JWST peers at planetary smashups that may explain our moon

Using NASA's James Webb Space Telescope, astronomers have surveyed 21 young, violent planetary systems to learn what happens when worlds crash together. The findings, reported by Space.com, could shed light on the hypothetical collision between Earth and a Mars-sized body called Theia, which may have created the moon.

The team, led by Kate Su of the Space Science Institute in Boulder, Colorado, studied systems called extreme debris disks. These are protoplanetary disks with unusually large amounts of warm dust close to their young star, in the region where rocky planets form. They are rare; scientists estimate they appear around just 1% of stars. Our sun may have been one of them.

Before JWST, astronomers had little to go on. The now-retired Spitzer Space Telescope had spotted a handful of these disks, but the sample was too small to understand. The new survey, published in The Astrophysical Journal on Oct. 1, includes follow-ups on four of five extreme debris disks from Spitzer's archive, plus 16 from JWST, 12 of them newly observed.

"This is the first time we have gathered enough systems to truly understand this subclass that we call extreme debris disks," Su said in a statement, as reported by Space.com. "Before the JWST, we had limited information."

The astronomers found three things these disks have in common: they contain smaller dust grains than classic protoplanetary disks, a high concentration of warm dust, and irregular variations in brightness. To figure out what drives those features, the team examined the minerals in each disk. They split into two groups: silica-rich and silica-poor.

Silica-rich disks, the researchers think, come from high-energy impacts of Mars-sized bodies, collisions violent enough to vaporize most of the material. Silica-poor disks come from gentler grazing impacts between roughly moon-sized objects. About a third of the surveyed disks are silica-rich, and those appeared only around stars younger than 300 million years. Silica-poor disks show up around stars of all ages and vary more in brightness, likely because fresh debris is still being churned by more collisions.

Apply that to our own solar system, and a picture emerges. Simulations show the terrestrial planets should form within the first few hundred million years, which matches the ages of the silica-rich disks and lines up with the estimated formation of Earth and the moon roughly 100 million years after the sun formed. The solar system likely also went through a silica-poor phase during the Late Heavy Bombardment, when collisions may have pushed the gas giants outward.

Team member Agnes Kospal of Konkoly Observatory in Budapest called the JWST spectra the most exciting part of the work. "We have no other way to study these planetary embryos directly because they are too small," she said in the same statement.

The research is published in The Astrophysical Journal. As Su put it: "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."

• extreme debris disks
• JWST
• moon
• planetary collisions
• planet formation
• Theia