With the James Webb Space Telescope (JWST) continuing to grab headlines, it’s easy to forget the Hubble Space Telescope (HST) is still zipping around Earth, monitoring the stars. First launched in 1990, the telescope has witnessed quite a bit in its 35-year life, and recently, it captured a never-before-seen astronomical event—the aftermath of two cosmic collisions.
An international team of astronomers aimed the HST at Fomalhaut, the brightest star in the constellation Piscis Austrinus, surrounded by a dusty cloud and orbited by a potential exoplanet, Fomalhaut b. The particularly bright Fomoalhaut b has puzzled researchers for years as they’ve struggled to determine if it’s an actual exoplanet or just a larger cloud of dust. When they took a peek in 2023, they made a startling discovery. Fomalhaut b had vanished. Instead, they found another bright spot in a different part of the system.
So what happened? The astronomers now believe the disappearing Fomalhaut b was never an exoplanet at all, rather it was a cloud of dust they’re now calling Fomalhaut cs1. The mystifying appearance of the other bright spot, Fomalhaut cs2, is another cloud of cosmic debris. Incredibly, both are the remnants of smashups between small rocky bodies called “planetesimals” (a portmanteau of “planet” and “infinitessimal”). They published their findings today in Science.
“Spotting a new light source in the dust belt around a star was surprising. We did not expect that at all,” co-author Jason Wang, an astrophysicist at Northwestern University, said in a statement. “Our primary hypothesis is that we saw two collisions of planetesimals—small rocky objects, like asteroids—over the last two decades.”
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It’s an incredibly rare occurrence to observe once, let alone twice, and it suggests the planetesimals zooming through Fomalhaut’s dust cloud are engaged in a game of cosmic billiards.
“Theory suggests that there should be one collision every 100,000 years, or longer. Here, in 20 years, we've seen two,” study co-author Paul Kalas of the University of California, Berkeley, 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 planetesimals are the building blocks of planets, astronomers believe this new research will help shed light on how planets form. The next step? Get a better picture of Fomhault’s chaotic crash zone using better equipment.
“Due to Hubble’s age, it can no longer collect reliable data of the system,” Wang added. “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.”
Whether it’s exoplanets or space telescopes, it’s always out with the old, in with the new. 
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Lead image: NASA, ESA, STScI, Ralf Crawford (STScI)
