Astronomers announced earlier this year that an enormous 62-mile (100-kilometer) object was creeping inward from the fringes of the solar system. We didn’t know what this mysterious visitor was at the time, but a recent development has shed light on the subject. The object, now officially designated C/2014 UN271, began forming a coma or tail in recent weeks. That confirms it’s a comet, and it may be the largest one ever discovered.
UN271 first appeared in astronomical observations in 2014, but it wasn’t until June of this year that scientists were able to identify it in the old data. Because it was (and still is) so far away, detecting the comet was a computationally expensive problem. It’s a bit more visible now, and it will become more so as it slides closer to the sun.
The data released earlier this summer came from observations made in 2020 when the object was about 29 AU away. That put it around the orbit of Neptune. Following the announcement, astronomers rushed to get updated images of UN271. The latest data from the Las Cumbres Observatory in South Africa shows a coma around C/2014 UN271 at its current location 19 AU away.
In the image above from the Las Cumbres Observatory, you can see UN271 in the center. The bright spot in the center is the comet’s record-breaking nucleus. The haze around that point of light is the coma, composed of material vaporized by exposure to sunlight. You can compare the fuzzy appearance of UN271 with the star visible just above it, which has sharp edges and no haze. That’s a dead giveaway for a comet.
Further observations have also helped to nail down the comet’s orbit. Astronomers now estimate that UN271’s orbit takes it as much as 40,000 AU away from Earth. That’s almost 6 trillion kilometers or 0.63 light-years, which is well outside the pocket of space we think of as our solar system and smack in the middle of the Oort Cloud. This theoretical cloud of frozen material is believed to be left over from the solar system’s formation. When something gets knocked out of orbit in the Oort Cloud, it becomes what we know as a comet.
We also know that C/2014 UN271 will continue getting closer until 2031, at which time it will start drifting back out into deep space. At its closest approach, it will be 10.9 AU distant — about at the orbit of Saturn. The coma will make it much brighter, but still not bright enough to be seen by the unaided eye. However, you can bet telescopes around the globe will be trained on this gargantuan comet.