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Alex Parker

A Centaur’s shadow reveals bright rings

Posted by Alex Parker

27-03-2014 9:28 CDT

Topics: explaining science, trojans and centaurs

In 1610, Galileo Galilei first observed Saturn’s ring system through his modest telescope. Saturn remained the only planet known to host rings until 1977, when James Elliot, Edward Dunham, and Douglas Mink discovered the ring system of Uranus. They did so not as Galileo did — who saw Saturn’s rings in reflected light — but rather by detecting the rings’ effect on the light of a background star. Aboard the Kuiper Airborne Observatory, they watched the light from a bright star as Uranus drifted in front of it,  with the goal of studying the properties of Uranus’ atmosphere by analyzing the starlight that was filtered through it. To their surprise, they noticed that before and after the planet dimmed the light of the star, the star “blinked” several times, as if blocked by narrow, partially-opaque structures moving in concert with Uranus. These structures were identified as a series of narrow rings circling the planet.

Discovery of Uranus' rings

Adapted from Elliot et al. (1977)

Discovery of Uranus' rings
Brightness of a background star observed from Kuiper Airborne Observatory in 1977 as Uranus was approaching it, showing sharp dips when five of Uranus' narrow rings swept across the star as seen from the observatory.

Since then, rings have been discovered around Jupiter and Neptune, leaving every giant planet in our solar system clad with rings. Despite substantial search efforts, no natural rings have ever been detected around any of the terrestrial planets.

Yesterday, another team of astronomers announced that they discovered a very similar set of rings with the same technique. However, these rings are not around a giant planet in our solar system or another, nor even a major planet at all. They are hosted by the Centaur Chariklo, an asteroid-like body that currently resides in the unstable region between the orbits of Saturn and Uranus.

Like Uranus, these rings were spotted when Chariklo was observed passing in front of a bright star. A network of telescopes deployed across South America where turned to witness the event,  called a “stellar occultation,” with hopes of accurately measuring the size and shape of Chariklo by measuring the size and shape of its shadow as it swept across the continent. Unexpectedly, before and after Chariklo’s shadow reached many of the telescope sites the star was seen to rapidly blink twice.

Ringed Chariklo

© Don Davis

Ringed Chariklo
Chariklo is the largest Centaur, an icy world in the realm between asteroids and comets. In 2013, it was discovered to have not one, but two, rings. Chariklo is about 250 km or so in diameter. Its inner ring is at 391 kilometers from the center of Chariklo and is 7 kilometers wide. Its outer ring is at 405 kilometers and is 3 kilometers wide. The gap between the rings is 8 kilometers wide.
Evidence of rings in starlight: The rings of Chariklo

Adapted from Braga-Ribas et al. (2014)

Evidence of rings in starlight: The rings of Chariklo
This graph shows how the brightness of the star Chariklo passed in front of as time passed. In addition to the shadow of Chariklo, four other narrow features from the shadows of Chariklo’s rings are visible.

In a paper published today in the journal Nature, lead by astronomer Felipe Braga-Ribas, these events were shown to be the flickers created by the passing shadows of two narrow rings about Chariklo. This is an unprecedented discovery; while Chariklo is the largest known Centaur, it is only 250 kilometers across, with a surface area comparable to the state of Texas. A giant planet it is not, and yet like a giant planet, it hosts a system of rings!

Given its large average distance from Earth, Chariklo is too small to appear as anything but a moving point of light in any of today’s best instruments. Without the fine details provided by the occultation observations, it is likely that Chariklo’s rings would never have been discovered.

Chariklo

ESO / Delsanti

Chariklo
Chariklo is the largest Centaur, a comet-like object that orbits among the giant planets. Images taken April 13, 2010 at the European Southern Observatory's New Technology Telescope.

By modeling the observations from seven different observatories that successfully observed the occultation event, the astronomers determined that the rings orbit at almost exactly one Chariklo diameter above its surface. They are narrow (a few kilometers wide each), close to one another (with a inter-ring gap of only 8 km), and dense, rendering the innermost ring over 30% opaque.

