Astronomers led by Kate Su of the Space Science Institute in Boulder, Colorado, have assembled 21 extreme debris disks, five from the archive of NASA’s retired Spitzer Space Telescope and 16 from the James Webb Space Telescope, and read the dust in them for signs of how violent the collisions behind it were. About a third of the disks carry the chemical signature of impacts between bodies the size of Mars.
The other two-thirds point to something gentler: glancing blows between objects about the size of the Moon. The results appeared in The Astrophysical Journal on October 1.
Twenty-one disks, two chemistries
An extreme debris disk is a ring of dust around a young star that is far brighter, in warm infrared light, than normal debris should be. In NASA’s account of the study, the 16 Webb systems break down into 12 newly observed stars plus follow-up observations of four Spitzer targets, which is how the sample reaches 21 without double-counting. The team used mid-infrared spectroscopy to read the dust, and three traits turned up across the sample: dust grains smaller than in ordinary debris disks, warm dust packed close to the star, and brightness that wobbles irregularly.
Su put the value of the collection plainly. In her words, it is the first time enough of these systems have been gathered together to truly understand the subclass she calls extreme debris disks.
The sorting variable was silica. Roughly one third of the disks are silica-rich, with dust resembling volcanic obsidian, and the remaining two thirds are silica-poor, dominated by minerals such as forsterite. Co-authors Agnes Kospal and Attila Moor of Konkoly Observatory in Budapest worked on the paper with Su. A NASA graphic of disk composition against stellar age, credited to Joseph Olmsted of the Space Telescope Science Institute, plots the two groups as black dots for silica-rich and purple for silica-poor.
Mars-sized impacts and Moon-sized grazes
The Mars-sized result belongs to the silica-rich minority. According to the team, those disks come from high-energy impacts between Mars-sized bodies, hits violent enough to vaporize a large share of the rock, which then recondenses into silica-rich dust. They appear only around stars younger than 300 million years. The size of the bodies is inferred from the chemistry and energy of the dust, not photographed: no one has watched two Mars-sized worlds meet.
Silica-poor disks are the majority, and they tell a quieter story. They arise from less energetic encounters between Moon-sized objects, and they persist across a broad range of stellar ages with larger swings in brightness, which Nanowerk’s write-up of the release reads as ongoing smaller-scale collisions rather than a single catastrophe. Mars-sized impacts therefore account for roughly one disk in three in this sample, with smaller bodies behind the rest.
The Mars-sized scale is not arbitrary. Newswise’s copy of the release links it to the leading theory that Earth was struck by a Mars-sized body called Theia early in its history, a collision that threw out the material that formed the Moon.
Early solar system parallels in the dust
Su tied the sample to the solar system’s own history. Nanowerk quotes her saying that how rocky planets formed and giant planets evolved are part of the broader story of the solar system’s formation. The silica-rich disks line up with Earth’s formation, which took roughly 100 million years after the Sun formed, while the silica-poor disks, with their flickering brightness, suggest orbital instability of the kind described in the Late Heavy Bombardment hypothesis, in which migrating giant planets are thought to have flung rocky debris inward.
Earlier searches had turned up individual cases. The catalogue of candidate extrasolar collisions includes HD 172555, a star about 23 million years old whose silica dust and gas have been attributed to a giant hypervelocity collision, and NGC 2547-ID8, where observers inferred a likely grazing or hit-and-run event. Those were single systems studied one at a time. The new work lines up 21 of them under a shared chemical test, so a disk can be classed as a Mars-scale or Moon-scale event from its spectrum alone.
What the study cannot yet say is how often each kind of collision happens across all young stars, since the 21 disks were selected because they are extreme. Whether the one-third and two-thirds shares would hold in a larger sample is not something a set of 21 can settle, and the release makes no such claim. The retired observatory still matters here: NASA’s Spitzer mission page notes that archived data from the telescope, which operated from 2003 until it was decommissioned on January 30, 2020, continues to yield discoveries, and five of these 21 disks come straight from that archive. Webb’s follow-up of four Spitzer targets means some of those same stars have now been observed by both telescopes.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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