Uranus' Epsilon ring is unlike any other known ring system

Uranus has 13 rings, grouped into two sets

Uranus's densest ring, Epsilon, is unlike any other known planetary ring system within our solar system and lacks the common small, dust-sized particles.

That's according to a new study by scientists from the University of California, Berkeley, and the University of Leicester.

Uranus' rings are notoriously hard to spot and were only discovered in 1977. There are 13 rings in total, which have been grouped into two sets: the inner set of 11 and the outer set of two. Epsilon is the outermost of the inner set of rings.

In 2017, scientists used the Atacama Large Millimetre Array (ALMA) and Very Large Telescope (VLT) to capture thermal images of Uranus and its cold, rock-strewn rings. Instead of using the reflected sunlight to image those structures, astronomers tried to take thermal images of the millimetre and mid-infrared 'glow' that was being emitted by the cold particles of the rings.

The rings appeared surprisingly bright in the heat images captured by ALMA and VLT. Scientists analysed those pictures in detail and concluded that the temperature of Uranus' rings was 77 Kelvin (about -196°C/ -320°F).

The data also revealed that the Epsilon ring differs from other known planetary ring in our solar system. It is made up of rocks, about the size of golf balls or larger, and lacks the smaller, dust-sized particles commonly found in planetary ring systems.

For example, Saturn's icy rings are made up of particle ranging from micron-sized dust particles (present in the innermost ring) to tens of metres in size in the main rings. Jupiter's rings also feature mostly micron-sized particles.

"We already know that the Epsilon ring is a bit weird, because we don't see the smaller stuff," said graduate student Edward Molter.

Epsilon is the widest of Uranus' rings. Its width varies from 20km to 100km.

Saturn's rings are wider than Uranus', ranging from 100s to tens of thousands of kilometres in width.

ALMA observations in the current study were led by De Pater and Molter from UC Berkeley. Leigh Fletcher and Michael Roman from the University of Leicester led the VLT observations.

The detailed findings of the study will be published this month in the Astrophysical Journal.