Scientists think one of the most important events in the early solar system was a colossal collision between the young Earth and a Mars-sized body known as Theia. The impact likely vaporized huge quantities of rock and hurled material into space. Some of that debris eventually came together to form the Moon, where NASA’s Artemis program is returning humans, preparing for Mars, and shaping the future of space exploration.
That ancient impact dramatically altered Earth. Now, astronomers are using NASA’s James Webb Space Telescope to investigate young star systems that appear to be experiencing similarly violent events. By studying these systems, researchers can estimate how energetic the collisions were and learn more about how rocky planets form and evolve.
The team’s findings published Oct 1. in The Astrophysical Journal.
Webb Targets Rare Extreme Debris Disks
The material surrounding a star changes significantly over time. Young stars begin with a juvenile, gas-rich protoplanetary disk where forming planets can reside. As the system matures, that environment develops into a gas-poor debris disk.
During its years of operation, NASA’s retired Spitzer Space Telescope studied these debris disks and identified an unusual category known as extreme debris disks. These systems contain exceptionally large amounts of warm dust close to their stars, roughly in the same region where rocky planets orbit in our own solar system.
Kate Su of the Space Science Institute in Boulder, Colorado, led a team of astronomers that used Webb to investigate these unusual systems in greater detail.
Theoretical models suggest extreme debris disks should be relatively common, but observations tell a different story. Based on data gathered so far, scientists estimate that only about 1% of young stars display observable signs of this stage. Our own solar system may also have passed through such a phase while it was forming.
Even though these systems are uncommon, the researchers assembled a sample of 21 extreme debris disks. Five came from archival Spitzer observations, and 16 were studied with Webb. Of the Webb sample, 12 disks were newly observed, while four were follow-up observations of systems previously examined by Spitzer.
“This is the first time we have gathered enough systems to truly understand this subclass that we call extreme debris disks,” said Su, lead author of the paper. “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.”
Dust Reveals the Nature of Planetary Collisions
The researchers confirmed that extreme debris disks have three defining characteristics. Their dust grains are smaller than those found in protoplanetary or more typical debris disks, they contain unusually high concentrations of warm dust, and their brightness changes irregularly over time. Webb and Spitzer revealed these features through mid-infrared spectra.
To understand what might be producing these unusual traits, the researchers examined the minerals present in the disks. Their analysis showed that the systems could be divided into two broad groups: silica-rich disks and silica-poor disks.
On Earth, volcanic glass such as obsidian is an example of silica-rich material. The silica-poor mineral forsterite, meanwhile, can be seen as green sand grains on certain beaches in Hawaii.
Whether a disk is rich or poor in silica can reveal important information about the collision that created its debris. The distinction may also help explain why some of the disks fluctuate more dramatically in infrared brightness.
“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, and a coauthor of the study. “We have no other way to study these planetary embryos directly because they are too small.”
Mars-Sized Worlds May Be Smashing Together
Roughly one-third of the disks in the sample are silica-rich. The researchers say these systems likely formed after extremely energetic collisions between Mars-sized bodies. Such impacts would be powerful enough to vaporize a substantial amount of rocky material.
The other two-thirds are silica-poor. These systems appear to result from lower-energy collisions, including grazing impacts between Moon-sized objects.
The researchers also found an important age difference between the two groups. Silica-rich disks have only been identified around stars younger than 300 million years. Silica-poor disks, however, occur around stars spanning a much wider range of ages and tend to show stronger changes in brightness.
The team suggests that this variability may come from the rapid evolution of newly created debris. Changes in the material’s orbit, along with additional collisions, could cause the infrared brightness to rise and fall over time.
The findings may also help scientists reconstruct the history of our own solar system, which could have passed through more than one extreme debris disk phase.
“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 Su. “Our work on extreme debris disks helps us bring together the big picture of what we currently understand.”
Clues to the Collision That Formed the Moon
Computer simulations suggest terrestrial planets, including Earth, should emerge within the first few hundred million years after a solar system begins forming.
That timeline matches the ages of the silica-rich extreme debris disks observed so far. It is also consistent with estimates that Earth and the Moon formed roughly 100 million years after the Sun, with the Moon likely being the result of a collision between Earth and a Mars-sized object.
Scientists are also interested in whether the Sun may once have passed through a silica-poor extreme debris disk phase.
If the older silica-poor disks and their seemingly random periods of changing infrared brightness are caused by orbital instability, the pattern would be broadly compatible with the Late Heavy Bombardment hypothesis for our solar system.
Under that scenario, the giant planets shifted significantly from their original positions. Their movement disturbed the orbits of smaller objects, setting off catastrophic collisions and producing brief periods filled with large amounts of dust, similar to what astronomers now observe in extreme debris disks.
“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.”
The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).
