While comets are best known for their tails, which can arc for millions of kilometres across the Solar System, their icy cores are usually only a few kilometres across. However, Hubble has revealed that a gargantuan comet is currently making its way through the Solar System, after recent observations showed the icy heart of C/2014 UN271 is over 135km across – making it the largest comet nucleus known.
Comets are the icy leftovers of planet formation that have survived on the outskirts of our Solar System. Those with long periods, such as this one – which takes three million years to orbit the Sun – are thought to originate from the hypothetical Oort Cloud, which is too far away to see directly.
“This comet is the tip of the iceberg for thousands of comets that are too faint to see in the more distant parts of the Solar System,” says David Jewitt from the University of California, who took part in the study. “We suspected this comet had to be big because it is so bright at such a large distance. Now we confirm it is.”