A Centaur Between Giants

The spaces between the major planets are occupied by a variety of populations of minor planets. Between Mars and Jupiter lies the Main Asteroid Belt; leading and trailing Jupiter are the Jovian Trojan swarms. Exterior to Neptune lies the Edgeworth-Kuiper Belt, where Pluto resides. The spaces between the orbits of Jupiter, Saturn, Uranus, and Neptune are populated by the unstable Centaurs, which filter in from exterior to Neptune and are buffeted around by the giant planets’ gravity fields until either becoming comets or being ejected outward once again. These bodies are small, asteroid-like, and icy in composition.

Uranus rings and shepherds

NASA/JPL

Uranus rings and shepherds
Voyager 2 discovered that Uranus' brightest Epsilon ring is shepherded by two satellites, Cordelia (1986U7) and Ophelia (1986U8), each only about 40 kilometers in diameter.

While the Centaur Chariklo currently resides on an orbit between Saturn and Uranus, it likely originated further out in the Edgeworth-Kuiper Belt before close encounters with the giant planets emplaced it on its current orbit. It is still subjected to close encounters with the giant planets, and its days in its current orbit are numbered; on timescales of order tens of millions of years it will likely be ejected from this region of space by its interactions with Uranus.

Short-lived or Shepherded?

Chariklo’s rings are narrow, confined to just a few kilometers in width. In the absence of active shepherding, planetary rings will spread due to collisions between ring particles. Given the altitude of Chariklo’s rings and the size of Chariklo itself, its rings should spread out beyond their current extent in matters of a few thousand years, unless there are small, as-yet-unseen moons there to guide the rings. Such shepherd moons have been seen in planetary ring systems like Saturn’s and Uranus’. According to the analysis presented by Braga-Ribas and co-authors, shepherd satellites only a few kilometers in radius would be sufficient to sustain the features observed in Chariklo’s rings. Such satellites would be extremely faint and close to Chariklo, and would be extremely difficult to detect on their own.

Origin and Survival of Rings

At first glance, it might not seem unusual to find rings around objects like Chariklo. Centaurs and Kuiper Belt objects are known to frequently exist in binary systems, and the destruction or cratering of a binary companion could produce ring material. In fact, this fact caused a great deal of concern when the small satellites of Pluto were first identified; impacts could dredge up material from these small moons and produce rings of debris that could be dangerous for New Horizons during its flyby of the system. An extensive effort was made to identify any such hazards in the Pluto system, and to date none have been found — much to the relief of the New Horizons team!

Yet finding this pair of rings around Chariklo was a shock to many planetary scientists. Many other asteroids and Edgeworth-Kuiper Belt objects have been studied in similar occultations, and none of them have showed unambiguous evidence of long-lived rings; that the first rings to be seen around an asteroid-like object would look so much like the rings of a giant planet — well-defined, dense, closely-spaced, and icy — is a surprise, and may indicate that ring formation is more generic and robust than perhaps assumed. The origin of Chariklo’s rings and their architecture will likely be a question hotly pursued by dynamicists in the coming months. Collisions between moons or other bodies passing through Chariklo’s region of space could produce the icy debris that makes up in the rings, but such a collision would have to occur at a low velocity for the ring structure to form. Braga-Ribas and co-authors suggest that this is more likely to have occurred while the object was still in the Edgeworth-Kuiper Belt, before it became a Centaur. Because Chariklo’s rings are so close to its surface, they are relatively tightly-bound to it, and it would take extremely close flybys of the giant planets to damage them; thus, the rings could likely survive the transition from the Edgeworth-Kuiper Belt into the Centaur region.

Odd Behavior Explained

The authors note that Chariklo had exhibited strange trends between 1997 to 2008; dimming slowly and showing ever-decreasing quantities of water ice in its spectrum. They demonstrate that an ice-rich ring system would neatly account for this; the rings would have appeared edge-on from Earth in 2008, and would have appeared their dimmest then, causing the combined light of Chariklo and its rings to dim as it did. If the rings are water-rich, they would contribute a water-ice component to Chariklo’s spectrum, but this would also fade as the rings moved toward an edge-on configuration in 2008.

