Astronomers were abuzz last week following the discovery of an active asteroid in Jupiter’s orbit, in what was thought to be a new type of celestial object. Follow-up investigations have revealed it to be a regular comet, albeit one in a misleading orbit.
As Gizmodo reported last week, object 2019 LD2 was initially identified as the first known active Trojan asteroid on account of its comet-like tail. The object was identified as such by astronomers from the University of Hawaii’s Asteroid Terrestrial-impact Last Alert System (ATLAS), and a collaborative team from Queen’s University Belfast. The University of Hawaii issued a correction notice on Tuesday, noting that the object has been re-classified as a true comet, but “with a chaotically changing orbit currently resembling that of a Trojan asteroid.”
Trojan asteroids are located in the same orbital path as Jupiter, with one big clump in front and one behind the gas giant. Trojans are inert, dead asteroids showing no signs of surface activity, which is why the apparent discovery of an active Trojan was such a big deal. Active asteroids, though rare, are similar to comets in that they shed volatile surface materials, like gas and dust.
That 2019 LD2 might be a comet, and not an active Trojan asteroid, was suggested by amateur astronomers Sam Deen and Tony Dunn late last week on the Minor Planet Mailing List, according to the University of Hawaii press release. The object’s cometary nature was then confirmed by ATLAS astronomers as well as Japanese astronomer Shuichi Nanako. The object has since been renamed comet P/2019 LD2 (ATLAS), per the International Astronomical Union’s Minor Planet Center.
P/2019 LD2 is a Jupiter-family comet, and not a member of the Trojan community, but it’s still under the gravitational influence of the gas giant. Jupiter-family comets have extended orbits that take them all the way to the outer solar system and then back again to near Jupiter. They’re considered short-period comets, which have orbital periods of less than 20 years.
P/2019 LD2’s orbit brings it close to Jupiter every few decades, but each time it swings past the gas giant its orbital path undergoes a major readjustment, hence the term “chaotically changing orbit.” The object is currently in an orbital position that closely matches those of Trojan asteroids. In a few decades, the orbital path of P/2019 LD2 will change again, after which time it will most certainly not be mistaken for a Trojan.
This is all a bit disappointing, of course, but it’s how science works. If anything, the episode confirms the status quo nature of Jupiter’s Trojans as boring, inert objects. We’ll learn more about these objects in a few years, though. NASA is planning to send a spacecraft called Lucy to explore the Trojans between 2027 and 2033.