Why Do Planets Have Rings?

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Saturn's bright, distinctive rings are the most prominent in our solar system. Webb's infrared vision will reveal new detail.
For centuries, Saturn was our solar system's famous ringed planet, encircled by wide, sweeping structures of water ice. Today we know that all four of our solar system's giant planets have rings, but only Saturn's have been studied in-depth.

The Webb telescope's infrared view will provide Earth with its best look at the composition and evolving motions of the outer planets' rings. Webb will examine the dark, difficult-to-see rings of Uranus, Neptune, and Jupiter, which shine brighter in infrared than visible light, while extending our studies of the rings of Saturn.

Though we think of rings as being passive, decorative elements to a planet, they're actually more like an extra planetary surface. Rings are active, changing structures that undergo daily temperature shifts, seasons, and even give birth to moons.

Saturn's bright rings can be observed from Earth and have been examined by the Cassini mission, in orbit around the planet from 2004 until about 2017. But rings, like planets, have seasons. As Saturn's orbit brings its rings into a tilt, the side turned toward the Sun warms while the other cools. Saturn's rings are made up of water-ice particles that range in size from specks smaller than household dust to boulders the size of houses. As the planet and its rings travel around the Sun, the tilt of the rings causes seasonal differences. These seasonal differences include temperature changes in the ice and spokes (microscopic dust particles lofted by electrostatic forces). Ring particles, which stick to each other at different rates depending on how warm or cold they are, coalesce into small objects that then fall apart again when conditions change. When the Cassini mission is over, we'll only have observed around half of Saturn's 28-Earth-year seasonal cycle. Combined, Cassini and Webb will allow us to capture an entire Saturn year.

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Uranus' rings are thought to be made primarily of large pieces of rock. Webb will investigate their composition.
Astronomers will see how these ring dynamics play out both around Saturn and neighboring giant planets. Saturn's rings, once thought to be young due to their brightness, have been shown by observations to actually be quite ancient. Some astronomers theorize that the rings formed in the young solar system when moons similar in size and ice content to Saturn's largest moon, Titan, were drawn toward the planet by gravity and subsequently broke apart. New moons can also form from the particles in Saturn's rings, coalescing and sweeping up material in the rings as they orbit the planet. This makes Saturn's rings a sort of recycling system in which new moons arise from the remains of the old.

Other Ring Worlds

But it remains to be seen whether the less dense rings around planets like Neptune, Uranus, and Jupiter perform a similar function. Their ring systems are quite different from those of Saturn. Jupiter's rings are made of very small dust particles, and their structure is dependent on Jupiter's magnetic fields. Neptune has dark rings made of methane and ammonia ice. Uranus' rings are dominated by chunky boulders and may consist primarily of rock.

Webb will observe the way the rings change around their planets, and look for small moons, new rings, and material around the giant planets. Webb will be able to study "occultations" of the planets' rings, watching as the planets move in front of stars and observing how the rings block starlight. This will show whether the rings are dense, porous, and how their structure changes over time. Most importantly, Webb will conduct spectroscopic observations of the rings, examining the light emitted by these objects for clues to their chemical composition. Spectroscopy will tell astronomers the age of the ice in the rings, and what it’s made of. From that, astronomers will be able to ascertain whether the rings’ origins are planetary moons that broke up, or objects from the distant Kuiper Belt that were drawn to the planet by gravity and then torn apart.

Finally, Webb will look at some puzzling features of rings that astronomers are still working to understand: spoke-like structures that appear in Saturn's rings and have rarely been observed by Cassini and Hubble; and intermittent regions of thickly clustered particles, or "ring arcs," around Neptune that have been seen to split and evolve.

Updated: Jun 01, 2016