Takeaway

There are now five worlds in the entire Universe confirmed to host systems of rings; in order of discovery, Saturn, Uranus, Jupiter, Neptune, and Chariklo. One ring system has been detected around a low-mass object orbiting another star — 1SWASP J140747.93-394542.6 — but it is unclear if these rings circle a massive planet or a low-mass star. Finding a ring system around a small world like Chariklo was unexpected and demonstrates how diverse the minor planets in our solar system are. We have recently witnessed minor planets collide with one another, sprout plumes of water, rip themselves apart by spinning too quickly, and now host planet-like systems of rings.

Asteroids, Centaurs, and Edgeworth-Kuiper Belt objects are demonstrating that they are complex and dynamic worlds in their own right. Next year we visit two dwarf planets for the first time; Dawn will reach Ceres in the asteroid belt, and New Horizons will reach Pluto on the fringes of the Edgeworth-Kuiper Belt. Given the surprises we’ve been given lately by worlds smaller than these, I can only imagine what remains in store for us just around the corner.

 
See other posts from March 2014

 

Or read more blog entries about: explaining science, trojans and centaurs

Comments:

Gene Van Buren: 03/27/2014 01:05 CDT

Very interesting. It seems to me that the presented numbers for radii and width of the rings (and gap) are merely minima for these, under the assumption that we are viewing the rings from an along-the-axis perspective. If viewed obliquely, and the line along which occultation occurs is not along the major axis of the ellipse of the rings in the oblique view, then the observed radii and widths will be smaller than reality. And that's the more likely scenario...unless I'm missing something? Thanks for any additional explanations.

Alex Parker: 03/27/2014 04:20 CDT

There were many other occultation chords other than the one I show in the article, which crossed the rings at a variety of locations. All together, these allowed the authors to determine the exact orientation of the rings (with one non-preferred mirror-degenerate solution). The sizes they quote are quite precise, and not subject to any kind of unfounded assumptions on orientation.

Gene Van Buren: 03/27/2014 08:30 CDT

Thanks, Alex. I see this now in the original article. I just didn't expect there to be enough parallax to do this. Cool!

Robert Little: 03/28/2014 01:19 CDT

Those gaps in the ring system fascinate me. Could they be the result of shepherd moons (which I'm leaning towards), or the 4:3 resonance in Chariklo's orbit?

Lindsay Dawson: 03/28/2014 04:57 CDT

Yes I am wondering about the gap also. Would it be similar to the Daphnis ploughed Keeler Gap in Saturn’s A Ring and if so how large (or massive) would any moon need to be to clear the 9 kilometer wide gap? On the other hand could the rings be more akin to Saturn’s Prometheus braided F ring? I suppose these analogies with Saturn’s rings must be tempered by the difference in scale of the bodies that the rings orbit. Wouldn’t there be differing tidal strengths to consider? With a diameter of some 250km Chariklo must almost certainly be irregularly shaped. I wonder if the occultation last year shed any light on that. A rough analogue would be the moons Phoebe or Hyperion being of similar size. Phoebe is actually fairly roughly round in a severely battered kind of way but more interestingly is probably a captured Centaur object and so could be very similar indeed to Chariklo. While being far from an expert in ring dynamics, a rotating irregular shaped body (even a roughly rounded Phoebe shape) would surely impart significant perturbations on any orbiting ring particles and literally make waves in them. That alone could well be a factor that shapes the rings with some kind of ring orbital resonant effects with Chariklo’s rotation period coming into play. All that in concert with any shepherding satellites and the smaller scale of the system could possibly result in some dramatic wakes and waveforms departing from the ring orbital plane. It certainly would not be a boring place to visit but realistically the chances of any flyby coming about at this time would probably be about the same as Buckley’s. It is a long way away. Still we can always live in hope.

Daniel Machacek: 03/28/2014 04:18 CDT

Multiple missions were proposed to some Centaurs or even Chariklo himself. For example "Chariklo Orbiter with Eurysaces Flyby Mission" or "Centaur Flyby SmallSat mission". Unfortunately such mission is not possible without reasonable budgets for New Frontiers or Discovery programs.

Saxton Hale: 03/30/2014 11:58 CDT

So wouldn't a near-miss occultation by an unknown ringed body be much more probable than a clean eclipse?

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