In our solar system, the ring system of Saturn dwarfs all others. While such a large and complex ring system is rare for our solar system, it’s reasonable to presume that similar ring systems exist throughout the cosmos. We know, for example, that Saturn’s rings are stable against things like meteor impacts, and that they are likely as old as the planet itself. Now we know of at least one exoplanet with a large and complex ring system.

Brightness of the eclipsed star (red) compared with the ring model (green). Credit: Kenworthy & Mamajek

It is a planet known as J1407b, which is a brown dwarf that orbits a young sunlike star (J1407). In 2007 the star went through a series of eclipses, where something passed in front of it. Earlier studies of these eclipses hinted at the existence of a ring system around J1407b. As the planet passed near the star from our perspective, the rings of the planet occulted the star, causing it to vary in brightness over a period of more than 50 days. Now a new paper being published in the Astrophysical Journal confirms these rings. The new work also demonstrates that the ring system is complex, much like Saturn’s rings.

The team compared the occultation data with models of ring systems to determine the basic structure of the rings. They found it spans a distance 200 times that of Saturn’s rings, and contains about an Earth-mass of material. It even has what appears to be a large gap in the ring system. Such gaps are known to form in Saturn’s rings, for example, when a moon or moons clears its orbital path. If that’s the case for this gap, it could contain a moon somewhere between the size of Mars and Earth.

The J1407 system is only about 16 million years old, so it is quite possible that thing ring system is in the process of forming moons. If that’s the case the rings may gradually diminish as the moon system forms. For now, it is the greatest ringed planet we know.

Paper: Matthew A. Kenworthy & Eric E. Mamajek. Modeling giant extrasolar ring systems in eclipse and the case of J1407b: sculpting by exomoons? arXiv:1501.05652 [astro-ph.SR] (2015)

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Saturn is everyone’s favorite ringed planet. It’s ring system is both complex and extraordinarily bright. Just why it is so bright has been a bit of a mystery. It’s brightness is due to the fact that the rings are composed almost entirely of water ice, and the fact that there isn’t a great deal of dust in the rings. Because of this, it was once thought that Saturn’s rings were generally young. If the ring system were old, one would expect it to darken over time as dust and other debris from the solar system accumulate over time. This also agreed with the idea that such a complex ring system would likely be unstable over millions of years.

Since then we’ve come to understand how complex interactions can occur within Saturn’s rings, and computer simulation have shown that complex ring systems can be stable over billions of years, and observational evidence has even shown Saturn’s rings can withstand cometary or asteroid bombardment. Spectral analysis of the rings show that its composition varies with distance, just as the moons of Saturn do, and this suggests the ring system formed around the same time as Saturn itself. But if Saturn’s rings are indeed old, how is it that they remain so bright? We’ve also observed that ice particles within the rings are continually clumping and breaking apart, which would help keep the rings looking “fresh”, but this doesn’t seem to be enough to keep the rings bright over billions of years.

Now new results announced at the University of Colorado may have solved the mystery. They analyzed the number dust particles striking the Cassini probe as it orbits Saturn. By determining the trajectories of these particles, the team could determine the rate at which material from the solar system is captured by Saturn’s rings. What they found was that the accumulation rate is smaller than previously supposed by a factor of 20. This means Saturn’s rings could remain bright for much longer than we had thought.  Based upon the accumulation rate, the team estimated Saturn’s rings to be about 4.4 billion years. This is within the range of Saturn’s formation.

So it seems that Saturn has always been a ringed planet.

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The rings of Saturn are among the more beautiful objects in our solar system. With their subtle variations they seem fragile, as if the slightest disruption would cause them to fall apart like a house of cards. In fact the rings are quite old, dating back about four billion years or more. They have evolved over time, and were likely much more extensive in the past, but they are hardly fragile.

Just how disturbances can affect the rings isn’t fully understood, but we have observed some disturbances in action. One of these you can see in the image below. The image below shows a section of the rings of Saturn, in particular the inner portion known as the C-ring (the brighter part in the middle and upper left) and the D-ring (the dimmer part in the lower right).

Credit: NASA/JPL/Space Science Institute

You might notice that in the central region there are these bands of lighter and darker regions. These variations are not caused by varying thickness within the ring, but rather by ripples in the ring itself. As the orientation of the ripples vary, the brightness of the ring varies. What’s interesting is that these variations weren’t observed by Voyager I and II when they passed Saturn in 1980 and 1981, which means something must have caused them between then and now. It must have been something pretty dramatic to cause such a large effect.

So what could it be? The most obvious suspect is an asteroid or comet, but to have such an effect it would need to have struck a wide region of the rings. The observed ripples span the entire ring at that distance, so it couldn’t have been a small, local collision. A wide collision would mean that the rings would become just slightly tilted relative to Saturn. The tiny difference in orientation between Saturn and its rings would then cause a ripple to form in the ring over time. You can see how this effect would work in the animation here.

While it might seem odd that a small comet or asteroid could shift the rings enough to have an effect, we’ve actually seen this happen before. In 1994 the comet Shoemaker-Levy 9 collided with Jupiter. By the time Galileo reached Jupiter, a similar spiraling pattern had formed in the faint rings of Jupiter. By modeling these ripples, it could be determined that they had started forming between June and September of 1994. The Shoemaker-Levy 9 collision occurred in July.

