Young star systems are chaotic environments where space rocks constantly collide. Asteroids, comets, and larger bodies crash into one another, sometimes sticking together and slowly transforming clouds of dust and ice into planets and moons. Although small impacts are common, the largest collisions are thought to be extremely rare during the hundreds of millions of years it takes for a planetary system to fully form -- possibly occurring only once every 100,000 years.

Yet astronomers have now identified evidence of two enormous collisions around a nearby star named Fomalhaut, all within just 20 years. The finding suggests either an extraordinary stroke of luck or that massive impacts may happen more often during planet formation than scientists previously believed.

The two events -- first seen in 2004 and again in 2023 -- represent the first time astronomers have directly imaged collisions between large objects in a planetary system beyond our own.

Catching the Aftermath of a Cosmic Impact

"We just witnessed the collision of two planetesimals and the dust cloud that gets spewed out of that violent event, which begins reflecting light from the host star," said Paul Kalas, an adjunct professor of astronomy at the University of California, Berkeley, and lead author of the study. "We do not directly see the two objects that crashed into each other, but we can spot the aftermath of this enormous impact."

Over tens of thousands of years, Kalas explained, the region around Fomalhaut would appear to be filled with glowing debris, "sparkling with these collisions" -- similar to twinkling holiday lights.

A Young Star That Mirrors the Early Solar System

Kalas began studying Fomalhaut in 1993 while searching for dusty disks that remain after planet formation. Located just 25 light years from Earth, Fomalhaut is relatively young -- about 440 million years old -- making it a useful stand-in for what the solar system looked like early in its history.

Using NASA's Hubble Space Telescope (HST), Kalas eventually detected a broad disk of debris around the star. In 2008, he also reported spotting a bright object near the disk that appeared to be a planet. This marked the first time an exoplanet had been directly imaged at optical wavelengths, and it was named Fomalhaut b following standard naming rules.

That apparent planet has since vanished. Researchers now believe the object was not a planet at all, but a cloud of dust created when two large bodies collided.

When Dust Clouds Look Like Planets

"This is a new phenomenon, a point source that appears in a planetary system and then over 10 years or more slowly disappears," Kalas said. "It's masquerading as a planet because planets also look like tiny dots orbiting nearby stars."

Based on how bright the 2004 and 2023 events appeared, scientists estimate that the colliding bodies were at least 60 kilometers (37 miles) wide. That is more than four times the size of the asteroid that struck Earth 66 million years ago and led to the extinction of the dinosaurs. Objects of this scale are known as planetesimals, similar in size to many asteroids and comets in our solar system, but far smaller than dwarf planets such as Pluto.