Uranian ring system

Uranus has thirteen known rings, eleven inner and two outer ones. These rings are mostly very narrow and faint. Uranus’s ring system is less complex than Saturn’s but more so than that of Jupiter.

Overview

When Uranus was first observed (perhaps as early as 1690), many astronomers considered it to be a star rather than a planet. British astronomerWilliam Herschel studied Uranus in 1781 when he thought he had discovered a new comet. After two years of further study, astronomers agreed that Uranus was, in fact, a planet.

Herschel appears to have observed the rings of Uranus in February of 1789. He sketched an image of Uranus in his journal, making a note that the planet had rings of a faint reddish hue. Rings around Uranus remained an open issue for a long time. The existence of Uranus’s ring system was finally confirmed, albeit accidentally, in 1977. Astronomers James Elliot, Edward Dunham, and Douglas Mink set out to study Uranus’s atmosphere by observing its occultation (obscuring) of the star SAO 158687. They noticed that this star briefly disappeared from view five times before and after passing behind the planet and concluded this was due to planetary rings around Uranus. When the researchers published their work, they referenced these rings using the Greek letters Alpha, Beta, Gamma, Delta, and Epsilon. In 1978, another group of scientists found four additional rings. The Eta ring was found between the Beta and Gamma rings. The other three were discovered inside the orbit of the Alpha ring and were named Six, Five, and Four (in that order).

The National Aeronautics and Space Administration’s (NASA’s) robotic Voyager 2 probe flew by Uranus in 1986, taking the first photographs of the Uranian ring system. Voyager 2 also discovered two more faint rings, Lambda and 1986U2R/Zeta, bringing the total number of known rings around the planet to eleven. In 2003, the Hubble Space Telescope discovered, and in 2005, confirmed, the existence of an additional pair of rings. They form an outer ring system that is separate from the other eleven rings. These two outer rings have an orbital radius of more than 100,000 kilometers from Uranus’s center—double that of the inner rings. In addition to the twelfth and thirteenth rings, Hubble found two satellites. Mab, which is only 24 kilometers in diameter, shares an orbit with the outermost ring. Every time a meteoroid impacts the small satellite, dust particles and other debris ejected become part of the Mu ring. The Nu ring lies between Uranus’s small satellites, Rosalind and Portia.

The Mu and Nu rings are very different from the inner rings. With widths of 17,000 kilometers and 30,000 kilometers, respectively, the Mu and Nu rings are much broader. These two rings are also much fainter than the others, but they can be seen in the Voyager 2 photographs.

Many similarities exist between Uranus’s outer rings and Saturn’s E and G rings. The E ring includes Enceladus, which contributes dust to it the same way Mab is believed to contribute to the Mu ring. The Nu ring, like Saturn’s G ring, contains no embedded “shepherding” satellites and is composed of dust and larger particles. Scientists working with the Keck telescopes in Hawaii studied the rings at near-infrared wavelengths. The Nu ring was visible, meaning it had a reddish hue. This possibly gives some credit to Herschel’s original claim about observing Uranus’s rings, despite critics’ claims that the rings are too faint for him to have seen. The Mu ring was not visible, meaning that its small dust particles appeared blue in color. Red is a typical color for planetary rings. Blue, however, is not. Saturn’s E ring is the only other ring known to have the unusual blue hue.

The inner ring system contains two types of rings: narrow and dusty. The closest ring to Uranus is 1986U2R. It was discovered in 1986 by Voyager 2. This ring is only about 12,000 kilometers above the cloud tops of Uranus. The 1986U2R (or Zeta) ring was observed in 2003 and 2004 using the Keck telescopes. Scientists found the ring to be broad, very faint, and composed of dust grains.

The next set of rings is Six, Five, and Four, which were named for the occultations that led to their discoveries. They are the faintest of Uranus’s narrow main rings. These three lie outside Uranus’s equatorial plane by 0.06, 0.05, and 0.03 degrees, respectively. Six, Five, and Four do not contain dust and are the thinnest of Uranus’s narrow rings.

After the Epsilon ring, the Alpha and Beta rings are the brightest of Uranus’s rings. Alpha and Beta are narrowest and faintest at their closest points to Uranus. At their farthest, the two rings are their broadest and brightest. Like all of Uranus’s rings, Alpha and Beta are composed of extremely dark material. They are much darker than Uranus’s inner satellites, meaning that the rings cannot be composed of pure water ice. The composition of the rings is thus unknown, but astronomers think it is probably a mixture of dark materials and ices.

The seventh ring outward from Uranus’s core is Eta, at 47,176 kilometers. Eta has both a narrow part and a broader, dustier section. When Voyager 2 photographed the ring in forward scattered light, it appeared very bright, indicating a large amount of dust. Eta has an inclination and eccentricity of zero, meaning that the ring lies in the planet’s equatorial plane, and the ring particles execute circular orbits. In 2007, Uranus’s rings were viewed edge-on for the first time. The Eta ring appeared to be the second brightest, which was a significant increase. This finding has led planetary scientists to believe that, while the ring is optically narrow, it is geometrically thick. The next chance for astronomers to view the rings in that unique geometry will occur in 2049.

