Webb survey of extreme debris discs sorts planetary collisions by impact energy

Early in the history of the Solar System, a Mars-sized object called Theia struck the infant Earth, vapourising large amounts of rock and throwing 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, gaining clues to the amount of energy in their collisions. The results offer insights into the composition and evolution of these chaotic systems.

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

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

Theoretical predictions suggest many extreme debris discs should be observed, but observations indicate that these environments are rare. Based on the data collected so far, scientists estimate that only about 1% of young stars show observable signatures of this phase, possibly including the Solar System during its formation. Despite that rarity, the team compiled a sample of 21 extreme debris discs: five from Spitzer's archival data and 16 from Webb, made up of 12 newly observed discs and follow-up observations of 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 three were revealed by mid-infrared spectra from Webb and Spitzer.

To find what drives these qualities, the team studied the mineralogical makeup of the discs. They found that their sample could be divided into silica-rich and silica-poor discs. Volcanic glass such as obsidian is one example of silica-rich material found on Earth, while the silica-poor mineral forsterite appears as green sand grains. An extreme debris disc's category carries 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."

About one-third of the sample is silica-rich, suggesting these discs are produced by high-energy impacts between Mars-sized bodies in which a significant portion of the material is vapourised. The remaining two-thirds is silica-poor, indicating that the collisions are occurring on smaller scales, such as grazing impacts 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.

The findings can be applied to the Solar System, which may have gone through 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 planetary system's formation. That period fits the ages of the silica-rich extreme debris discs observed so far, and matches the estimate that Earth and the Moon formed around 100 million years after the Sun, with the Moon likely the result of a collision between Earth and a Mars-sized object.

As for whether the Sun went through 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 the 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 seen in extreme debris discs.

Coauthor Attila Moor of Konkoly Observatory noted that much about these discs remains unknown.

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