Astronomers call it an extreme debris disk: a violent stage that only around 1% of young stars appear to show any trace of, even though theory predicts it should be more common. A new survey using NASA's James Webb Space Telescope doesn’t resolve that gap, but it does begin to reveal what happens inside these uncommon, dust-filled systems. Some of them appear to be the wreckage left behind when Mars-sized worlds slammed into one another.
The scenario may ring a bell. Scientists believe the early Earth was struck by a Mars-sized body known as Theia. That collision probably vaporized vast quantities of rock and hurled material into space, and some of that debris eventually came together to form the Moon.
Published Oct. 1 in The Astrophysical Journal, the study marks the first time researchers have gathered enough of these systems to examine them as a class rather than as a scattered collection of oddities.
Two kinds of dust, two kinds of crashes
The minerals provide the most revealing result. Once the researchers analyzed the composition of the dust, the disks fell into two broad camps: silica-rich and silica-poor.
About one-third of the sample falls into the silica-rich group. According to the researchers, these systems likely arose from extremely energetic collisions between Mars-sized bodies, with impacts powerful enough to vaporize a substantial amount of rocky material.
The remaining two-thirds are silica-poor. These seem to originate from lower-energy collisions, among them grazing impacts between Moon-sized objects.
Earthly rocks offer a sense of the contrast. Obsidian and other volcanic glass is silica-rich. Forsterite, a silica-poor mineral, appears as green sand grains on certain Hawaiian beaches.
A disk’s silica content reveals something about the collision that produced its debris. It could also account for why some disks vary more dramatically in infrared brightness than others.
“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, Hungary, a coauthor of the study. “We have no other way to study these planetary embryos directly because they are too small.”
Age is the tell
The division follows a pattern. So far, silica-rich disks have been detected only around stars younger than 300 million years. Silica-poor disks, by contrast, appear around stars spanning a far broader range of ages, and they typically display stronger brightness changes.
The team proposes that this flickering stems from freshly produced debris evolving rapidly. Changes in the material’s orbit, along with additional collisions, could drive the infrared brightness up and down over time.
This age boundary is what brings the Moon back into the picture. Computer simulations indicate that terrestrial planets, Earth among them, should form within the first few hundred million years after a solar system begins taking shape. That timeframe lines up with the ages of the silica-rich disks observed to date. It is also consistent with estimates that Earth and the Moon formed roughly 100 million years after the Sun, with the Moon probably born from a collision between Earth and a Mars-sized object.
Our solar system may have done this twice
The silica-poor disks prompt a separate question: did the Sun once pass through that stage as well?
Should the older silica-poor disks and their apparently random bursts of infrared brightness stem from orbital instability, the pattern would be broadly in line with the Late Heavy Bombardment hypothesis. Under that scenario, the giant planets shifted significantly from their birthplaces. Their migration disrupted the orbits of smaller bodies, setting off catastrophic collisions and short, dust-laden episodes resembling what astronomers now observe in extreme debris disks.
That means our solar system might have gone through more than one extreme debris disk phase. It remains a hypothesis rather than a finding, and the researchers treat it with caution.
“How rocky planets formed and giant planets evolved are part of the broader story of the solar system’s formation. It’s all one story,” said Kate Su of the Space Science Institute in Boulder, Colorado, who led the team. “Our work on extreme debris disks helps us bring together the big picture of what we currently understand.”
What makes a disk “extreme”
A bit of context is useful. The material surrounding a star evolves as the star gets older. Young stars begin with a gas-rich protoplanetary disk in which planets can form, and that setting later turns into a gas-poor debris disk.
NASA’s Spitzer Space Telescope, now retired, examined those debris disks and identified an unusual category. Extreme debris disks contain exceptionally large quantities of warm dust near their stars, in about the same region where the rocky planets of our own solar system orbit.
The new research confirms three defining characteristics. Their dust grains are smaller than those found in protoplanetary or more ordinary debris disks. The disks hold unusually high concentrations of warm dust. And their brightness varies irregularly over time. Webb and Spitzer detected all three via mid-infrared spectra.
Given how rare these systems are, assembling a sample required effort. The team compiled 21 extreme debris disks: five drawn from archival Spitzer observations and 16 examined with Webb. Within the Webb set, 12 were observed for the first time and four were follow-ups on systems Spitzer had previously studied.
“This is the first time we have gathered enough systems to truly understand this subclass that we call extreme debris disks,” Su said. “Before Webb, we had limited information. We knew that they are weird and very different from the typical cold debris disks that we know, like Vega and Fomalhaut. Now that we have more data, we can pin down what these disks represent for planet formation and evolution.”
The sample is still thin
Twenty-one disks represents genuine progress for the field. Still, the argument that older systems shouldn’t be silica-rich relies on very few data points, and the team acknowledges as much.
“Of course, there’s many things we still don’t know about these disks,” said Attila Moor of Konkoly Observatory, a coauthor of the study. “We expect no silica-rich systems among older extreme debris disks. We only have three disks in our sample that fit that age criteria, so it’ll be nice to observe more of these systems to confirm our hypothesis.”
Just three disks — that’s the figure worth tracking. Should Webb spot a silica-rich disk orbiting an older star, the neat connection between high-energy, Mars-sized collisions and the planet-forming era would collapse. If it doesn’t, astronomers will gain a working model of the type of impact that created our Moon, observed unfolding around other stars.