NASA’s Hubble Space Telescope discovered new clues around a nearby star that strengthen the idea that the object previously seen there was not a planet, but rather the result of a giant cosmic collision.
A team of astronomers discovered a new, faint spot of light near the inner edge of the vast dust ring surrounding the star Fomalhaut in 2023. This object is very similar to a previous detection in the mid-2000s, which slowly faded over time.
The two objects appeared in locations that were in accordance with scientists’ estimates as places where debris was formed due to high-speed collisions between large planetesimals, namely the initial chunks of rock that were the building blocks for planets outside the solar system.
Capturing such a rare event was called “amazing” by Paul Kalas, lead researcher from the University of California, Berkeley.
Together, these two detections provide direct evidence that large cosmic collisions are still occurring in mature planetary systems.
By observing these impacts in near real time, scientists can estimate how often such collisions occur, how much material is released, and how debris disks—and the planets that might form from them—continue to evolve long after a star forms.
“This is truly the first time I have seen a point of light just appear in an extrasolar planetary system,” Kalas said in a statement.
“This object was not visible in all previous Hubble images, which means we have just witnessed a devastating collision between two massive objects and the formation of a giant debris cloud that is incomparable to anything in our solar system today.”
Fomalhaut is about 25 light years from Earth in the constellation Piscis Austrinus, or Southern Fish, and is one of the brightest stars in the night sky.
This star is surrounded by several belts of dust and debris, leftover material from the planet formation process, similar to the Kuiper Belt beyond the orbit of Neptune in our solar system.
In 2004, Hubble spotted a compact light source within the belt that was later named Fomalhaut b. At that time, scientists debated whether the object was a planet covered in dust or something completely different.
In 2008, some researchers even believed it could be the first exoplanet discovery made with a visible light telescope.
However, as time goes by, the behavior of the object raises doubts. The mysterious source dimmed instead of getting brighter, seemed to extend outward, and finally disappeared.
These changes are more in line with the characteristics of debris clouds that form when two large objects collide and then slowly spread.
When astronomers returned to observe the system nearly 20 years later, they no longer found the initial object.
Instead, they saw a new source nearby in the same dust ring, indicating that a second major collision occurred in roughly the same region. The results of this research were published in the journal Science.
“What we learned,” Kalas said, “is that large dust clouds can masquerade as planets for years.”
What was considered strange was that the research team saw two clouds of debris quite close together. If the collisions occurred randomly, experts would expect their locations to be randomly distributed as well.
Researchers also cannot explain why these two large collisions occurred in a relatively short time span. Previous theories suggested that a collision of this magnitude should only occur once every 100,000 years or so.
“If you had a film about the last 3,000 years and played it fast so that each year lasted only a fraction of a second, imagine how many flashes of light you would see,” Kalas said. “The Fomalhaut planetary system would appear to glow with these impacts.”
The dust cloud looks shining because it reflects starlight, so it can be detected by telescopes like Hubble.
However, the same starlight also pushes against tiny dust grains, causing the cloud to spread outward and fade. This process explains why the first cloud disappeared and why the second cloud will also likely fade.
Based on the brightness of the debris, researchers estimate the colliding object was about 60 kilometers wide, larger than most asteroids involved in known collisions in our solar system.
Such impacts release enormous amounts of dust, which temporarily bring to light a normally invisible event.
For astronomers, this discovery provides a rare opportunity to witness directly the destructive events that once shaped—and may still shape—planetary systems across the galaxy, said study co-author Mark Wyatt of the University of Cambridge, England.
The research team is looking forward to additional insights from the James Webb Space Telescope, which observes the universe in invisible infrared light, to reveal the size and composition of the dust.
“This system is a natural laboratory for researching how planetesimals behave in collisions,” Wyatt said in the statement, “which ultimately tells us about what they are made of and how they formed.”