Chariklo’s rings changed between 2017 and 2022, the inner one denser and the outer one fading, and an unseen moonlet may be feeding both.<\/p>\n
Rings were a feature of giant planets until 2013, when a star blinked twice on its way behind a world that is only about 160 miles (250 kilometers) wide. That world, Chariklo, orbits between Saturn and Uranus, and astronomers have since treated its two thin rings as steady. They aren\u2019t.<\/p>\n
Between 2017 and 2022, the inner ring became much more opaque while the outer one nearly faded from view. It is the first sign that a small body\u2019s rings can change in just a few years.<\/p>\n
The James Webb Space Telescope made the measurement on October 18, 2022, in the first stellar occultation ever planned for Webb<\/a>. <\/p>\n Part of the difference might be Webb seeing in infrared where most earlier telescopes saw visible light, and the team said as much. But when the researchers modeled that, it couldn\u2019t explain everything.<\/p>\n Pablo Santos-Sanz is a planetary scientist at the Instituto de Astrof\u00edsica de Andaluc\u00eda (IAA-CSIC<\/a>) in Granada, Spain, and leader of the study. <\/p>\n \u201cWe do not yet know the physical mechanism responsible for the opposite behavior of the two rings, and I think that is one of the most intriguing aspects of our result,\u201d he told Earth.com.<\/p>\n A star blinked behind Chariklo\u2019s rings<\/p>\n Chariklo<\/a> is the largest of the centaurs<\/a>, icy bodies orbiting among the giant planets. It orbits about 1.6 billion miles (2.5 billion kilometers) from the Sun, nearly 17 times as far out as Earth. <\/p>\n Its two rings circle about 240 and 250 miles (390 and 405 kilometers) from its center, and they\u2019re far too thin and distant to photograph, even with Webb.<\/p>\n So astronomers wait for Chariklo to pass in front of a star and time how the starlight dips<\/a>. Each ring blocks the light briefly, and the dip\u2019s depth and length give its width and density. That is a stellar occultation, and it is how the rings were found in 2013<\/a>.<\/p>\n Webb\u2019s target was a faint star from Europe\u2019s Gaia<\/a> catalog. The telescope recorded its brightness more than three times a second, in two infrared colors at once. Seen from Webb, the star\u2019s path missed Chariklo\u2019s edge by about 4.6 miles (7.4 kilometers).<\/p>\n \u201cUltimately Chariklo itself missed the star, but both narrow rings crossed it,\u201d Santos-Sanz said.<\/p>\n Predicting that path was hard, because Webb orbits a point nearly a million miles<\/a> (1.5 million kilometers) from Earth. In the two months before the event, he said, the prediction shifted by about 70 miles (110 kilometers).<\/p>\n Chariklo\u2019s inner ring blocked more starlight<\/p>\n The rings have names, C1R for the inner one and C2R for the outer. Ground telescopes had timed them in 2013, 2014, and 2017, so the team knew what to expect. The inner ring has been the dense one since the discovery, the outer one the faint one.<\/p>\n C1R came through clearly, with sharp spikes at each edge of its dip, indicating that the ring has sharp edges. And it blocked more light than before: about 43% of the starlight, compared with 30% measured in earlier occultations.<\/p>\n C2R did the opposite: in Webb\u2019s shorter-wavelength color it was barely there, and in the longer one it didn\u2019t show up at all. <\/p>\n Both cameras watched the same stretch of ring at once. The material along Webb\u2019s line through it came out about 60% lower than the 2017 measurements.<\/p>\n \u201cWe knew very well what Chariklo\u2019s rings had looked like in previous stellar occultations, so when we first inspected the data it was immediately striking that C2R was barely visible, while the inner ring, C1R, remained very clear,\u201d Santos-Sanz said in his answers to Earth.com. <\/p>\n His team first checked that it wasn\u2019t an instrument problem or a slip in the analysis, but after careful comparison with earlier occultations, it remained, he said.<\/p>\n A patchy ring can\u2019t explain the change<\/p>\n The team weighed three explanations. The rings could be patchy, so Webb might have crossed a dense stretch of C1R and a thin stretch of C2R.<\/p>\n The rings could also have gained or lost material. Or the grains could block infrared light differently from visible light, making the same rings look different to Webb.<\/p>\n The researchers tested patchiness first, simulating rings of varying density around the circle and drawing random samples 10 million times. <\/p>\n Fewer than one draw in a thousand produced values as low as Webb\u2019s for C2R, and C1R\u2019s high readings were similarly unlikely. Patchiness alone probably isn\u2019t it.