In 2013, using ground-based telescopes, theastronomerastronomer Brazilian Felipe Braga-Ribas and his collaborators had discovered that the centaur (10199) Chariklo, a small body whoseorbitorbit lies between those of SaturnSaturn and D’UranusUranus, hosts a system of two thin rings (see our 2014 article below). Previously, the only known rings were those around the four giant planetsgiant planets (Jupiter, Saturn, Uranus and Neptune).
To locate the rings of Chariklo, astronomers had observed the passage of the small body in front of a star. To their great surprise, the luminous star had disappeared twice before then, twice, after its occultationoccultation by Chariklo, revealing the presence of the rings. The latter orbit about 400 kilometers from the center of Chariklo, which measures about 250 kilometers in diameter, making it the largest known centaur.
A first concealment for James-Webb
On October 18, 2022, Pablo Santos-Sanz, of the Institute ofastrophysicsastrophysics of Andalusia (GrenadeGrenadeSpain), and his colleagues used the same technique to re-observe these rings, but this time using the near camera infraredinfrared (NIRCamNIRCam) from the James Webb Space Telescope. This was the first attempt to observe a stellar occultation with this telescope. The event light curve, i.e. the graph of the brightnessbrightness of the’starstar (ici GaiaGaia DR3 6873519665992128512) over time shows that the “shadows” produced by Chariklo’s rings were clearly detected, demonstrating a new way of using Webb to explore objects in the Solar systemSolar system. As expected after the last trajectory maneuver of WebbChariklo did not occult the star himself.
Santos-Sanz explained: « As we dig deeper into the data, we will explore whether we resolve both rings cleanly. From the shape of the ring occultation light curves, we will also explore the thickness of the rings, the size and the colorcolor ring particles, and more. We hope to understand why this small body has rings, and perhaps detect new, fainter rings. »
The rings are probably composed of small particles of water ice mixed with a materialmaterial dark, debris from an icy body that collided with Chariklo in the past. Chariklo is too small and too distant for even James-Webb to directly image the rings separated from the main body; occultations are therefore the only way to characterize the rings by themselves.
Crystalline water ice in the Chariklo system
Shortly after the occultation, Webb again targeted Chariklo, this time to gather observations of sunlight reflected from Chariklo and its rings. the spectrespectre of the system shows three bands ofabsorptionabsorption water ice. Noemí Pinilla-Alonso, who led these spectroscopic observations, explained that ” spectra from ground-based telescopes suggested the presence of this ice (Duffard et al. 2014), but the exquisite quality of Webb’s spectrum revealed the clear signature of crystalline ice for the first time “. Dean Hines, the principal investigator of this second program, added: “ Since particles with high energyenergy transform ice from a crystalline state to a amorphousamorphousdetection of crystalline ice indicates that the Chariklo system is undergoing continuous micro-collisions that expose pristine material or initiate crystallization processes. »
Most of the reflected lightreflected light in the spectrum comes from Chariklo itself: the models suggest that the area of the observed ring, as seen by JWST during these observations, is probably one-fifth of the area of the body itself. Pinilla-Alonso commented: “ By observing Chariklo with Webb over several years as the angle of view of the rings changes, we may be able to isolate the contribution of the rings themselves. »
Chariklo, the asteroid with two rings
Article by Xavier Demeersman, published on 27 mars 2014
During the occultation of a star by the largest asteroidasteroid known to the family of centaurs, astronomers who sought to characterize it noted with surprise the presence of two rings. Confirmed by the joint observation of seven telescopes, their existence is a first in the history of astronomy which, until now, only knew of them in the environment of the four largest planets of the Solar System.
In the Solar System, we all know that magnificent rings surround Saturn (a spectacle that one never tires of observing). Less famous and more tenuous, three others surround the giant planets JupiterJupiterUranus and NeptuneNeptune. It has only been thirty years since those of the last two were discovered (respectively in 1977 and 1984). But as nature still has many surprises in store for us, we will now have to rely on rings around an asteroid! Who could have imagined this before?
