MANILA, Philippines โ What astronomers once hailed as a potential new exoplanet has now been unmasked as cosmic collision: the fading glow of a violent, head-on crash between rocky bodies in deep space.
A study published in Science on Friday, December 19, reveals that the mysterious object known for years as Fomalhaut b was never a planet at all. Instead, it was the dusty aftermath of a collision between planetesimalsโsmall, rocky building blocks of planets.
The discovery was made possible through fresh, high-resolution observations from the Hubble Space Telescope.
A 20-year mystery
The saga began in 2008, when astronomers spotted an unusually bright object 25 light-years away in the constellation Piscis Austrinus. Unsure of its nature, they tagged it as Fomalhaut b. Early data hinted at a massive exoplanet, but its swirling debris cloud raised doubts.
โThe system has one of the largest dust belts that we know of. That makes it an easy target to study,โ said Jason Wang, astrophysicist at Northwestern University and co-author of the study.
In 2023, Wangโs team revisited the object using Hubbleโs powerful imaging tools. But when the telescope pointed to Fomalhaut bโs expected location, the object had disappeared. Instead, a new bright spot appeared nearby.
โWe assumed the bright light was Fomalhaut b because thatโs the known source in the system,โ Wang said. โBut after comparing our new images with older ones, we realized it could not be the same source.โ
Paul Kalas, an astronomer at the University of California, Berkeley and co-author, said the discovery stunned the team. โThis is certainly the first time Iโve ever seen a point of light appear out of nowhere in an exoplanetary system,โ he said.
Not one, but cosmic collision
The newly detected objectโnow called Fomalhaut cs2โshows the same characteristics as the earlier Fomalhaut cs1 event. Both are now believed to be the remnants of separate collisions between planetesimals.
What shocked researchers is the frequency. Previous models suggested such collisions should occur only once every 100,000 years. But the Fomalhaut system has produced two in just two decades.
โHere, in 20 years, weโve seen two,โ Kalas said. โIf you had a movie of the last 3,000 years, sped up so every year was a fraction of a second, imagine how many flashes youโd see. Fomalhautโs planetary system would be sparkling with these collisions.โ
Four independent analyses confirmed the findings: the system recently hosted two short-lived, dust-producing impacts.
โThis is the first time weโre seeing something like this,โ Wang added.
A warning for future planet hunters
The discovery offers a cautionary tale for astronomers searching for new worlds. Dust clouds from collisions can mimic the appearance of exoplanets, especially in reflected light.
โFomalhaut cs2 looks exactly like an extrasolar planet reflecting starlight,โ Kalas said. โWhat we learned from studying cs1 is that a large dust cloud can masquerade as a planet for many years.โ
With the mystery finally resolved, scientists say the data will help refine models of how planets formโand sharpen the tools used to identify real exoplanets amid the cosmic noise.
Image Credit: NASA, ESA, STScI, Ralf Crawford (STScI)
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