1977

Uranian Rings Discovered

James L. Elliot (1943–2011), Edward W. Dunham (b. 1952), Douglas J. Mink (b. 1951)

In 1659, the Dutch astronomer Christiaan Huygens observed and explained the rings of Saturn as a thin disk of material circling the sixth planet. Saturn appeared to be the only planet with a ring system until 1789, when, shortly after discovering Uranus, the English astronomer William Herschel thought he detected a faint ring around that planet as well. But subsequent visual and photographic observations of Uranus couldn’t verify Herschel’s claim.

Nearly two hundred years later, on March 10, 1977, the American team of planetary scientists James Elliot, Ted Dunham, and Douglas Mink was preparing to observe Uranus pass in front of a relatively bright star—an eclipse of sorts known as an occultation. To ensure that they were in the precise location to catch the rare occultation, the team observed the event from the Kuiper Airborne Observatory, a telescope mounted in a NASA C-141A jet that flew missions in the stratosphere, above most of our atmosphere’s clouds and water vapor.

The occultation yielded an exciting surprise. Just before the star was eclipsed by Uranus, its brightness briefly dropped dramatically, five separate times. Then, just after the star reemerged from the eclipse, it happened five times again. Analysis by Elliot and his team revealed that the drops in starlight occurred at the same radial distance from Uranus on either side of the planet. They had discovered a faint set of narrow rings—meaning that the seventh planet had become the second known planet with rings.

Follow-up observations revealed four more narrow rings around Uranus, bringing the total to nine. Two more rings were discovered during Voyager 2’s 1986 flyby—which revealed that the rings are extremely dark and are likely made of centimeter- to meter-size icy blocks that have been darkened by interactions between Uranus’s magnetic field and icy, organic molecules—and two more were discovered in the early twenty-first century by astronomers using the Hubble Space Telescope, bringing the total to 13. Faint rings were later also discovered around Jupiter and Neptune (also in Voyager data), meaning that all the giant planets in our solar system—not just Saturn—are ring worlds.

Voyager 2 image of the rings of Uranus, acquired during the probe’s flyby in January 1986. Background stars appear as streaks in this long-exposure photograph. The blinking on and off of blocked starlight passing through the rings is what earlier had led to their discovery in 1977.