Asteroids venturing near white dwarfs don’t break directly into dust, study reveals

When asteroids travel too close to white dwarfs, the dense remnants of stars that have exhausted their nuclear fuel, powerful gravitational forces can tear them apart. However, new research from Planetary Science Institute (PSI) suggests that these asteroids don’t immediately break into dust. Instead, most of the mass initially remains in larger fragments, revealing a previously overlooked step in how debris forms around dying stars.

The study, led by PSI senior scientist Jordan Steckloff, identifies a “kilometer-size barrier” that limits just how much asteroids can break apart when subjected to the powerful gravitational forces of white dwarfs.

White dwarfs are often surrounded by disks of planetary debris, including dust from asteroids and other objects pulled apart by the star’s gravity. Previous models have generally assumed that these objects behave like loosely bound piles of rubble with no internal strength. This allows them to break into increasingly smaller pieces and eventually turn into dust.

However, those models did not take into account that rubble-pile asteroids have some strength. Tiny attractive forces between their grains and particles help hold them together, preventing them from breaking apart into smaller pieces.

“Nothing is truly strengthless,” Steckloff said. “Tiny attractions between grains called van der Waals forces give comets and asteroids a minimum strength 10 to 100 times weaker than pulling apart freshly fallen snow.”

While using a physical model that accounts for an asteroid’s internal strength, density and the gravitational forces acting on it, Steckloff and his team found that most debris mass remains in pieces approximately 0.1 to 1 kilometer (0.06 to 0.6 miles) in size.

These fragments are too large to be significantly affected by Poynting-Robertson drag, a process in which radiation from a star gradually causes small particles to spiral inward. Instead, the fragments must undergo additional events or processes, such as collisions or rotational breakup, before they can become the fine dust that we observe around white dwarfs.

There have been examples of these findings in our own solar system. When Comet Shoemaker-Levy 9 traveled near Jupiter in 1992, the planet’s gravity tore it into these large ~100–1,000 m (330–3,300 feet) fragments. Those large fragments later collided with the planet. Similar tidal fragmentation has also produced families of comets that pass extremely close to the sun.

The study also found that asteroid fragments remain on nearly identical orbits after breaking apart. These orbits are potentially keeping debris more concentrated around white dwarfs than previous models predicted.

“Understanding the details of how debris around white dwarfs is produced and evolves is important for putting the observations into context,” says co-author Kat Volk. “This will help us understand what happens to the small bodies in orbit around a star as it undergoes the end stages of its life.”

The paper, “Size Limits on Tidal Debris around White Dwarfs: The Kilometer-size Barrier,” is published in The Astrophysical Journal Letters. Co-authors include PSI senior scientist Volk and Dimitri Veras of the University of Warwick.

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