Astronomers using the James Webb Space Telescope have found that the rings around a small solar system body named Chariklo are changing over time. Chariklo orbits the sun between Saturn and Uranus, at a distance of about 17 times Earth’s distance from the sun. The asteroid is only about 155 miles (250 kilometers) wide and belongs to a group of objects called Centaurs.
Despite its small size, Chariklo has two dense and narrow rings around it, which was unexpected when they were first discovered, according to Space.com. The rings were first seen in 2013 using ground-based telescopes during a star occultation event, according to a report from martincid.com. For years, scientists thought the rings were stable and did not change much over time. The two rings orbit about 390 and 405 kilometers from Chariklo’s center and are only a few kilometers wide.
But new data from Webb shows that one ring is more opaque while the other is fading. The two rings changed in opposite directions over the period covered by the observations, an unexpected result. The research team, led by scientists from Spain’s Institute of Astrophysics of Andalusia, began observing Chariklo with Webb in October 2022. The occultation was observed on October 18, 2022 and was the first stellar occultation specifically planned for Webb and successfully observed with the telescope.
They used the telescope’s NIRCam instrument to monitor a star as Chariklo’s rings passed in front of it. Chariklo itself did not pass in front of the star from Webb’s viewpoint. This event allowed Webb to measure the rings with much higher detail than ever before. The occultation provided spatial resolution on the order of one kilometer. At the time, Chariklo was moving relative to Webb at only about 2.5 kilometers per second, which helped produce the unusually high resolution. NASA reported that NIRSpec was used later on October 31 to collect a spectrum of the Chariklo system, not for the October 18 occultation.
This produced the highest spatial resolution yet achieved in observations of Chariklo’s rings. The team compared this new data with observations taken over the past decade using ground-based telescopes. They found that the inner ring, called C1R, became more opaque in 2022 compared to earlier years. According to the Paris Observatory, C1R appeared about 50 percent more opaque than before.
At the same time, the outer ring, C2R, became less visible. Its opacity appeared to decrease by about 60 percent compared with 2017, according to the Paris Observatory. These changes were not seen in earlier studies, which had suggested the rings were stable. The findings challenge what scientists believed about how ring systems behave around small bodies in space. The positions of the two rings themselves remained essentially unchanged.
Previous observations had not revealed changes this large on such short timescales. But now, the data shows that even small objects can have evolving ring systems. Scientists are unsure what is causing these changes in the rings.
One idea is that material moved between the two rings over time. Another possibility is a collision or gravitational effect from a moon, if one exists. It’s also possible that the change is due to how light behaves at different wavelengths. Sky & Telescope reported that the researchers are considering mass transfer between the rings and the possible influence of an unseen shepherding moon if the changes are physical.
The rings may look different depending on which part of the spectrum is being observed. Another explanation combines both real changes and observational differences. The lead author of the study, Pablo Santos-Sanz, said that the exact cause is still unknown.
The Webb occultation produced the most detailed spatial measurements yet of Chariklo’s rings. However, more observations are needed to fully understand what is happening. Future occultations, particularly at visible wavelengths, could show whether the changes continue or whether some of the difference is caused by wavelength.
The results were published in the journal Science Advances on September 9. This discovery shows that even small objects in the solar system can have complex and changing features. It also raises new questions about how rings form and stay stable over time.
The research team hopes that future data will help answer these questions. They are careful not to favor any single explanation yet. The study is an important step in understanding how ring systems work across the solar system.
It shows that even distant and small objects can surprise scientists with new discoveries.
IMAGE: Observations at many sites in South America, including ESO’s La Silla Observatory, have made the surprise discovery that the remote asteroid Chariklo is surrounded by two dense and narrow rings. This is the smallest object by far found to have rings and only the fifth body in the Solar System — after the much larger planets Jupiter, Saturn, Uranus and Neptune — to have this feature. The origin of these rings remains a mystery, but they may be the result of a collision that created a disc of debris.
This artist’s impression shows how the rings might look from close to the surface of Chariklo.. Photo: ESO/L. Calçada/Nick Risinger (skysurvey.org) / Wikimedia, taken 2014-03-26, CC BY 4.0
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