Collisions between large bodies play a central role in how planets form, but direct observations are rare. Astronomers have now imaged the dusty aftermath of two powerful collisions around the nearby young star Fomalhaut, marking the first directly imaged impacts between large objects in any system beyond the solar system. The two events — one seen in images from 2004 and 2006 and another revealed in a 2023 HST image — are interpreted as separate impacts that produced expanding dust clouds.
Hubble originally showed a bright point near a dusty belt that was reported as a possible planet in 2008 and named Fomalhaut b. Follow-up HST images in 2010, 2012 and 2013 revealed motion that later curved away from the star, behavior consistent with small dust particles pushed by starlight rather than a planet. The 2023 image revealed a second bright spot, now called Fomalhaut cs2, and analysis of the 2023 and a poorer 2024 image led the team to conclude both bright points are light reflected from expanding debris produced by planetesimal collisions.
From the brightness, the colliding bodies were at least 30 kilometers (18 miles) across, at least twice the size of the object that struck Earth 66 million years ago. The researchers estimate roughly 300 million similar objects in the Fomalhaut belt, and earlier detections of carbon monoxide indicate these planetesimals are volatile-rich, similar to icy comets in the solar system. The Fomalhaut dust cloud is about a billion times larger than the cloud created by NASA’s DART mission in 2022, and the team said they were surprised to see a second collision so soon, suggesting such impacts may be more common in young systems than previously thought.
Next steps include tracking the cloud with the James Webb Space Telescope’s NIRCam and with HST over the coming three years to measure its expansion and orbit. Observers are also warned that dust clouds can mimic planets when future telescopes search for exoplanets. Support for the work came from NASA, and coauthors are from several universities and observatories.
Difficult words
- collision — a violent meeting of two objectscollisions
- debris — small pieces left after an explosion
- planetesimal — a small solid body that can form planetsplanetesimals
- volatile-rich — containing substances that evaporate easily
- expand — to become larger in size or areaexpanding
- mimic — to look like or behave like something else
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Discussion questions
- How could astronomers use follow-up observations to tell dust clouds apart from real planets? Give one or two methods and reasons.
- What does a large number of planetesimals in a belt suggest about planet formation in that system?
- If collisions are common in young systems, how might that affect the future detection of exoplanets and our interpretation of young planetary systems?
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