NASA's Hubble Space Telescope has spotted fresh clues around a nearby star that strengthen the case that an object previously seen there was not a planet but an enormous space crash.
A team of astronomers observed a new, faint point of light near the inner edge of a broad ring of dust circling Fomalhaut in 2023. The object closely resembles an earlier detection in the mid-2000s, which gradually faded.
Both objects appear at locations where scientists would expect debris from high-speed collisions between large planetesimals, the early rocky building blocks of exoplanets. Capturing such a rare event is "amazing," said Paul Kalas, the principal investigator from UC Berkeley.
Together the two detections provide direct evidence that large cosmic collisions are still happening in mature planetary systems. By observing these impacts almost in real time, scientists can estimate how often these kinds of crashes happen, how much material they release, and how debris disks — and the planets that may emerge from them — continue to evolve long after a star forms.
"This is certainly the first time I’ve ever seen a point of light appear out of nowhere in an exoplanetary system," Kalas said in a statement. "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."
Fomalhaut lies about 25 light-years away in the constellation Piscis Austrinus, aka the Southern Fish, and is one of the brightest stars in the night sky. It is surrounded by several belts of dust and debris, material left over from the planet-building process, similar to our solar system’s Kuiper Belt beyond Neptune.
In 2004, Hubble saw a compact source inside this belt, dubbed Fomalhaut b. At the time, scientists debated whether it was a planet surrounded by dust or something else entirely. By 2008, some believed it could be the first exoplanet discovery made with a visible light telescope.
But over the ensuing years, the object’s behavior raised doubts. The mysterious source dimmed instead of brightened, appeared to stretch outward, and eventually vanished. Those changes better matched what scientists would expect from a cloud of debris created when two large bodies smash and then slowly disperse.
When astronomers had another look at the system nearly 20 years later, they did not see the original object at all. Instead, they found a new source nearby along the same dust ring, suggesting that a second major collision had occurred in roughly the same region. The study results appear in the journal Science.
"What we learned," Kalas said, "is that a large dust cloud can masquerade as a planet for many years."
What's strange is that the team is seeing the two debris clouds in close proximity. If these collisions were haphazard, experts would think they'd appear in totally random places. The researchers also can't yet explain why these two crashes happened within such a short span of time. Previous theories would suggest a collision of this magnitude should only happen once in 100,000 years or so.
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"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," Kalas said. "Fomalhaut’s planetary system would be sparkling with these collisions."
The dust clouds shine by reflecting starlight, making them visible to telescopes like Hubble. But that same starlight also pushes on the tiny dust grains, causing the clouds to spread outward and fade. This process explains why the first cloud disappeared and why the second may also fade.
Based on the brightness of the debris, researchers estimate that the colliding objects were likely 37 miles wide — larger than most asteroids involved in known crashes in our own solar system. Such impacts release enormous amounts of dust, briefly lighting up otherwise invisible events.
For astronomers, this discovery offers a rare chance to witness the kinds of destructive events that once shaped — and may still shape — planetary systems across the galaxy, said coauthor Mark Wyatt, who is based at the University of Cambridge in the United Kingdom. The team looks forward to what additional insight the James Webb Space Telescope, which observes in invisible infrared light, can reveal about the size and makeup of the dust.
"The system is a natural laboratory to probe how planetesimals behave when undergoing collisions," said Wyatt in a statement, "which in turn tells us about what they are made of and how they formed."
