Webb Studies 21 'Extreme Debris Disks' Where Planetary Embryos Smash Together

Astronomers using NASA's James Webb Space Telescope have assembled the largest sample yet of a rare kind of young star system where planet-sized bodies appear to be smashing into one another. The results, published Thursday in The Astrophysical Journal, offer a rough accounting of the energy in those collisions and a glimpse of what the early solar system may have looked like.

Scientists theorize that early in the solar system's history a Mars-sized object called Theia struck the infant Earth, vaporizing huge amounts of rock and blasting it into space. Some of that material coalesced into the moon. The new work looks at distant systems that show signs of similar upheavals.

A star's surroundings change as it ages. It begins with a young, gas-rich protoplanetary disk where planets can form, and later evolves into a gas-poor debris disk. NASA's retired Spitzer Space Telescope studied the debris disk stage and found a subclass called extreme debris disks. These systems hold unusually large amounts of warm dust close to the star, in the region comparable to where rocky planets orbit in our own solar system.

A team led by Kate Su of the Space Science Institute in Boulder, Colorado, examined these objects with Webb. Theory predicts we should see many of them, but observations show they are rare. Based on the data so far, scientists estimate only about 1% of young stars show signs of this phase. The team compiled a sample of 21 extreme debris disks, five from Spitzer's archive and 16 from Webb. Twelve of the Webb disks were newly observed, and four were follow-ups of Spitzer targets.

"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."

The team confirmed that the disks share three properties: smaller dust grains than those in protoplanetary or classic debris disks, a high concentration of warm dust, and irregular swings in brightness. All were revealed by mid-infrared spectra from Webb and Spitzer.

To find the cause, the researchers studied the minerals in the dust. The sample split into silica-rich and silica-poor disks. Volcanic glass such as obsidian is a silica-rich material on Earth, while the silica-poor mineral forsterite appears as green sand on some Hawaiian beaches. About one-third of the sample is silica-rich, which suggests high-energy impacts between Mars-sized bodies that vaporize a significant share of the material. The other two-thirds are silica-poor, pointing to smaller-scale collisions such as grazing impacts between moon-sized objects.

Silica-rich disks turned up only around stars younger than 300 million years. Silica-poor disks persist across a wide range of ages and often vary more in brightness, which the team attributes to fresh debris evolving quickly through orbital changes and further impacts.

"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 co-author Agnes Kospal of Konkoly Observatory in Budapest. "We have no other way to study these planetary embryos directly because they are too small."

The timing fits what we know of Earth. Simulations suggest terrestrial planets form within the first few hundred million years, and Earth and the moon formed about 100 million years after the sun. The authors say the sun may have gone through more than one such phase, and that older silica-poor disks would be broadly consistent with the Late Heavy Bombardment hypothesis. Co-author Attila Moor said the team expects no silica-rich systems among older disks but has only three older examples so far, so more observations are needed.