Disclosed on March 26, the announcement had its effect within the scientific community. The co-discoverer Felipe Braga-Ribas (national observatory, MCTI, in Rio de Janeiro) who signed the article published in the online edition of the journal Natureconcedes: he and his entire team of researchers were not looking for a ring. “We were also far from thinking that small bodies such as Chariklo had them. So this discovery — like the surprising amount of detail observed — turned out to be a real surprise. ! » They really didn’t expect it…
An unexpected satellite
Informed that the large asteroid (10199) Chariklo, then about two billion kilometers from Earth, would pass in front of the distant star designated UCAC248-108672 during the night of June 3, 2013, astronomers mobilized to go to the only terrestrial region where the phenomenon would be observable, a narrow band in the north of Chile. No less than seven large telescopes in the region, including the Danish telescope (1.54 meters in diameter) and the Belgian Trappist telescope (TRAnsiting Planets and PlanetesImals Small Telescope) installed at the La Silla Observatory (Eso), recorded the event.
The occultation of a star by an asteroid, a cometcomet or even a dwarf planetdwarf planet is always a great opportunity for researchers to characterize the shapes, sizes and compositions of these small bodies that belong to our solar system. On this subject, moreover, no later than March 20, a fraction of Americans were able to follow the transittransit of the asteroid (163) Erigone (73 km in diameter) in front of the brightest star of the LionLion, RegulusRegulus (of Leo, CorCor of Leo).
Protean Centaur
Discovered in 1977, Chariklo is known to be the largest asteroid — about 250 km at its greatest length — in the protean Centaur family. Remember that, like the half-human half-horse mythological figures of the same name, the celestial objects which have the particularity of circulating in irregular orbits between Saturn and Uranus (constantly disturbed by gravitation, they probably originate from the Kuiper beltKuiper belton the margins of Neptune) share their identity between comet and asteroid.
What was the surprise of the team to notice irregularities during the moments which preceded and followed the occultation! Indeed, the brightness variations of this magnitude 12 star betrayed the existence of material around the main body. By cross-checking all the available data, the astronomers were able to distinguish two rings of different density around Chariklo. Reflecting on the unusual landscape, Uffe Gråe Jørgensen from the University of Copenhagen says: “I try to imagine myself standing on the surface of this icy object — small enough for a carcar racer can reach escape velocity and soar into space — observing a system of rings 20 km wide and 1,000 times closer than the LuneLune of the earth »
Two thin rings escorted by small satellites
Arbitrarily named after the Brazilian rivers Oyapock and Chuí, the rings extend 391 km and 405 km respectively from the center of the asteroid. The closest is also the densest. Its width is estimated at 7 km and it would have 12 times more material than its neighbour, which occupies a width of 3 km. About 9 km separate these two rings. Their spectral study showed that they are rich in water. If they could be aggregated into a single body, the densest would be enough to form a frozen sphere 2 km in diameter, while the smallest would only represent a ball with a diameter twice as small.
Finally, to explain their unexpected presence, the researchers have identified several possible scenarios, recognizing however that it is still too early to decide. It can of course be debris amassed after one or more collisions between two asteroids or comets, residues of a comet, a dispersion of materials, etc. Despite everything, the team suspects that two small natural satellites, still well hidden, played a decisive role in the formation of the rings. Considered real dogsdogs of a shepherd, following the example of what is observed at the edge of the rings of saturnrings of saturn, they would be the guarantors of the good performance of the rings that surround Chariklo. It is not excluded that eventually, in more or less ten million years, they will form into small satellites.
In the meantime, it remains for astronomers to flush out the two potential small moons. “Like the rings, one or more small satellites are certainly waiting to be discovered around Chariklo”, says Felipe Braga-Ribas. So many elements that will allow in the future to refine the models of the formation of bodies in the immediate environment of objects larger and more massive than them, like our Moon.
