The Largest Comet We've Ever Seen Just Delivered a Curious Surprise

schwit1 shares a report from ScienceAlert: The comet Bernardinelli-Bernstein (BB) -- the largest our telescopes have ever spotted -- is on a journey from the outer reaches of our Solar System that will see it flying relatively close to Saturn's orbit. Now, a new analysis of the data we've collected on BB has revealed something rather surprising. Digging into readings logged by the Transient Exoplanet Survey Satellite (TESS) between 2018 and 2020, researchers have discovered that BB became active much earlier, and much farther out from the Sun, than was previously thought. A comet becomes active when light from the Sun heats its icy surface, turning ice to vapor and releasing trapped dust and grit. The resulting haze, called a coma, can be useful for astronomers in working out exactly what a particular comet is made out of. In the case of BB, it's still too far out for water to sublimate. Based on studies of comets at similar distances, it's likely that the emerging fog is driven instead by a slow release of carbon monoxide. Only one active comet has previously been directly observed at a greater distance from the Sun, and it was much smaller than BB. "These observations are pushing the distances for active comets dramatically farther than we have previously known," says astronomer Tony Farnham, from the University of Maryland (UMD). "We make the assumption that comet BB was probably active even farther out, but we just didn't see it before this. What we don't know yet is if there's some cut-off point where we can start to see these things in cold storage before they become active." The research has been published in the Planetary Science Journal.

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