Pocket Full of Posies, Ring Around the…Centaur?

Usually, a centaur refers to a mythical half horse half person creature. In studies of the solar system, and the remainder of this post, a Centaur is one of the objects orbiting the sun between Jupiter and Neptune. They don’t always fall neatly into categories of ‘asteroid’ or ‘comet’ (though a large portion of them appear to be comet-like).
Like centaurs, mention of rings around bodies in the solar system conjures up one image: the gas giant planet Saturn. While Saturn has by far the most impressive ring system, all of the outer four planets (Saturn, Jupiter, Uranus and Neptune) have rings. For decades these were the only known rings in our solar system; but in 2014, a Centaur object known as Chairklo also was found to have rings.
While Saturn's rings are known to most, the rings of the other gas giant planets are often ignored. Uranus and its rings are shown in in a composite. Image credit: W. M. Keck Observatory (Marcos van Dam)/NASA.
While Saturn’s rings are known to most, the rings of the other gas giant planets are often ignored. Uranus and its rings are shown in in a composite. Image credit: W. M. Keck Observatory/NASA.
Jupiter orbits the sun at a distance of 5.2 times the Earth-Sun distance (an astronomical unit, or AU). Saturn orbits at 9.54 AU, Uranus at 19.1, and distant Neptune at 30.1 AU. Chairklo keeps an orbit that varies from 13 and 19 AU. A second Centaur object, which possibly also has rings, orbits between 8 and 19 AU. In other words, the three dense ring systems–Saturn, Uranus and Chairklo–all fall between 8 and 20 AU.
That could be coincidence. Perhaps we simply have not seen such ring systems on bodies outside these ranges yet. There have been many observations of many objects, but we certainly have not cataloged and organized every object in our solar system! Perhaps it simply tells us more about the formation of each unique planet.
But what if this isn’t a coincidence, but a pattern?
This question is exactly what M. M. Hedman investigates in their article “Why are dense planetary rings only found between 8 and 20 AU?“. Published in The Astrophysical Journal Letters in 2015, the author looks to see if there is something about this region of our solar system that makes it preferential for dense ring systems.
Both Saturn and Uranus have rings close by, then moons in more distant orbits, with some area of overlap between. Jupiter and Neptune do not follow this pattern. One possibility here is that the material around Saturn and Uranus are weaker than their outer solar system counterparts. Strength and weakness here are references to how well a material can withstand outside forces; in this context, being pulled apart. The author ran through some math; since this blog is meant for nonscientists, I’m going to skip the equations and get to the results. The materials around Jupiter and Neptune were likely stronger than those around Saturn and Uranus, and thus formed moons instead of broken up chunks of rock and ice to form rings. This leads to a suggestion that weak material is needed to form dense rings.
Also of note is the temperature of the rings. Saturn’s rings have been measured, and have an average temperature of 66-75 Kelvin (-207 C/-341 F to -198 C/-325 F). From the albedo (a measure of reflected sunlight) of Uranus’ rings, they are about 65 Kelvin. The rings of Chairklo have an albedo, and thus likely temperature, in between that of Saturn and Uranus’ rings. In other words, all the ring temperatures center around 70 Kelvin. In science, that kind of coincidence is suspicious.
Given the two items noted above, perhaps this means that icy bodies are especially weak around 70 Kelvin, and thus more prone to forming dense rings. One problem with this idea is that there are plenty of icy moons, including ones around Saturn, that are not weak, and even retain the shapes of features such as craters. Very weak materials could not do this. Perhaps then it has to do with size: larger objects are stronger, while smaller objects are naturally more porous (have small holes in them) and thus weaker. Some of Saturn’s moons fit this concept, but without comparison to the moons of Jupiter, Uranus and Neptune, there is too little data to give us a good indication either way.
Then again, maybe this is only a look at dense rings seen so far. Three sets of data points on rings is not a lot to go by, and this makes it difficult to draw conclusions. Finding other dense rings, both within and outside our solar system, could give us more clues. Also, most studies on icy materials do not test them as cold as 70 Kelvin; if there is something special about this temperature for porous ices, we may not have seen it yet.
Luckily for us, in science, it is okay to be unsure and keep searching for clues.
Bibliography
Bennett, Jeffrey O., ed. The Cosmic Perspective: the Solar System. 7th ed. Boston: Addison-Wesley, 2014. Print.
Textbook used for the distances of the outer planets.
Dunford, Bill. Davis, Phillips, ed. Uranus Composite Ring Image.  National Aeronautics and Space Administration, 14 Dec. 2012. Web.
The source of today’s image post.
Greicius, Tony, ed. NEOWISE Eyes the Enigmatic CentaursNational Aeronatuics and Space Administration. 28 July 2013. Web.
A brief explanation about Centaurs, and how they were found to be mostly comet-like.
Hedman, M. M. “Why are dense planetary rings only found between 8 and 20 AU?.” The Astrophysical Journal Letters 801.2 (2015): L33.
Today’s main article.
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