Webb Finds Two Types of Planet-Shattering Collisions

Billions of years ago, a Mars-sized world slammed into the infant Earth so hard it vaporized rock and flung debris into space, material that eventually became the Moon. For a long time, scientists could only theorize about that event. Now, thanks to NASA’s James Webb Space Telescope, they can watch something similar happening around other stars.

A Rare Window Into Violent Youth

As stars age, the material surrounding them evolves from a young, gas-rich protoplanetary disk, where planets can form, into an older, gas-poor debris disk. NASA’s retired Spitzer Space Telescope previously identified a distinctive subclass within that debris disk stage, known as extreme debris disks, systems carrying unusually large amounts of warm dust close to the star, in roughly the same region where rocky planets like Earth and Mars orbit in our own solar system. A team led by Kate Su of the Space Science Institute in Boulder, Colorado, used Webb to study these objects in far greater detail than ever before.

Rarer Than Expected

Theoretical models predicted astronomers should find plenty of extreme debris disks, but observations tell a different story. Scientists now estimate that only about 1 percent of young stars show observable signs of this phase, a stage our own solar system may well have passed through during its early formation. Even with that rarity working against them, Su’s team managed to compile a sample of 21 extreme debris disks in total, combining 5 systems from Spitzer’s archival data with 16 observed by Webb, including 12 brand-new detections and follow-up observations on 4 of the original Spitzer targets. Su said this is the first time researchers have gathered enough systems to truly understand this subclass, since earlier information was far more limited.

Two Flavors of Cosmic Wreckage

Using Webb’s mid-infrared spectra, the team confirmed that extreme debris disks share three defining traits: smaller dust grains than typical protoplanetary or classic debris disks, a notably high concentration of warm dust, and irregular changes in brightness over time. Digging into the mineral makeup of these disks revealed a clear split into two categories. Silica-rich disks contain material similar to volcanic glass, like the obsidian found on Earth, while silica-poor disks are dominated by a mineral called forsterite, the same green mineral that gives some Hawaiian beaches their distinctive sand color. Agnes Kospal of the Konkoly Observatory in Budapest, a coauthor of the study, said being able to directly identify the composition of these otherwise impossible-to-study planetary embryos through Webb’s spectral data was the most exciting part of the research for her.

What the Debris Reveals About the Collision

About one-third of the sample turned out to be silica-rich, a composition the team links to high-energy impacts between Mars-sized bodies violent enough to vaporize a significant portion of the colliding material. The remaining two-thirds were silica-poor, pointing instead to smaller-scale, grazing collisions between Moon-sized objects. The two categories also behave differently over time: silica-rich disks only appear around stars younger than 300 million years, while silica-poor disks persist across a much broader range of ages and tend to show greater variability in brightness, a pattern the researchers attribute to ongoing orbital changes and repeated impacts gradually reshaping fresh debris.

What This Means for Our Own Solar System

The silica-rich disks’ age range lines up well with current models suggesting Earth and the Moon formed roughly 100 million years after the Sun itself, through a collision much like the one that produced silica-rich debris in these other systems. Whether the early solar system also passed through a silica-poor phase remains an open question, but if older, silica-poor disks and their irregular brightness do reflect genuine orbital instability, that would broadly support the Late Heavy Bombardment hypothesis, the idea that migrating giant planets gravitationally disrupted smaller bodies and triggered a wave of catastrophic collisions. Attila Moor of the Konkoly Observatory, another coauthor, noted that the team currently has only three disks old enough to directly test whether silica-rich systems truly disappear with age, and said observing more such systems would help confirm the pattern.

Sources

This report draws on NASA’s own announcement of the study by Kate Su and colleagues, published in The Astrophysical Journal.