The Gamma ring is narrow, with an inclination close to zero. Gamma’s width varies from 3.6 to 4.7 kilometers. The ring was not visible during the 2007 ring plane crossing. This means that Gamma is both optically and geometrically narrow. The ring also does not contain any dust. Scientists are uncertain about what holds this small ring together.

Like the Eta ring, Delta has both a narrow and a broad component. The thinner part varies from 4.1 to 6.1 kilometers wide, while the thicker part ranges from 10 to 12 kilometers. The wide section is composed of dust, unlike the narrow part. In 2007, only the broad area of Delta was visible. At the outer edge of the Delta ring, a small satellite named Cordelia orbits Uranus.

The Lambda ring lies between Cordelia (a shepherd satellite) and the Epsilon ring. Lambda is faint and narrow even when backlit. The dusty ring was first detected by Voyager 2 during stellar occultation observations, but only at ultraviolet wavelengths. The Lambda ring is composed of micrometer-sized dust, which was confirmed in 2007 when it appeared very bright.

The brightest and densest of Uranus’s rings is Epsilon. It is the outermost ring of the inner system. Epsilon reflects two-thirds of all the light visible from Uranus’s rings. It is only one of two rings that Voyager 2 was able to photograph clearly. Epsilon is the most eccentric of the rings but has a near-zero orbital inclination. The dense ring has particles ranging in size from 0.2 to 20 meters in diameter. In 2007, the ring was not observable because of its lack of dust. Epsilon contains many dense, narrow ringlets and possibly partial arcs. The ring may stay so compact because of its shepherd satellites: Cordelia on the inner side and Ophelia on the outer.

Knowledge Gained

The first rings of Uranus were officially discovered by accident in 1977. Elliot, Dunham, and Mink were using the Kuiper Airborne Observatory (KAO) to study Uranus’s atmosphere during five stellar occultations. The KAO is an airplane with a 36-inch (91.5-centimeter) telescope mounted on the side. With the KAO, scientists can conduct research while flying 14 kilometers above the Earth’s surface, thereby making infrared observations readily. At that elevation, there is significantly less atmospheric water vapor, which blocks infrared wavelengths from reaching the surface of the Earth. The KAO, therefore, combines many benefits of a space telescope with the accessibility of a ground-based telescope.

Launched in 1977, Voyager 2 is the only spacecraft to have visited Uranus. It came within 81,500 kilometers of the planet on January 24, 1986. The spacecraft had several instruments on board, including cameras, magnetometers, and spectroscopes. At the time, Uranus’s south pole was pointed toward the Sun. Voyager discovered ten satellites as well as the Lambda and Zeta (1986U2R) rings. The Mu and Nu rings have since been located on Voyager 2 photographs.

The Hubble Space Telescope was studying Uranus in 2003 when it discovered the Mu and Nu rings. Scientists were able to confirm the finding in 2005. When the Keck telescope studied the two rings at near-infrared wavelengths, only the Nu ring was visible. This means that the Nu ring has a reddish color. The Mu ring, therefore, has a bluish tint because it was not visible.

In 2007, astronomers were able to view Uranus’s rings edge-on. Teams of scientists used the Keck II telescope in Hawaii, the Hubble Space Telescope, and the European Southern Observatory’s Very Large Telescope in Chile to study the event. Images taken with the Keck telescope show that the rings have changed since Voyager 2 visited the planet more than two decades ago. The broad, dusty inner Zeta ring appears very different. If it is the same ring discovered by Voyager, Zeta has moved several thousand kilometers away from Uranus. Similar shifts have been detected in the ring systems of Saturn and Neptune.

Scientists continued to search for answers about Uranus’s rings as the twenty-first century progressed. Launched in 2021, the James T. Webb Space Telescope has imaged incredibly clear pictures of Uranus’s rings, storm clouds, and polar caps, providing increasing data about the workings of the planet.

Context

All of the Jovian planets in our solar system have ring systems. Each set of rings is unique. Neptune’s rings are simpler than Uranus’s, containing only five rings and partial arcs. Saturn’s ring system, on the other hand, is more complex. Jupiter has but two faint rings.

The thirteen rings of Uranus are mostly faint and narrow. They are in two groupings—an inner system of eleven rings and an outer set of two. In 2007, the Nu ring was determined to be red in color, and the Mu ring was found to be blue. Red seems to be a typical color for planetary rings, like Saturn’s G ring. The blue color of the Mu ring, however, is not common. The only other example in the solar system is Saturn’s E ring. What causes this odd blue color is still a mystery to scientists. The rings around Uranus are believed to be made of debris from collisions between Uranus’s satellites.

Scientists can learn more about the formation and evolution of the solar system by investigating planetary ring systems. As ground-based and space telescopes improve, astronomers could unlock the secrets of Uranus’s rings and the solar system itself.

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