Billions of years ago, a cataclysmic encounter between the young Earth and a Mars-sized planet may have created the Moon—and now astronomers are finding new potential clues about such planetary-scale collisions around other stars with help from NASA’s James Webb Space Telescope.
Recent Webb investigations of what planetary scientists refer to as “extreme debris disks,” an uncommon environment in which large volumes of warm dust are observed close to their host star, have revealed some of the deepest insights yet into these unique areas, which roughly correspond to the locations where rocky planets normally orbit in our Solar System.
Presently, only around 1% of all young stars display signatures that are consistent with this highly chaotic phase in stellar development. In observations of more than 20 extreme debris discs collected by Webb and NASA’s retired Spitzer Space Telescope, a team led by Kate Su of the Space Science Institute has compiled enough data to enable a truly deeper understanding of these odd environments.
“This is the first time we have gathered enough systems to truly understand this subclass that we call extreme debris discs,” Su said in a statement.
Calling Cards of an Extreme Debris Disk
Through the team’s observations, they found three main characteristics that are common in these dusty regions. These include the presence of very fine dust grains that are smaller than those usually observed by astronomers, as well as much higher concentrations of warm dust.
Finally, the team also identified irregular variations in infrared brightness, which points to the existence of large volumes of material within these regions that, unlike light at optical wavelengths, infrared light is generally more capable of passing through.
Additionally, and perhaps even more significantly, the mid-infrared observations by Webb allowed Su and her colleagues to examine the mineral composition of the debris, which revealed two very distinctive categories.
The Theia Hypothesis
Massive collision events like this would be capable of generating enough power to vaporize significant quantities of material, and could also potentially resemble the kind of ancient collision that is suspected of having given origin to our Moon.
This hypothetical collision between Earth and an ancient protoplanet, which researchers refer to as the Theia hypothesis, is supported by modern simulations that point to further evidence of an ancient impact event. Primarily, this involves a pair of very large, low-shear-velocity regions present within Earth’s lower mantle, which scientists believe could be the remnants of this ancient object.
In around one-third of 21 disks that the team observed, an abundance of silica was detected, which the researchers say points to extremely energetic collisions between Mars-sized planetary bodies similar to what proponents of the Theia hypothesis suggest about Earth’s ancient past.
However, the remaining two-thirds were found to be silica-poor, which seems to indicate far less energetic events, such as grazing collisions that might occur between objects roughly the same size as Earth’s Moon.
“We have no other way to study these planetary embryos directly because they are too small,” said study Agnes Kospal of Konkoly Observatory and one of the study’s coauthors in a statement.
Early Planetary Clues
Another intriguing component of the new research is that silica-rich discs were detected only in a few instances, which involved stars younger than 300 million years.
This is an important clue as well, since the timeframe in question corresponds closely with simulations that point to the formation of terrestrial planets occurring within the first few hundred million years of a planetary system’s existence.
By comparison, Earth and the Moon are only thought to have formed sometime around 100 million years after the Sun itself. Meanwhile, discs poor in silica occur around stars spanning a much wider range of ages, and often display greater variations in brightness, potentially as a result of ongoing collisions over time, and the rapid redistribution of newly created debris.
Overall, the team’s findings may help to shed light on the early stages of planetary evolution, which, for now, remain invisible to astronomers. The research may also help to shed light on the processes that occurred in the remote past—perhaps billions of years ago—within our own Solar System.
“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,” Su said, adding that she and her team’s work exploring these extreme debris discs “helps us bring together the big picture of what we currently understand.”
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.