Ring in the new

Planets are no longer the only Solar System bodies sporting ring systems. Two dense rings have been detected encircling a Centaur object — a relatively small, icy interloper from the distant reaches of the Solar System. See Letter p.72

To date, rings containing icy and rocky particles have been known to orbit only the four giant planets in the Solar System, most notably Saturn¹. Now, continuing nature's excellent track record of upending the firm beliefs of planetary theorists, a fully developed ring system has been detected surrounding the Centaur object (10199) Chariklo, as Braga-Ribas et al.² report in a paper published on page 72 today.

Centaurs are relatively small icy bodies that travel along short-lived, moderately elliptical orbits in the region of the giant planets. The discovery was made when Braga-Ribas and colleagues stationed an armada of small telescopes across a 1,500-kilometre-wide swathe of South America to watch as the enigmatic Chariklo crossed in front of a star (Fig. 1). Astronomers use such stellar occultations just as a motorist gauges an obstacle's location and silhouette on a dimly lit highway by watching background lights flicker as the obstacle passes before them. Here, the authors sought to use this technique to refine Chariklo's size and to determine its shape during a predicted occultation. But they — and we — got much more.

Figure 1: Chariklo's rings.

When a Solar System body transits in front of a star, blocking its light, the star will blink off for an interval proportional to the length of the chord across the body. Observing such a stellar occultation, Braga-Ribas et al.² detected two dark rings surrounding Chariklo, a Centaur object with a radius of about 125 kilometres. The inner ring, centred at 391 km, is about 7 km across and extinguishes roughly 40% of the star's light. A clear gap of about 9 km separates it from the outer ring, which is centred at 405 km, is roughly 3 km wide and removes about 5% of the starlight. The star's track (dotted lines) crossed previously unknown rings as seen from several separate sites, including the La Silla Observatory in Chile, which obtained the highest time resolution. The star's path (dashed line) seen from another observatory missed the rings. Chariklo is darker than drawn here. (Illustration adapted from Fig. 2 of the paper².)

Chariklo's rings are not the first to be discovered by occultation. Although planetary experts³ had maintained for decades that special circumstances had led Saturn, alone among the planets, to sport rings, a complex ring system was detected^4,5nestled within a few radii of Uranus during an occultation in 1977. In the following decade, many occultations probed Neptune's surroundings, but only occasionally was the star observed to flicker, and then on just one side of the planet⁶. Eventually, the realization struck that the hit-and-miss nature of these few successful detections could be explained if Neptune's rings were restricted to localized arcs covering about 10% of the rings' circumference⁷. Images obtained with the Voyager 2 spacecraft confirmed⁸ this unexpected morphology in 1989.

This is not the first time that Chariklo has surprised observers. Following its discovery in 1997, the object's brightness systematically and mysteriously dropped by 40% (ref. 9), and its strong water-ice signature gradually faded¹⁰, only to have these trends reverse since 2008. The identification of Chariklo's rings now explains these puzzling decreases in brightness and spectral intensity: they occur as ice-rich rings, which have 15% of Chariklo's surface area but 3 times its reflectivity, become edge-on when viewed from Earth. Like so many baffling observations in science, the answer is obvious once its explanation is known.

Of the four planetary ring systems, that of Uranus provides the closest analogue to Chariklo's pair. Its dozen or so isolated, coal-black rings — most only a few kilometres wide, with crisp edges and sometimes slightly variable widths — are separated by clear gaps. Small moons known as shepherd satellites were suggested¹¹ to prise open lanes in the Uranian rings and to confine their edges. In this mechanism, high-order gravitational perturbations of moons generate repulsive torques on nearby disks of orbiting material.

To be effective at their task, the shepherds need to have masses comparable to those of the ringlets they herd. In other words, the moons should be perhaps a few kilometres in radius — too small to be visible through a ground-based telescope. When Voyagers 1 and 2 subsequently spied two tiny moons herding Saturn's F ring, another pair guiding particles in Uranus' ε ring, and also the small moon Pan in the Encke gap in Saturn's A ring, this clever mechanism became enshrined as fact¹. It is now routinely called upon to explain openings and confined rings in all sorts of astrophysical disks, even in the protoplanetary disks within which planets form.

Naturally, then, Braga-Ribas and co-authors hypothesize that hidden small shepherd satellites account for the gap between Chariklo's rings as well as the crisp peripheries of both rings. But a dirty secret of planetary rings should be exposed: following exhaustive searches since 2004 using the Cassini spacecraft, it is almost certain that none of the numerous gaps in Saturn's C ring and in its Cassini Division (a low-density band between Saturn's main A and B rings) harbour any shepherds of the requisite size¹².

Perhaps the physics missing in our attempts to explain such ring features as gaps will be revealed by investigations of Chariklo's much simpler system. Its rings extend a mere three-thousandth the dimensions of Saturn's; indeed, its entire retinue could slip — with much room to spare — through the largest gaps at the outer and inner edges of Saturn's rings. Circular orbital speeds in the ring regions surrounding the planets and Chariklo scale with the size of the central body, and are merely tens of metres per second for Chariklo's ring particles. Relative speeds between adjacent particles in Chariklo's rings will allow collisions that thicken the rings, whereas Saturn's rings, with their higher orbital speeds, are much thinner. The reduced gravity, low speeds and resulting gentle collisions at Chariklo permit ring dynamics to be investigated in a previously unimagined regime.

How might such a diminutive ring system have formed? Given Chariklo's relatively small gravitational influence, it seems improbable that its rings formed contemporaneously with the processes that gave birth to the Centaur itself. A more likely scenario is one similar to that proposed for the origin of our Moon¹³, in which a nearly catastrophic collision between the primordial Earth and a Mars-sized object lofted copious impact ejecta into an orbiting disk, from which the Moon subsequently formed. Whatever its cause, as this disk spreads out by a few Chariklo radii, the largest shards in the disk could shepherd the remaining disk material.

An alternative model arises from the fact that about five per cent of Centaurs and Trans-Neptunian Objects have small companions, possibly obtained as a result of three-body interactions¹⁴. Such satellites may be gently disrupted by impacts of interplanetary debris. At Chariklo's ring distances, orbital speeds are low, but some of the impact ejecta will depart from their source even more slowly, implying that a ring-like tube of fragments might enshroud the satellite's orbit¹⁵.

The detection of rings around Chariklo will startle many planetary theorists. But so it has always been in planetary exploration: theoretical ideas rarely generate searches that lead to discovery — rather, discoveries such as this prompt us to new understandings.