A bizarre object that sometimes gets as near the sun as Saturn, and other times retreats as far out as Uranus, has been discovered to have a reworking disk of dust around it that changes shape and might even mimic rings.
Minor planet 2060 Chiron is what’s often called a Centaur, that are captured cometary objects that travel across the sun on looping orbits between Jupiter and Neptune. Chiron is just 218 kilometers (135 miles) across and sometimes has outbursts like a comet. To this point, nonetheless, no spacecraft has ever visited a Centaur.
In 2011, Chiron passed in front of a faint star from our viewpoint here on Earth. Such events are known as “stellar occultations,” and based on how an object equivalent to Chiron blocks a star’s light, the occulting object’s shape and size may be determined through deduction. Throughout the 2011 occultation, it was noticed that the star’s light dimmed barely — twice before Chiron itself occulted the star, and two further times after Chiron had moved past the star. This statement was interpreted as Chiron having a double-ring system of dust.
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Then, Chiron occulted one other star on Nov. 28, 2018, in an event taken advantage of by Amanda Sickafoose, who’s a senior scientist on the Planetary Science Institute in Tucson, Arizona. Because Chiron’s shadow forged by the star is so small, it crossed only a narrow region of the Earth, clipping southern Africa. Sickafoose subsequently led a team who used the 1.9-meter (6.2 feet) telescope on the South African Astronomical Observatory in Sutherland, South Africa, to look at the occultation.
Their results, published exactly five years later, tell a rather different story to 2011.
“We detected dips within the starlight because it was blocked by Chiron’s nucleus in addition to by material situated between 300 to 400 kilometers on either side,” Sickafoose said in a statement.
Specifically, as Chiron moved over the star, Sickafoose’s team observed dips in starlight produced by dusty material at radii of 352, 344 and 316 kilometers (roughly 219, 214 and 196 miles) from the middle of Chiron. In other words, that was between about 100 to 130 kilometers (60 to 80 miles) above the Centaur’s surface. After Chiron had moved away from the star, the scientists then witnessed two further dips at 357 and 364 kilometers (221 and 226 miles) from Chiron’s center.
If Chiron had only two stable rings, one would expect just two pairs of symmetrical dips in light on either side of Chiron. The anomalous third dip on one side of the Centaur is evidence that the situation will not be so clear-cut. Moreover, the dips brought on by the mystery material seemingly occurred tens of kilometers away from the locations of the rings as measured in 2011 (although they do fall inside the margin of error of the 2011 observations). The magnitude of the dips in starlight brought on by the fabric, nonetheless, can be different.
“The locations and amounts of fabric that were detected around Chiron are different enough from previous observations to suggest that there will not be a stable ring system but relatively surrounding material that’s currently evolving,” said Sickafoose.
One other stellar occultation by Chiron on Dec. 15, 2022, was observed on the Kottamia Astronomical Observatory in Egypt by a team led by Jose Luis Ortiz of the Instituto de Astrofísica de Andalucía in Spain. They found the fabric around Chiron had modified again, detecting three symmetrical structures on either side of Chiron. Two of the features are narrow and one is broad, and that together they appear to form a large disk 580 kilometers (360 miles) across.
The origin and composition of this material around Chiron stays unknown, although the chances are it comes from Chiron itself, perhaps blasted into space by cometary outbursts equivalent to those witnessed in the summertime of 2021 when Chiron brightened by 0.6 magnitudes. For context, a magnitude 0 object is 100 times brighter than a magnitude 5 object on this brightness system. One other Centaur, the 250-kilometer (160 miles) wide 10199 Chariklo, was also previously shown to have rings during a stellar occultation in 2013, and confirmed during one other stellar occultation witnessed by the James Webb Space Telescope on Oct. 18, 2022. The JWST even detected water-ice on Chariklo. Nevertheless, the findings at Chiron call into query the character of the structures around Chariklo, too.
Occultations of faint stars by Chiron occur fairly recurrently, and future observations now have a challenge on their hands to clarify what is going on across the distant, apparently morphing, Centaur.
The outcomes were published on Nov. 28in The Planetary Science Journal.