Astronomers using NASA's Hubble Space Telescope (HST) have witnessed the surprising aftermath of massive space rocks smashing into each other in a nearby planetary system. What initially looked like a reflective exoplanet turned out to be something far more dramatic.

Researchers first noticed a bright point of light and assumed it was a dust covered planet reflecting its star's glow. That interpretation fell apart when the object vanished and a different bright source appeared nearby. The international research team, which included Northwestern University astrophysicist Jason Wang, realized they were not seeing planets at all. Instead, the light came from glowing clouds of debris created by violent collisions.

The observations reveal two separate and powerful impacts that produced expanding clouds of dust within the same planetary system. Catching these events in real time gives scientists a rare window into how planets form and what kinds of materials come together to build new worlds.

The findings were published on Dec. 18) in the journal Science.

"Spotting a new light source in the dust belt around a star was surprising. We did not expect that at all," Wang said. "Our primary hypothesis is that we saw two collisions of planetesimals -- small rocky objects, like asteroids -- over the last two decades. Collisions of planetesimals are extremely rare events, and this marks the first time we have seen one outside our solar system. Studying planetesimal collisions is important for understanding how planets form. It also can tell us about the structure of asteroids, which is important information for planetary defense programs like the Double Asteroid Redirection Test (DART)."

"This is certainly the first time I've ever seen a point of light appear out of nowhere in an exoplanetary system," said lead author Paul Kalas, an astronomer at the University of California, Berkeley. "It's absent in all of our previous Hubble images, which means that we just witnessed a violent collision between two massive objects and a huge debris cloud unlike anything in our own solar system today."

Wang specializes in imaging exoplanets and is an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences, as well as a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).

Fomalhaut and Its Puzzling Dusty Environment

The collisions occurred in the planetary system surrounding the star Fomalhaut, located about 25 light-years from Earth in the Piscis Austrinus constellation. Fomalhaut is larger than the sun and surrounded by an extensive and complex set of dusty debris belts.

"The system has one of the largest dust belts that we know of," Wang said. "That makes it an easy target to study."

For years, astronomers have debated the nature of a bright object known as Fomalhaut b, first reported in 2008 just outside the star's main dust belt. Some researchers thought it was a planet, while others suspected it was a spreading cloud of dust from a collision.