Space and Physics
PUBLISHED
Researchers observing the distant world of Chariklo have found that not only does it have a ring system, but that those rings have changed over the course of just a few years.
We know we're biased as a science website, but astronomers are not given enough credit for the ridiculous ingenuity they show in their work. For an obvious and relevant example, they have been getting silly with stellar occultation – observing an object or planet as it passes in front of a star.
"When a celestial object such as an asteroid passes in front of a star, the brief dimming of starlight reveals information about the occulting body’s size, shape, and surroundings," the team, led by scientists at the Institute of Astrophysics of Andalusia (IAA-CSIC), explain in their new paper, adding that this method has been used to detect tenuous atmospheres on these objects.
As well as this, observations during stellar occultations have led to the discovery of ring systems on Centaurs Chariklo and Chiron, dwarf planet Haumea, and trans-Neptunian (TNO) object Quaoar. By observing the dimming of the stars behind them, astronomers have found these objects to have delicate rings around them, once thought to be exclusively found around giant planets.
Chariklo was amongst the first of these smaller bodies to have their rings discovered, when an IAA-CSIC-led team used a new technique to look at the shadows cast by Chariklo, finding the previously invisible ring system on a surprisingly small world.
"With a diameter of about 188 miles (302 kilometers), Chariklo is the largest member of an asteroid class known as the Centaurs. It orbits between more than 2 billion miles away beyond the orbit of Saturn," NASA explains.
"Chariklo is about ~51 times smaller than Earth in diameter, and its rings orbit at a distance of about 250 miles (400 kilometers) from the center of the body. The rings are believed to be between two and four miles (3-7 kilometers) wide."
At that distance, stellar occultation is the only way to observe the ring system, monitoring for dips in light to "see" them indirectly. In further observations with the JWST, the team was able to take advantage of the fact that Chariklo was moving at a low relative speed to it.
At just 2.5 kilometers per second, the team was able to provide much better spatial resolution on the ring system. While they had expected ring systems on these smaller worlds to be relatively stable, in these follow-up observations the team was surprised to find that they appear to have changed over the course of a few short years.
“By comparing JWST observations with those obtained during other stellar occultations over the last decade, we discovered opposite changes in the two rings: while the inner ring shows significantly higher opacity, the outer ring shows lower opacity,” Pablo Santos-Sanz, an IAA-CSIC researcher who lead the study, said in a statement.
That's pretty surprising, probably more so to astronomers used to working on astronomical timescales, and suggests that these ring system may be a lot more dynamic than previously thought.
“Our results force us to rethink how they form, how they evolve, and what mechanisms maintain their stability. The ability to detect these changes opens a new window for understanding the evolution of these systems and, possibly, that of other ring systems in the Solar System,” Santos-Sanz added.
There are a few other mysteries to be solved here. The team believes that the inner ring shows genuine signs of being more opaque than before, suggesting more material within it.
The outer ring, meanwhile has shown signs of being less opaque, though this could be due to the opacity being dependent on wavelength. If it is not due to wavelength, the team suggests that the particles that make it up must be teeny tiny in scale. If that were correct, they would also be dispersed relatively quickly due to radiation pressure, distorting the rings on short timescales.
Yet looking at the rings, neither of them appear to have been disturbed as you would expect, "suggesting that a stabilizing or replenishing mechanism must be operating."
The team writes that it was thought that the ring system was held in place by orbital resonance, where a particle orbits Chariklo once for every three rotations of the object. However, this resonance would also drive an outward migration of the rings.
Another team suggested instead that the rings are held in place with another "shepherd" moonlet, which would also provide more material to the ring system. It is this explanation that is consistent with the team's data, based on estimates of how material would spread in both versions.
As always, further observations are needed. Perhaps we will see more changes to the ring system in future, and answer further questions about them.
"The unexpected changes detected in Chariklo’s rings point to dynamical behavior that had remained entirely hidden until now, although they do not yet provide a definitive physical picture," the team writes in their discussion.
"Whether these changes primarily reflect temporal evolution, wavelength-dependent opacity, or a combination of both, their physical origin remains an open question, offering an unprecedented window into the physical processes shaping ring systems around minor bodies."
The study is published in Science Advances.
