In young star systems, violent collisions are common as rocks, comets, asteroids, and larger objects crash and merge, gradually forming planets and moons from the primordial dust and ice of a stellar nebula. The largest of these collisions are expected to be rare, occurring perhaps every 100,000 years over the millions of years it takes to form a planetary system.
Astronomers have recently observed the aftermath of two significant collisions within two decades around the nearby star Fomalhaut. These observations suggest either an unusual stroke of luck or that such collisions are more frequent than previously predicted during the process of planet formation.
The first collision was detected in 2004, and the second in 2023—making them the first large-object collisions ever directly imaged in a solar system other than our own.
“We’ve witnessed a collision between two planetesimals and the resulting dust cloud reflecting light from the host star,” said Paul Kalas, an adjunct professor of astronomy at the University of California, Berkeley, and the report’s lead author. “We don’t directly see the colliding objects, but we can see the impact’s aftermath.”
Kalas noted that over tens of thousands of years, the dust around Fomalhaut would “sparkle with these collisions,” much like twinkling holiday lights.
Kalas began searching for a dusty disk around Fomalhaut in 1993, intending to observe debris remaining after planet formation for the first time. Situated 25 light-years from Earth, the young star—about 440 million years old—serves as a stand-in for what our solar system looked like in its early years. NASA’s Hubble Space Telescope (HST) eventually helped him find such a disk around the star, and in 2008, he reported discovering a bright spot near the disk, believed to be a planet and the first directly imaged at visible wavelengths, named Fomalhaut b.
However, the planet discovery turned out to be a dust cloud generated by colliding planetesimals rather than an actual planet.
“This is a new phenomenon—a point source appearing in a planetary system, slowly disappearing over 10 years or more,” he said. “It masquerades as a planet because planets also appear as tiny dots orbiting nearby stars.”
The brightness of the 2004 and 2023 events suggests that the colliding objects are at least 60 kilometers (37 miles) across—four times larger than the object that collided with Earth 66 million years ago, leading to the dinosaurs’ extinction. Such objects are known as planetesimals—comparable in size to many asteroids and comets in our solar system but smaller than a dwarf planet like Pluto.
“Fomalhaut, much younger than our solar system, resembles our solar system at 440 million years old, when planetesimals frequently collided,” said Kalas. “We’re observing a time when small worlds experienced violent collisions or were destroyed and reformed. It’s like looking back at our solar system’s turbulent period, less than a billion years old.”
The 2023 Fomalhaut observations are detailed in a paper published online in the journal Science on Dec. 18.
“The Fomalhaut system serves as a natural laboratory for studying planetesimal collision behavior, revealing composition and formation details,” said Kalas’s colleague, Mark Wyatt, a theorist and professor of astronomy at the University of Cambridge. “This observation is exciting because it allows us to estimate colliding bodies’ size and quantity within the disk, information nearly impossible to obtain otherwise.”
He estimates approximately 300 million objects around Fomalhaut are similar in size to the ones that collided, creating bright dust clouds. Previous star observations detected carbon monoxide gas, indicating these planetesimals are volatile-rich and similar in composition to our solar system’s icy comets.
Dust clouds mimicking exoplanets
Fomalhaut, found in the southern constellation Piscis Austrinus, is 16 times more luminous than our sun and one of the sky’s brightest stars. Kalas began observing it with HST in 2004 and discovered a large dusty debris belt 133 astronomical units (AU) from the star,
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