Around some young stars, primitive planets smash into each other so hard they vaporize rock, leaving behind clouds of glassy bits in space.
Softer glancing blows grind rock into something finer, like the green sand on some Hawaiian beaches. Astronomers found both types of crash debris while studying a sample of 21 young star systems.
Each of these stars carries an unusually heavy load of warm dust in the same zone where Earth and the other rocky planets — Mercury, Venus, and Mars — orbit the sun. Astronomers call these “extreme debris disks,” the detritus left over from smashups between rocky worlds about the size of our moon or Mars.
These systems give scientists a window into the rough final stretch of building terrestrial planets, when scuffles between big bodies are common. By matching each system’s dust to its age and behavior, researchers can learn when giant impacts happen, how violent they get, and which systems are still shuffling the positions of their planets. Our own solar system may have gone through this stage. Scientists think one of those collisions, involving a Mars-size world dubbed Theia, created the moon after slamming into early Earth.
“How rocky planets formed and giant planets evolved are part of the broader story of the solar system’s formation,” said Kate Su, first author of the study, in a statement. “Our work on extreme debris disks helps us bring together the big picture.”
Scientists used two NASA observatories to find systems in this violent phase. The Spitzer Space Telescope, now retired, first noticed these odd disks near the end of its main mission. By then, astronomers had little time to collect examples.
But the James Webb Space Telescope, which NASA runs with the European and Canadian space agencies, recently picked up where Spitzer left off. With Webb, researchers were able to more than double the number of systems with detailed measurements. The larger sample size allowed scientists to study them as a group for the first time.
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By seeing their mid-infrared emissions and light patterns, the researchers were able to identify what was in the disks, said coauthor Agnes Kospal of Konkoly Observatory in Budapest. Their findings were published in The Astrophysical Journal.
It turns out the dust mix splits the systems into two camps. About one-third brimmed with silica, the main ingredient in volcanic glass, like obsidian. The researchers tie that type of dust to the most violent crashes, between Mars-size objects, where the impact actually vaporizes rock. The other two-thirds have very little silica content and more of a green mineral, called forsterite, a form of olivine better known in its gem form as peridot. Those likely came from smaller crashes that only grazed the worlds involved.
The glassy debris appears only around stars younger than about 300 million years. That matches computer simulations suggesting that rocky planets like Earth take shape within the first few hundred million years of a system.
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To find the samples was incredibly challenging. Only about 1 percent of young stars show this dusty phase, far fewer than theories predicted, according to the paper.
A green sand beach at Ka Lae, or South Point, on Hawaii’s Big Island gets its color from olivine, a mineral better known in its gem form as peridot.
Credit: Ted Soqui / Corbis / Getty Images
The dust clouds also flicker — dimming and brightening over weeks, months, or years as the dust thickens and thins. The systems packed with the sandy dust flicker the most, and some of them circle stars that should have finished building planets long ago.
Researchers suspect hidden planets in those systems keep shifting orbits and hurling smaller bodies into each other. If their hypothesis is correct, this kind of dust could help astronomers spot planetary systems in the middle of a critical juncture in their formation. Our solar system likely went through a similar upheaval in the ancient past, when its giant planets migrated and set off a wave of collisions like cosmic billiards.
But big questions remain. Though astronomers have more data to work with now, they still only have three examples with stars older than 300 million years, said Attila Moor, a coauthor from the Konkoly Observatory. More observations could show whether the high-energy crashes with glassy dust really die out after that age.
“There’s many things we still don’t know about these disks,” Moor said.
