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Hubble Reveals Two Collision Clouds at Fomalhaut, Resolving the ‘Fomalhaut b’ Puzzle

A Science paper reclassifies Fomalhaut b as a dust cloud—JWST will track a newly seen collision.

An artist's rendition of a collision between two planetesimals in the debris disk around a young star. Collisions like these occur over hundreds of millions of years as gas and dust slowly coalesces into planets and moons.
Credit: Paul Kalas
Image
The Hubble Space Telescope observed the debris belt and dust clouds around the star Fomalhaut, which was blotted out to allow the fainter features around it to be seen.

Overview

  • The peer-reviewed study, published December 18 in Science, reports Hubble images of two transient sources in Fomalhaut’s outer debris belt interpreted as luminous dust clouds from separate planetesimal collisions.
  • The long-debated object once labeled Fomalhaut b is now designated cs1 and identified as a dissipating dust cloud rather than a bona fide exoplanet.
  • A second source, cs2, appeared in 2023 near the same belt and shows similar properties to cs1, marking the first directly imaged planetesimal smashups outside the Solar System.
  • Analyses of the clouds’ brightness and evolution indicate parent bodies tens of kilometers across, with estimates clustering around roughly 30–60 kilometers in diameter.
  • Two observed events within about 20 years contrast with theoretical expectations of one per 100,000 years, and approved JWST NIRCam follow-up will probe cs2’s color and composition to refine collision rates and avoid mistaking dust clouds for planets.