If the same model is applied to the ripples in Saturn’s inner rings, you get a collision sometime in 1983, which confirms that it would have occurred after the Voyager missions. The collision was likely similar to Shoemaker-Levy 9, which broke apart before reaching Jupiter so that its material was extended over a wide region. A similar collision with Saturn and its rings would have caused the ripples we now observe.

So rather than being a fragile thing, the rings of Saturn are an old relic with a history of collisions written upon it.

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All of the outer planets (and at least one asteroid) have ring systems, but none are nearly so bright and extensive as those of Saturn.  Saturn will always be known as the ringed planet.  Saturn’s rings were a surprising mystery when first observed by Galileo in 1610, and in many ways they remain a mystery.

Early observations of Saturn by Galileo and others. Credit: Huygens’s Systema Saturnium

When Galileo observed Saturn through his telescope, he could clearly see it was not a circular disk like the other planets.  But his telescope wasn’t powerful enough to resolve Saturn as a planet with rings.  He speculated that Saturn perhaps had two large and close moons. As Galileo improved upon his telescopes, he was eventually able to resolve the rings.  Later observations by Huygens and Cassini showed that the ring system had a structure to it, including an inner and outer region separated by what is now known as the Cassini division.

Vertical spires in the rings cast shadows. Credit: NASA/JPL

Modern observations of Saturn’s rings show a complex system of thousands of ringlets within groups of rings.  This structure is mediated by small shepherd moons which, through a complex gravitational dance, keep these rings from dispersing. We see vertical structures rising from the rings, and spokes that move with the rotation of Saturn.

One of the biggest mysteries about Saturn’s rings that remains unsolved is their age.  The rings are largely composed of water ice, and are unusually bright.  This would suggest a rather young age of about 100 million years or so, since dust within Saturn’s moon system would gradually mix with the ice and darken the rings.  In this case the rings likely formed when two moons collided, or when a moon drifted too close to Saturn and was ripped apart by the planet’s gravity.

But recent analysis of the composition of the moons and rings of Saturn show that they have chemical similarities that vary with their distance from Saturn.  This suggests that the rings are much older, and likely formed at the same time as the moons about 4 billion years ago.  This would also explain the complex and subtle interactions that can occur between moons and rings.  The unusual brightness of the rings would still need to be explained, but one idea is that ice particles within the rings are continually clumping and breaking apart, which would help keep the rings looking “fresh”.

If the rings are truly old, then it is likely that Saturn’s ring will remain for much of the lifetime of the solar system, thus we can rest assured that future generations will always have a view of such an iconic planet.

Up next: Uranus

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Astronomers have found an asteroid with a ring system, and the results have been published Nature. While this is the first discovery of a ring system about an asteroid, such a thing isn’t entirely unexpected.  We have known for a while that asteroids can have moons, for example.  But what’s particularly interesting is how they discovered these rings.

The asteroid in question is known as  Chariklo, a 130 kilometer wide object that orbits between the orbits of Saturn and Uranus.  It is the largest example of the Centaur group, which are likely Kuiper belt objects that were captured by the gravitational pull of the outer planets.  In the Summer of 2013, Chariklo passed in front of a dim, 12th-magnitude star as seen from parts of South America.  It is a phenomenon known as an occultation.

Brightness of the background star during occultation.
Credit: F. Braga-Ribas, B. Sicardy, et al.

Such an event is a great opportunity to better determine the size and shape of an asteroid, so a team of astronomers dispersed through Brazil, Argentina, Uruguay and Chile made observations of the occultation.  Since each observation is made from a different vantage point, Chariklo’s occultation of the background star occurs at a slightly different orientation.  By combining these different observations the team could map the overall shape of the asteroid.

But what the team found was that from some vantage points the background star dimmed a couple times before being occulted by Chariklo, then dimmed a couple times after the occultation as well. Analysis of these pre and post dips makes it pretty clear that there are at least two rings around the asteroid.

The orientation of Saturn’s rings changes over time. Credit: NASA/HST

Knowing that Chariklo has a ring system, the team was also able to determine a bit about the composition of the rings.  As Chariklo (or any other ringed object) orbits the Sun, there are times when the rings are seen more edge on, and times when we see more of the rings.  The team compared spectral observations of the asteroid between 1997 and 2008, and found that the asteroid gradually dimmed during that period.  The observation of water ice in the spectrum also faded during that period.  This would be consistent with a ring system containing ice (which is very reflective) gradually shifting to an edge-on orientation.

One of the more surprising aspects of the discovery is that the rings are so sharp and dense.  Typically a ring system would diffuse over time unless there are shepherd moons to keep the rings in order.  So it may be that Chariklo has not only a ring system, but a moon system as well.

To determine whether that is the case, it will likely take some high resolution imaging.  But that remains to be seen.

Paper:  F. Braga-Ribas, B. Sicardy, et al. A ring system detected around the Centaur (10199) Chariklo. Nature (2014) doi:10.1038/nature13155.

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