<\/p>\n Testing other explanations<\/p>\n The infrared filter idea fit better, but only for the old data. Mixtures of water ice and rocky grains about a micrometer across, a hundredth the width of a hair, can reproduce the measurements from before Webb. <\/p>\n However, add Webb\u2019s two colors and no mixture of ice, rock and carbon the team tried fits them all.<\/p>\n On the researchers\u2019 reading, C1R\u2019s opacity has genuinely gone up, suggesting the presence of more material. For C2R they are less sure, because clean profiles of the outer ring are scarce.<\/p>\n Unseen moonlet could feed Chariklo\u2019s rings<\/p>\n The material didn\u2019t simply move from one ring to the other: C1R gained roughly ten times as much as C2R lost. More material could explain the gain, or collisions that shattered larger particles into many smaller ones.<\/p>\n Neither ring has moved in nearly a decade, so something is holding them in place and replenishing them. For C1R, the leading idea is a rhythm with Chariklo\u2019s spin: one orbit for every three turns of the body.<\/p>\n That rhythm can keep the ring narrow, but it also pushes the ring slowly outward, so an earlier study proposed a small, unseen moon just outside it.<\/p>\n Such a moonlet could share C2R\u2019s orbit and shed material into it. The team estimates that material released at one spot on C2R would spread around the ring in about four months. That fits a ring being resupplied a little at a time.<\/p>\n \u201cOne particularly interesting hypothesis is the presence of a small, still-undetected satellite interacting with the rings,\u201d Santos-Sanz said. It remains a hypothesis, he added, and more occultations from the ground and from space will be needed to understand it.<\/p>\n Other rings change too. Saturn<\/a>\u2018s faint D ring has shrunk over decades, and the team thinks Chariklo\u2019s outer ring may be a dusty, low-mass ring like it.<\/p>\n When the outer ring faded remains open<\/p>\n All of this rests on one observation that crossed each ring at just two points. The team can\u2019t tell whether C2R has been fading gradually since 2017 or dropped suddenly at some point in the five years after. <\/p>\n The earlier ground measurements are too sparse and too uncertain to settle it, and they don\u2019t rule out a decline that started before 2017.<\/p>\n If the outer ring really is losing material, that points to small grains, which sunlight\u2019s pressure can push onto stretched orbits that cross the inner ring. <\/p>\n Something similar may be underway at Chiron<\/a>, another centaur, where material has come and gone between occultations.<\/p>\n More monitoring is the plan<\/p>\n The team wants another occultation in visible light, so the comparison with older data is direct. Asked by Earth.com what comes next, Santos-Sanz said the plan is to keep monitoring. <\/p>\n \u201cIf we again find a more opaque C1R and a much weaker C2R, that would provide very strong evidence that the ring system itself is evolving with time,\u201d he said.<\/p>\n Until that happens, the case that Chariklo\u2019s rings are changing rests on 75 minutes of Webb data and ground measurements from 2013 to 2017 that don\u2019t match it.<\/p>\n The full study was published in the journal Science Advances<\/a>.<\/p>\n \u2014\u2013<\/p>\n Like what you read? Subscribe to our newsletter<\/a> for engaging articles, exclusive content, and the latest updates.<\/p>\n Check us out on EarthSnap<\/a>, a free app brought to you by Eric Ralls<\/a> and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"Chariklo’s rings changed between 2017 and 2022, the inner one denser and the outer one fading, and an…\n","protected":false},"author":2,"featured_media":616389,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[23],"tags":[111,139,69,147,392],"class_list":["post-616388","post","type-post","status-publish","format-standard","has-post-thumbnail","category-space","tag-new-zealand","tag-newzealand","tag-nz","tag-science","tag-space"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/616388","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/comments?post=616388"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/616388\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media\/616389"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media?parent=616388"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/categories?post=616388"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/tags?post=616388"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}