Using NASA's James Webb Space Telescope (JWST), astronomers are investigating turbulent planetary systems in which worlds violently collide. This investigation could shed light on the hypothetical collision between Earth and a Mars-sized planetary body called Theia, which is thought to have spilled matter around our planet that coalesced to forge the moon billions of years ago.
Young planetary systems are notoriously violent, featuring young stars surrounded by disks of gas and dust called protoplanetary disks. Within these disks, small bodies called planetesimals collide and stick together to begin the process of forming planets. However, NASA's now-retired Spitzer Space Telescope discovered that there is a class of protoplanetary disks that are even more violent than usual, so-called extreme debris disks. These systems contain unusually large amounts of warm dust close to the young star, in the region similar to that occupied by rocky planets in the solar system.
Investigating extreme debris disks is challenging, partially because they are so rare. Scientists estimate that such systems exist around just 1% of stars. It is suspected that our sun was one of these rare stars.
A team of scientists set about investigating 21 extreme debris disks with the incredible observing power of the JWST. This included follow-ups on four of the five of these environments from Spitzer's archival data and 16 from the JWST, with 12 newly observed disks
"This is the first time we have gathered enough systems to truly understand this subclass that we call extreme debris disks," team leader Kate Su of the Space Science Institute in Boulder, Colorado, said in a statement. "Before the JWST, 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.
"Now that we have more data, we can pin down what these disks represent for planet formation and evolution."
Extremely violent and just as rare
The researchers discovered that there are three properties common to extreme debris disks. They contain smaller dust grains than those found in "classic" protoplanetary disks, a high concentration of warm dust, and irregular variations in brightness.
The next step was to discover what was driving these features. To do this, the team investigated the minerals that composed the disks they investigated, finding they could be categorized as silica-rich or silica-poor.
This told the scientists which type of collisions were occurring in those disks and producing impact debris. These collisions may also help account for its variability in brightness seen in these systems.
"To just see their mid-infrared emission and beautiful spectral features with the JWST, which allowed us to identify their compositions, was the most exciting thing for me," team member Agnes Kospal of Konkoly Observatory in Budapest, Hungary, said. "We have no other way to study these planetary embryos directly because they are too small."
The researchers think that silica-rich disks are produced by high-energy impacts of Mars-sized objects, in which most of the material is vaporized. Silica-poor disks are created by less energetic events from roughly moon-sized bodies grazing each other.
From their sample, around one-third of extreme debris disks are silica-rich disks. These were found only around stars younger than 300 million years.
Silica-poor disks, on the other hand, were found around stars with a broad range of ages. They also often show greater brightness variability than silica-rich disks. The researchers propose that this variability in brightness is caused by the rapid evolution of fresh debris through further impacts and changes in the orbits of the system's occupants.
The findings can now be applied to the solar system to build a better picture of its 4.6-billion-year evolution. Simulations recreating the earliest era of the solar system show that terrestrial planets, such as Earth, should form within the first few hundred million years.
That lines up with the ages of observed silica-rich extreme debris disks and also aligns with the estimated formation of the Earth and the moon around 100 million years after the sun formed.
The solar system likely also went through a silica-poor extreme disk phase during a period called the Late Heavy Bombardment. It is during this period that scientists think collisions may have caused the solar system's gas giants to migrate from close to the sun to the positions they now occupy.
"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 disks helps us bring together the big picture of what we currently understand."
The team's research was published on Thursday (Oct. 1) in The Astrophysical Journal.
