Uranus and its rings seen by ALMA and VLT

Uranus and its rings seen by ALMA (Image ALMA (ESO/NAOJ/NRAO); Edward M. Molter and Imke de Pater))

An article published in “The Astronomical Journal” reports new observations at infrareds and millimeter wavelengths of the planet Uranus and its rings. Imke de Pater and Edward Molter conducted observations with the ALMA radio telescope while Michael Roman and Leigh Fletcher conducted observations with the VLT. For the first time the temperature of the rings was measured, which turned out to be around 77 Kelvin. These observations also help to better understand the rings’ composition and the differences compared to those of the other planets.

Uranus rings are not exactly as spectacular as those of Saturn, so much so that they were discovered only in 1977 by astronomers who were trying to study the planet’s atmosphere. They’re really dark and dim so only an analysis of the images led the astronomers to realize their existence.

Over the years, NASA’s Voyager 2 space probe’s flyby and follow-up observations led to the discovery of a complex and extensive system, again not at the level of Saturn’s but bigger than Jupiter’s and Neptune’s.

The mechanisms of ring formation are probably the same for all the planets with moons that get destroyed by the gravity of the planet if their orbit is disrupted and come too close. There may also be a part formed by fragments of comets and asteroids that came too close to the planet. A different composition of materials and different amounts can generate very different rings and the studies continue to understand the various processes in progress. For example, one recently published in the journal “Science” reports new details on Saturn rings and the moons that orbit among them to prove their influence.

One possibility to study Uranus rings is to observe the very little sunlight they reflect, instead in this new research the authors detected their natural brightness at infrareds and millimeter wavelengths. The ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope, inaugurated in March 2013, is generally used to study objects far away but sometimes it’s also useful for research on much closer objects. In this case, observations with the VLT (Very Large Telescope) were added.

This type of detection made it possible to measure Uranus rings’ temperature for the first time and it was found to be 77.3 ± 1.8 Kelvin, -196.15° Celsius or -320° Fahrenheit. They’re really cold and yet the researchers expected an even lower temperature for fast-rotating ring particles. As for the first observations, the researchers were studying Uranus’s atmosphere and were surprised by the quality of the rings’ images.

The ring called epsilon, the brightest and densest, turned out to be made up of stones as big as golf balls and larger rocks while dust is missing. It’s an oddity since normally planetary rings are composed of very different sized particles, including microscopic ones.

At the moment 13 rings are known around Uranus with some dust bands between the rings. Although they’re not spectacular, they’re interesting from the scientific point of view, also within more general research on planetary rings. These results show the possibilities of infrared observations that in the future could also be conducted with the James Webb Space Telescope, when it will be launched, at last.

Artist's concept of Uranus and its rings (Image NRAO/AUI/NSF; S. Dagnello)

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