Discovering the rings of Uranus

On March 10, 1977, James Elliot, Edward Dunham and Jessica Mink were aboard NASA’s Kuiper Airborne Observatory in order to study the occultation of a star by Uranus. By the time they landed, they had discovered the rings of Uranus. A.S.Ganesh revisits this serendipitous discovery…

Updated - November 10, 2021 12:18 pm IST

Published - March 10, 2019 01:59 am IST

Voyager 2 natural colour composite image of Uranus and its ring system, taken late on January 23, 1986. This view combines a single-frame colour image of Uranus and a 6 frame mosaic of its ring system taken a few hours later.

Voyager 2 natural colour composite image of Uranus and its ring system, taken late on January 23, 1986. This view combines a single-frame colour image of Uranus and a 6 frame mosaic of its ring system taken a few hours later.

Mention the words planet and rings, and the first name that comes to mind is Saturn. Granted, Saturn has a beautiful ring system that is both large and colourful. Saturn, however, isn’t the only planet in the solar system that is endowed with a ring system. Each of the gas giants – Jupiter, Saturn, Uranus and Neptune – has its own ring system.

While the rings of Saturn have been known since the 17th century, those of Jupiter, Uranus and Neptune were discovered and observed only in the second half of the 20th century. The rings of Uranus were discovered in 1977, followed by that of Jupiter in 1979 and Neptune’s in 1989.

A three-member team inadvertently discovered the rings of Uranus in 1977. James Elliot, an astronomer and the principal investigator, Edward Dunham, then working on his PhD thesis project on planetary occultations and Jessica Mink, a data analyst, got together for their research of planetary occultations.

Extremely rare events

While planetary occultations can be simply put as the phenomenon by which stars are blocked by planets when viewed from Earth, they are actually extremely rare. Observing them not only involved predicting when the planet will pass in front of a star, but also figuring out where on Earth its shadow will be cast.

On March 10, 1977, Uranus was due to occult a star called SAO 158687, affording an opportunity to study the atmosphere of Uranus. As a star gets blocked over time when a planet passes in front of it, it gives an idea of the temperature and pressure structure of the planet.

The three-member team from Cornell University, U.S. realised that the best place to observe this was over the Indian Ocean and therefore flew from California, where NASA’s Kuiper Airborne Observatory was based, to Perth, Australia.

On the day, they did a long, 11-hour flight, flying at night over a patch of the Indian Ocean where they had predicted the shadow of Uranus would be cast. They were to do their observations using a three-foot telescope mounted on the Kuiper Airborne Observatory, which was flying above the clouds to get an unobstructed view of the occultation

Start observing early

Being on a long flight, Elliot had the foresight to turn on the equipments early, not something that was done normally. As a result of this, they were able to observe a bunch of occultations before the main event. While the Uranus occultation affected SAO 158687 like they had expected, the star got blocked again after it had come out from behind the planet.

Though they were unsure how symmetric the blockages were, they were quite sure that it happened more or less the same distance before and after the planet’s passage. The series of slight dimmings that were recorded, before and after Uranus passed in front of the star, thereby turned out to be the first evidence of Uranus’ rings.

While those on board were cheering themselves for their unplanned discovery, the ground team were mobilising the air rescue service, fearing the worst. For the Kuiper Airborne Observatory had lost contact with the ground staff during this trip, prompting scares that eventually proved uncalled for.

Since this discovery, observations by Voyager 2 and the Hubble Space Telescope have enabled us to better understand the ring system of Uranus. A total of 13 rings have been observed and identified in Uranus’ system so far. We also now know that these are relatively dark and faint and also very young – not more than 600 million years old. And it wasn’t long back when we didn’t even know that Uranus had a system of rings...


More about Uranus’ rings

From what we know now, Uranus has two set of rings. The inner system comprises of nine rings and are mostly narrow, dark grey rings. There are two outer rings with the innermost one reddish and the outer ring blue.

In order of increasing distance from Uranus, the names of the 13 known rings are Zeta, 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta, Lambda, Epsilon, Nu and Mu.

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