The Kilometer Barrier: Only Large Chunks Survive from Asteroids ✦

Original: "Size limits on tidal debris around white dwarfs: the km-size barrier"

· Jordan K. Steckloff, Dimitri Veras, Kathryn Volk

arXiv:2606.02457v1 · 2026-06-01 · CC BY 4.0 · ⏱ 1 min · Exoplanets Stellar

Asteroids near white dwarfs don’t crumble to dust — they first shatter into kilometer-sized pieces.

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When a star turns into a white dwarf — a super-dense remnant the size of Earth — its tidal pull rips apart passing asteroids and comets from the remnants of planetary systems. It’s like squeezing a dry cookie in your fist: it breaks into large chunks, not crumbs.

The force holding particles together keeps fragments from becoming smaller than a few hundred meters — giving rise to a “kilometer barrier.” Only after countless collisions inside a dense swarm do they gradually grind down into cosmic dust.

The fragments move almost in sync — their speeds differ by a millionth, slower than fingernail growth — so the swarm doesn’t spread out for centuries.

This dust factory explains the star’s flickering: sometimes dust eclipses it. Astronomers track this via light analysis, brightness measurements, and eclipse observations. The James Webb Space Telescope will check the details. By the way, a white dwarf can’t be heavier than 1.4 Suns — a limit discovered by Chandrasekhar.

🎯 In the Solar System, sungrazing comets, as they approach the Sun, also break apart into kilometer-sized chunks — consistent with the model.

\frac{GM\rho s^4 r}{(r^2 - s^2/4)^2} = \sigma s^2 + G\rho^2 s^4

The left-hand side — the tidal force tearing apart a fragment of size s at distance r from a white dwarf of mass M; the right-hand side — the binding sum of cohesion (strength σ) and self-gravity. From this balance arises the kilometer-scale minimum size of a stable fragment.

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The Kilometer Barrier: Only Large Chunks Survive from Asteroids ✦