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.

In 2023, new Hubble observations added another twist. The original light source was gone, but a new bright object appeared in a slightly different part of the system.

“With these observations, our original intention was to monitor Fomalhaut b, which we initially thought was a planet,” Wang said. “We assumed the bright light was Fomalhaut b because that’s the known source in the system. But, upon carefully comparing our new images to past images, we realized it could not be the same source. That was both exciting and caused us to scratch our heads.”

Evidence for Two Separate Cosmic Smashups

The disappearance of the original object, now labeled Fomalhaut cs1, supports the idea that it was a dust cloud slowly dispersing after a collision. The appearance of a second bright source, called Fomalhaut cs2, strengthens the conclusion that neither object is a planet. Instead, both appear to be debris clouds created when large planetesimals crashed into each other.

Fomalhaut cs2 closely resembles how cs1 first appeared two decades ago, both in brightness and location. By studying these events, the team was able to estimate how often such collisions might occur in this system.

“Theory suggests that there should be one collision every 100,000 years, or longer. Here, in 20 years, we’ve seen two,” Kalas said. “If you had a movie of the last 3,000 years, and it was sped up so that every year was a fraction of a second, imagine how many flashes you’d see over that time. Fomalhaut’s planetary system would be sparkling with these collisions.”

Because the result was so unexpected, Wang performed one of four independent analyses to confirm the findings. Each analysis detected a new transient light source in roughly the same region, reinforcing the conclusion that two separate collisions were observed.

“This is the first time we’re seeing something like this,” Wang said. “So, we had to make sure we can trust our images and that we are measuring the properties of the collision properly. I crunched the numbers to show that the four independent analyses all confidently detect a new source around the vicinity of the star.”

Why Dust Clouds Can Fool Planet Hunters

Beyond revealing active collisions, the discovery also serves as a warning for future planet searches. Large clouds of dust can closely mimic the appearance of an exoplanet by reflecting starlight, potentially misleading astronomers.

“Fomalhaut cs2 looks exactly like an extrasolar planet reflecting starlight,” Kalas said. “What we learned from studying cs1 is that a large dust cloud can masquerade as a planet for many years. This is a cautionary note for future missions that aim to detect extrasolar planets in reflected light.”

As new observatories such as the Giant Magellan Telescope prepare to directly image Earth-like planets around nearby stars, distinguishing between real planets and temporary dust clouds will become increasingly important.

Turning to Webb for a Closer Look

Although Fomalhaut cs1 has faded, researchers plan to continue monitoring the system. Their next target is cs2, which may reveal more about how collisions unfold in young planetary systems.

Future observations will rely on the Near-Infrared Camera (NIRCam) aboard NASA’s James Webb Space Telescope (JWST). Unlike Hubble, NIRCam can capture detailed color information that helps scientists determine the size and composition of dust grains, including whether they contain water or ice.

“Due to Hubble’s age, it can no longer collect reliable data of the system,” Wang said. “Fortunately, we now have the JWST. We have an approved JWST program to follow up this planetesimal collision to understand the new circumstellar source and the nature of its two parent planetesimals that collided.”

The study, “A second violent planetesimal collision in the Fomalhaut system,” was supported by NASA (award number HST-GO-17139).

[H/T ScienceDaily]