In an unprecedented celestial event, NASA’s Hubble Space Telescope (HST) captured the dramatic aftermath of colliding space rocks within a nearby planetary system.
When astronomers initially spotted a bright object in the sky, they assumed it was a dust-covered exoplanet, reflecting starlight. But when the “exoplanet” disappeared and a new bright object appeared, the international team of astrophysicists — including Northwestern University’s Jason Wang — realized these were not planets at all. Instead, they were the illuminated remains of a cosmic fender bender.
Two distinct, violent collisions generated two luminous clouds of debris in the same planetary system. The discovery offers a unique real-time glimpse into the mechanisms of planet formation and the composition of materials that coalesce to form new worlds.
The study will be published on Thursday (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.”
An expert on imaging exoplanets, Wang is an assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).
Site of the smashup
For years, astronomers have puzzled over a bright object called Fomalhaut b, an exoplanet candidate residing just outside the star Fomalhaut. Located a mere 25 light-years from Earth in the Piscis Austrinus constellation, Fomalhaut is more massive than the sun and encircled by an intricate system of dusty debris belts.
“The system has one of the largest dust belts that we know of,” said Wang, who is part of the team that has monitored the system for two decades. “That makes it an easy target to study.”
Since discovering Fomalhaut b in 2008, astronomers have struggled to determine whether it is, indeed, an actual planet or a large expanding cloud of dust. In 2023, researchers used the HST to further examine the strange light source. Surprisingly, it was no longer there. But another bright point of light emerged in a slightly different location within the same 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.”
Double trouble
The disappearance of Fomalhaut b (now called Fomalhaut cs1) supports the hypothesis that it was a dissipating dust cloud, likely produced by a collision. The appearance of a second point of light (now called Fomalhaut cs2) further supports the theory that neither are planets, but the dusty remnants of dramatic smashups between planetesimals — the rocky building blocks of planets.
The location and brightness of Fomalhaut cs2 bear striking similarities to the initial observations of Fomalhaut cs1 two decades prior. By imaging the system, the team was able to calculate how frequent such planetesimal collisions occur.
“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.”
As unbelievable as it seemed, Wang helped substantiate the extraordinary occurrence. He provided one of four independent analyses to confirm the astronomers detected two transient events in Fomalhaut’s dust belt.
“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.”
A cautionary tale
While the discovery offers a rare laboratory for watching collisions in action, it also highlights the possibility of misinterpreting the dusty aftermath of such collisions as actual planets reflecting starlight. As next-generation telescopes, including the Giant Magellan Telescope, aim to directly image habitable-zone planets around nearby stars, understanding and distinguishing these transient collision clouds from genuine exoplanets will be critical.
“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."
Although Fomalhaut cs1 has faded from view, the research team will continue to observe the Fomalhaut system. They plan to track the evolution of Fomalhaut cs2 and potentially uncover more details about the dynamics of collisions in the stellar neighborhood.
For those observations, Wang, Kalas and their collaborators will use the Near-Infrared Camera (NIRCam) instrument on NASA’s James Webb Space Telescope (JWST). NIRCam can provide color information that HST’s spectrograph instrument could not. This color data can reveal the size and composition of the cloud’s dust grains, including whether the cloud contains water and 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).
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