Astronomers using the James Webb Space Telescope (JWST) have discovered that the ring system of the tiny solar system body Chariklo has changed over the past decade. Orbiting the sun between Saturn and Uranus at approximately 17 times the distance between Earth and the sun, Chariklo measures about 155 miles (250 kilometers) wide and is part of the Centaur family of asteroids. Despite its small size, the body possesses two thick rings. While giant planets such as Saturn, Uranus, Neptune, and Jupiter are famous for their ring systems, Chariklo and the smaller body Chiron demonstrate that diminutive bodies can also develop rings.
Discovering Changes in Chariklo’s Rings
A research team led by scientists from the Institute of Astrophysics of Andalusia (IAA-CSIC) began observing Chariklo with the JWST in October 2022 using stellar occultation, a technique that measures the decrease in light from a background star when an object passes in front of it. Team leader Pablo Santos-Sanz noted that by comparing JWST observations with data from stellar occultations gathered over the preceding decade, researchers found opposite changes in the two rings: the inner ring exhibits significantly higher opacity, whereas the outer ring shows lower opacity. The findings were published in Science Advances.
Precision Navigation and Mechanics
Achieving these unprecedented observations required extraordinary precision regarding the orbit of Chariklo, the position of the star, and the trajectory of the JWST itself around the L2 Lagrange point located about 1 million miles (1.5 million kilometers) beyond Earth, away from the sun. Team member Yücel Kilic of the IAA-CSIC explained that the telescope maintains its position around this region through periodic station-keeping maneuvers, utilizing star position data from the European Space Agency’s Gaia mission.
During the specific stellar occultation used in the research, Chariklo traveled at approximately 5,600 miles per hour (2.5 kilometers per second) relative to the JWST. Although fast by terrestrial standards, this speed is relatively slow for objects moving through the solar system, allowing the rings to be resolved in unprecedented detail. Because the asteroid is too small and distant for direct imaging even by the JWST, astronomers continue to rely on stellar occultation methods.
The Ghost Moon Hypothesis and Ring Composition
The observed changes suggest that Chariklo experiences more complex physics than previously understood, challenging the prior scientific belief that small bodies maintain relatively stable rings. Santos-Sanz stated that the results force a reevaluation of how such rings form, evolve, and maintain stability. The exact cause of the changes remains a mystery, though researchers have proposed hypotheses.
The leading explanation points to a small shepherd satellite sharing the outer ring’s orbit, which could explain the rings’ stability and sharp edges while shedding debris that replenishes the inner ring, designated C1R. Santos-Sanz noted that this satellite has not yet been detected. Furthermore, a computer model based on JWST data suggests compositional differences between the two rings, indicating that the inner ring consists of larger particles while the outer ring is more dusty, though researchers emphasize that this modeling remains a work in progress.
Broader Context Among Minor Bodies
Chariklo is not isolated in possessing a ring system. Rings are now documented around Chiron, the dwarf planet Haumea, and the trans-Neptunian object Quaoar. While giant-planet rings are known to shift over months and years—such as the measurable shrinkage of Saturn’s D ring and the rearrangement of Neptune’s Adams arcs—small bodies now appear to undergo similar transformations.
To separate genuine physical changes from wavelength effects, the research team aims to capture another Chariklo occultation utilizing visible light. Santos-Sanz described the current findings as a crucial piece of the puzzle that provides important clues for broader studies concerning ring systems around minor bodies and giant planets alike.
