while leaving most of its mass in fragments hundreds of meters across. Turning that wreckage into the dust astronomers observe requires another stage of destruction.<\/p>\n
The findings concern the debris disks surrounding some white dwarfs, the dense stellar remnants left after stars exhaust their nuclear fuel. These disks contain material from disrupted planetary bodies. Understanding how that material becomes dust is essential for reconstructing what happens to planetary systems after their stars die.<\/p>\n
Model of tidally elongated asteroid. We model an inwardly scattered planetesimal as a rectangular prism comprising two cubes, which simplifies the problem while providing a conservative estimate of the minimum characteristic stable size. (CREDIT: Jordan Steckloff et al, The Astrophysical Journal Letters 2026) Why weak rubble still holds together<\/p>\n
Many previous models treated asteroids as rubble piles without internal strength. In that picture, gravity holds the original collection together, but its individual components have no additional cohesion. A close encounter can therefore initiate repeated fragmentation without accounting for forces that bind neighboring grains.<\/p>\n
Real rubble piles are more complicated. Tiny attractions called van der Waals forces act between their particles, giving even loose material some resistance to being pulled apart. These forces are weak, yet their contribution becomes increasingly important as an object breaks into smaller pieces.<\/p>\n
\u201cNothing is truly strengthless,\u201d Steckloff said. \u201cTiny attractions between grains called Van der Waals forces<\/a> give comets and asteroids a minimum strength 10 to 100 times weaker than pulling apart freshly fallen snow.\u201d Such modest cohesion can substantially change the outcome of a destructive encounter.<\/p>\n The destructive force comes from a difference in gravitational pull. The side of an asteroid nearer the white dwarf feels a stronger attraction than the far side. These tidal forces stretch the body, and it breaks when that stretching overcomes its self-gravity and internal strength.<\/p>\n Calculating where fragmentation stops<\/p>\n The team built a physical model balancing those competing forces. It represents an elongated body as two joined cubes, allowing the researchers to calculate the pull separating its halves. They then determine which fragment sizes can remain stable at different distances from the star.<\/p>\n Researchers found that the size of asteroid fragments near white dwarfs depends largely on how strongly the asteroid\u2019s materials hold together and how close it gets to the star. Even the weakest objects would break into fragments a few hundred meters (several hundred feet) across. (CREDIT: Jordan Steckloff et al, The Astrophysical Journal Letters 2026) <\/p>\n For the main calculations, the researchers used a white dwarf with 60% of the Sun\u2019s mass. They tested material densities spanning ice to iron and cohesive strengths of 10 and 1,000 pascals. A pascal measures stress, or force applied across an area, in this case the resistance to separation.<\/p>\n The calculations produced characteristic fragment sizes of roughly 100 meters for the weaker material and 1 kilometer for the stronger material. Most of the disrupted body\u2019s mass would remain in pieces around these sizes. The result concerns where the mass resides, rather than a claim that no smaller particles form.<\/p>\n The \u201cbarrier\u201d also is not a universal minimum size. Stable sizes depend on material properties and distance from the white dwarf, with stronger tides<\/a> permitting smaller surviving fragments closer in. At sufficiently high temperatures, material can instead be destroyed by sublimation, the direct transition from solid to gas.<\/p>\n The calculation assumes the body spends enough time near its closest approach for breakup to occur. It also aligns the long axis toward the star, maximizing the tidal pull. These choices help isolate the physics, but they do not reproduce every possible asteroid shape, orientation or encounter history.<\/p>\n Dust needs another destructive step<\/p>\n This distinction matters because large fragments and fine dust respond differently to starlight. Poynting-Robertson drag gradually removes orbital energy from small particles, drawing them inward. Fragments hundreds of meters across are too large for that process to significantly shape their immediate evolution.<\/p>\n Models show how a white dwarf\u2019s gravity breaks apart asteroids into debris ranging from roughly 100 to 1,000 meters, depending on the material\u2019s strength. The size of the fragments also varies with their distance from the star. (CREDIT: Jordan Steckloff et al, The Astrophysical Journal Letters 2026) <\/p>\n Models therefore need to include the stage between tidal breakup and dust migration. Collisions could grind the larger pieces down, producing particles that radiation can influence more effectively. Another possibility is rotational breakup<\/a>, in which fragments spin fast enough to pull themselves apart.<\/p>\n The relative importance of these mechanisms remains uncertain. Radiation and escaping gas can alter an object\u2019s spin, but their effectiveness depends on conditions such as stellar luminosity and material composition. The study identifies the need for further processing without establishing one route for every debris disk.<\/p>\n Old, cool white dwarfs provide an important constraint. Some retain dusty disks despite being too faint for strongly heating-driven processes to remain equally effective. Their existence supports the importance of mechanisms such as collisions that can continue after the stellar remnant has cooled substantially.<\/p>\n A familiar example near Jupiter<\/p>\n A related breakup happened much closer to home. Comet Shoemaker-Levy 9<\/a> passed near Jupiter in 1992, when the planet\u2019s tidal forces tore it into large fragments. Those pieces later struck Jupiter in 1994, giving astronomers an unusually detailed view of a disrupted small body.<\/p>\n Estimates placed the comet\u2019s major fragments at approximately 100 to 1,000 meters across. That range is consistent with the characteristic sizes in the white dwarf calculations. The comparison offers a useful physical parallel, although a comet encountering Jupiter does not directly verify the model around another star.<\/p>\n Models show that asteroid fragments torn apart by a white dwarf experience tiny changes in orbital speed, typically around one part per million. As a result, the debris continues traveling along nearly identical orbits. (CREDIT: Jordan Steckloff et al, The Astrophysical Journal Letters 2026) <\/p>\n The researchers also discuss the Kreutz family of comets that pass extremely close to the Sun. Many known members are tiny, but most of the family\u2019s mass resides in larger bodies. That distinction reinforces why counting fragments alone can give a misleading impression of how thoroughly an object was destroyed.<\/p>\n