weic2618 — Science Release

Webb provides crash course on planet-shattering collisions

1 October 2026

In the early history of our Solar System, a Mars-sized object called Theia smashed into the infant Earth, vapourising massive amounts of rock and blasting it into space. Some of that material coalesced into the Moon. Astronomers have now used the NASA/ESA/CSA James Webb Space Telescope to examine a class of young stellar systems that show signs of similar upheavals, providing clues to the amount of energy in their collisions. The results offer insights into the composition and evolution of these chaotic systems.

The environment surrounding a star changes as it ages, beginning with a juvenile, gas-rich protoplanetary disc where forming planets can reside and ending with a gas-poor debris disc. During its mission lifetime, NASA’s retired Spitzer Space Telescope examined the debris disc stage in even more detail, discovering a subclass termed extreme debris discs. These systems harbour unusually large amounts of warm dust close to the star, in the region comparable to where rocky planets orbit in our Solar System. A team of astronomers led by Kate Su of the Space Science Institute in Boulder, Colorado investigated these intriguing objects with Webb.

The team’s findings have been published in The Astrophysical Journal.

Contrary to theoretical predictions, which suggest we should observe many extreme debris discs, observations indicate that these environments are rare. Scientists estimate roughly only 1% of young stars show observable signatures of this phase based on the data collected so far, including possibly our own Solar System during its formation. Despite their rarity, the team was able to compile a sample of 21 extreme debris discs, including five from Spitzer’s archival data and 16 from Webb, with 12 newly observed discs and follow-up observations on four of Spitzer’s.

“This is the first time we have gathered enough systems to truly understand this subclass that we call extreme debris discs,” 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 discs that we know, like Vega and Fomalhaut. Now that we have more data, we can pin down what these discs represent for planet formation and evolution.”

The team confirmed that extreme debris discs share three key properties: smaller dust grains than those in protoplanetary or classic debris discs, a high concentration of warm dust, and irregular brightness variations, all revealed by mid-infrared spectra from Webb and Spitzer.

To determine the driving factor for these qualities, the team studied the mineralogical makeup of the discs. They found that their sample could be categorised into silica-rich and silica-poor discs. Volcanic glass like obsidian is one example of silica-rich material found on Earth, whereas the silica-poor mineral forsterite appears as green sand grains. An extreme debris disc’s category relays information on the type of collisions producing the impact debris and may help account for its variability 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.”

Of their sample, about one-third is silica rich, suggesting these discs are produced by high-energy impacts between Mars-sized bodies where a significant portion of the material is vapourised. The remaining two-thirds of their sample is silica-poor, indicating that the collisions are occurring on smaller scales, like grazing, between Moon-sized objects. Silica-rich discs are found only around stars younger than 300 million years, while silica-poor discs persist across a broad range of ages and often show greater brightness variability. The team proposes that this variability is driven by the rapid orbital and collisional evolution of fresh debris produced by multiple impacts.

Their findings can be applied to our own Solar System, which may have experienced more than one extreme debris disc 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 discs helps us bring together the big picture of what we currently understand.”

Simulations suggest that terrestrial planets, such as Earth, should form within the first few hundred million years of a Solar System’s formation. This period fits with the ages of silica-rich extreme debris discs observed so far and aligns with the estimation that Earth and the Moon formed around 100 million years after the Sun formed, with the Moon likely being the result of a collision between Earth and a Mars-sized object.

As for whether our Sun underwent a silica-poor extreme disc phase, if older silica-poor discs and their random intervals of infrared brightness do reflect orbital instability, this would be broadly consistent with the Late Heavy Bombardment hypothesis for our Solar System. In that scenario, the gas giant planets migrated significant distances, gravitationally disrupting the orbits of smaller bodies and triggering catastrophic collisions that generated the short-lived, dust-rich phases observed in extreme debris discs.

“Of course, there’s many things we still don’t know about these discs,” said Attila Moor of Konkoly Observatory, a coauthor of the study. “We expect no silica-rich systems among older extreme debris discs. We only have three discs in our sample that fit that age criteria, so it’ll be nice to observe more of these systems to confirm our hypothesis.”

More information

Webb is the largest, most powerful telescope ever launched into space. Under an international collaboration agreement, ESA provided the telescope’s launch service, using the Ariane 5 launch vehicle. Working with partners, ESA was responsible for the development and qualification of Ariane 5 adaptations for the Webb mission and for the procurement of the launch service by Arianespace. ESA also provided the workhorse spectrograph NIRSpec and 50% of the mid-infrared instrument MIRI, which was designed and built by a consortium of nationally funded European Institutes (The MIRI European Consortium) in partnership with JPL and the University of Arizona.

Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).

Image Credit: NASA, ESA, CSA, J. Olmsted (STScI)