Why Do Planets Have Rings?

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Though we think of rings as being passive, decorative elements to a planet, they're actually more like an extra-planetary surface.

For centuries Saturn was famous as our solar system’s only 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 James Webb Space Telescope’s infrared instruments are capable of providing astronomers with their best look yet at the composition and motion of the outer planets’ rings. In visible light, the rings of Uranus, Neptune, and Jupiter are dark and indistinct, but they shine more clearly in infrared light. Webb will also continue the study of Saturn’s rings.

Hubble image of Saturn, March 22, 2004
Saturn’s bright, distinctive rings are the most prominent rings in our solar system. Webb’s infrared vision will reveal new detail. Credit: NASA, ESA and E. Karkoschka (University of Arizona).

Though we think of rings as being passive, decorative elements to a planet, they’re actually more like an extraplanetary surface. They are active, dynamic structures that undergo daily temperature shifts, seasons, and even give birth to moons. As Saturn and its rings travel around the Sun, the tilt of the rings causes seasonal changes.

The water-ice particles that make up Saturn’s rings range in size from specks smaller than household dust to boulders the size of houses. These ring particles stick to each other at different rates depending on how warm or cold they are, coalescing into small objects that then fall apart again when conditions change. At the conclusion of NASA’s 13-year Cassini mission at Saturn, the spacecraft only observed about half of the planet’s 28-Earth-year seasonal cycle. Combined, Cassini and Webb will allow us to capture an entire Saturnian year.

Once thought to be young due to their brightness, the Cassini spacecraft mission showed that Saturn’s rings are actually quite ancient. Some astronomers theorize that the rings formed when our solar system was young, 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 elements of the old. It remains to be seen whether the less dense rings around planets like Neptune, Uranus, and Jupiter perform a similar function.


Other Ring Worlds

Hubble image of Uranus, 2003
Uranus’ rings are thought to be made primarily of large pieces of rock. Webb will investigate their composition. CREDIT: NASA, ESA, and M. Showalter (SETI Institute).

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 how the rings change around their planets, and look for small moons, new rings, and other material around the giant planets. Webb will watch as the planets move in front of stars and observe how the rings block starlight. This will show whether the rings are dense or porous, and how their structures change over time. Most importantly, Webb will conduct spectroscopic observations of the rings, examining the light they emit for clues to their chemical composition. Spectroscopy will also tell astronomers the age of the ice in the rings. These clues will assist astronomers in determining 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. Gaps in our understanding of Solar System evolution will be filled in.

Finally, Webb will look at some puzzling features of rings that astronomers are still working to understand. The spoke-like structures that appear in Saturn’s rings and have rarely been observed by Cassini or the Hubble Space Telescope. Around Neptune, intermittent regions of thickly clustered particles, known as ring arcs, have been seen to split and evolve, but astronomers need more data to truly understand the processes taking place. Webb will provide a much clearer understanding of how the Solar System works, and likely open up new areas of study we haven’t yet imagined.

Last Updated: May 31, 2018

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