Giant Comet Springs to Life: Bernardinelli-Bernstein Activity
June 27, 2025 • 2 minute read.
Let’s dive in! 😉 Astronomers have just made a landmark discovery by detecting molecular activity—specifically carbon monoxide (CO) jets—in comet C/2014 UN271 “Bernardinelli-Bernstein,” the largest comet ever observed. This breakthrough, made possible by the Atacama Large Millimeter/Submillimeter Array (ALMA), was achieved when the comet was a chilly 16.6 astronomical units (AU) from the Sun—well beyond Saturn’s orbit.
ALMA’s Breakthrough Detection
In March 2024, ALMA’s powerful array of 66 radio dishes captured faint emissions of the CO (J=2–1) spectral line at 230 GHz from UN271’s coma and nucleus. This marks the first time molecular outgassing has been observed in this mega-comet, revealing active jets that evolve over time.
How ALMA Works
- High Sensitivity & Resolution: ALMA can detect extremely weak signals from cold, distant objects—crucial for observing a comet over 16 AU away.
- Spectral Imaging: By tuning into specific molecular lines (like CO), scientists can map outgassing regions and quantify production rates.
- Continuum Mapping: Thermal emissions from the dust coma and ∼140 km (∼87 mi) nucleus were also recorded, helping refine size and activity models.
What This Means for Oort Cloud Science
This first detection of molecular activity so far from the Sun reshapes our understanding of how comets from the distant Oort Cloud behave:
- Outgassing at Extreme Distances – Even in frigid conditions, ancient ices can sublimate, driving jets that shed light on composition and structure.
- Insights into Solar System Origins – CO and dust measurements reveal clues about primordial materials that formed planets and smaller bodies.
- Model Refinements – Data from UN271 helps improve thermal and dynamical models for other long-period comets.
As the National Radio Astronomy Observatory shared on X, “NEW: Largest Oort Cloud Comet Ever Observed Reveals Its Secrets with ALMA’s Powerful Gaze.”
NEW: Largest Oort Cloud Comet Ever Observed Reveals Its Secrets with ALMA’s Powerful Gaze A team of astronomers detected molecular activity in comet C/2014 UN271 (Bernardinelli-Bernstein)—the largest comet ever observed from the Oort Cloud! @almaobs public.nrao.edu/news/largest-o…
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Conclusion
The detection of CO jets in Bernardinelli-Bernstein at 16.6 AU is a testament to human curiosity and technological prowess. This distant world, once thought dormant, now whispers stories of the early Solar System—stories we’re only beginning to decode.
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