Webb sorts 21 extreme debris disks by the violence of the crashes that made them

In this study, collision energy is read from the minerals. Kate Su's team at the Space Science Institute in Boulder, reporting in The Astrophysical Journal, sorted each disk's dust into two groups, silica-rich and silica-poor ^[1][5]. On Earth, volcanic glass such as obsidian is silica-rich, while forsterite, a silica-poor mineral, forms the green sand on some Hawaiian beaches ^[6]. The team takes silica-rich dust as the product of high-energy impacts between Mars-sized bodies that vaporize a significant portion of the material ^[7]. The silica-poor two-thirds of the sample point to smaller collisions, such as grazing encounters between Moon-sized objects ^[8].

So the energy estimate comes in two bins. NASA describes the result as clues to the amount of energy in these collisions. The release does not put a figure on that energy in physical units ^[11]. About a third of 21 disks is roughly seven silica-rich systems, against about fourteen silica-poor ones ^[1]. The finding that silica-rich disks circle only stars younger than 300 million years rests on that group of about seven ^[9][1]. An age cutoff in a sample that small cannot, on its own, separate two cases: vaporizing impacts that stop after a star's youth, and silica-rich debris that stops being visible.

Su credits Webb with the sample size. "This is the first time we have gathered enough systems to truly understand this subclass that we call extreme debris disks," said Su, the paper's lead author ^[14]. "Before Webb, we had limited information," she said ^[19]. The dust in these disks is warm and sits close to the star, in the region comparable to where rocky planets orbit in our solar system ^[17]. "To just see their mid-infrared emission and beautiful spectral features with Webb, which allowed us to identify their compositions, was the most exciting thing for me," said Agnes Kospal of Konkoly Observatory in Budapest, a coauthor ^[15]. "We have no other way to study these planetary embryos directly because they are too small," she said ^[16].

The connection to the Moon comes through theory. Scientists theorize that a Mars-sized object called Theia hit the infant Earth and vaporized enough rock that some of it coalesced into the Moon ^[12]. The team's silica-rich disks match that scenario in impactor size and in outcome ^[7][12]. The Webb data describe other young stars. Using them to explain the Moon means treating those systems as stand-ins for the early solar system. The team makes that step, and says our solar system may have gone through more than one extreme debris disk phase ^[13].