[ESA]. 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.
A team of researchers compiled the largest sample of extreme debris discs to date thanks to data from the NASA/ESA/CSA James Webb and NASA’s Spitzer space telescopes. These young stellar systems are a subclass of debris discs, which follows the juvenile protoplanetary disc stage. Interested in the mineralogical makeup of these young stellar systems, the team separated their sample into silica-rich and silica-poor. Based on their categorisation, they were able to infer the type of collisions that are helping create these chaotic, dusty environments. Of their sample, eight are silica-rich discs (black dots), suggesting that they are produced by high-energy impacts between Mars-sized bodies where a substantial amount of the material is vaporised. Thirteen discs fall into the silica-poor category (purple dots), indicating that the collisions within these discs are less intense in nature and occur between Moon-sized objects. The team also noted the corresponding age of each disc’s star and spotted an interesting trend: The silica-rich discs in their sample are found only around stars younger than 300 million years. The stars with silica-poor discs span a broad range of ages. The study’s findings are shaping scientists’ understanding of our own Solar System, which may have experienced more than one extreme debris disc phase. Simulations suggest that terrestrial planets should form within the first few hundred million years. This period aligns with the distribution of the silica-rich extreme debris discs and with astronomers’ 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. The distribution of the silica-poor discs is broadly consistent with the Late Heavy Bombardment hypothesis for our Solar System, which proposes that the gas giant planets migrated significant distances and gravitationally disrupted the orbits of smaller bodies. Due to their movements, catastrophic collisions occurred and generated the short-lived, dust-rich phases observed in extreme debris discs. [Image description: Graphic titled Extreme Debris Discs, Composition Across Time showing a plot and corresponding timeline of the Solar System. The plot’s y-axis is labeled Silica with an up arrow labeled rich and a down arrow labeled poor. The x-axis is labeled Age (millions of years) and starts with 1 at the left and increases by factors of ten, ending with 1000 at right. A key at right has 3 symbols: black dot is Silica-rich disc, purple is Silica-poor disc, and orange is Protoplanetary disc. All 27 orange dots are within the first 10 million years and range in silica composition. The 8 black and 13 purple dots begin to appear around 10 million. The black dots stop around 100 million. The purple dots continue right. The timeline below has the same labels as the plot’s x-axis. A gray band before 100 marks the Moon-forming impact. A gray band before 1000 marks the Late heavy bombardment. 3 blue bands stretch from left to right: Giant planet formation, Terrestrial planet formation, and Giant planet migration/orbital instability.]
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.”
Extreme Debris Disks: Insights into Violent Collisions in Planet Formation and Destruction, The Astrophysical Journal (open access)
Astrobiology,
