An infrared view of Uranus over two days in July 2004. Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory

Researchers measuring heat in Uranus' atmosphere said they were "astonished" to find the warm glow of rings around the planet in thermal images.

They determined the rings' temperature for the first time: -320 degrees Fahrenheit.

Uranus' brightest ring is missing the tiny dust particles common in other rings throughout the solar system, and scientists don't know why.

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Four decades after their discovery, the 13 mysterious rings around Uranus surprised astronomers again this summer.

In June, new images captured their warm glow for the first time. Well, warm for Uranus.

At -320 degrees Fahrenheit, the rings are 10 degrees warmer than the planet's surface, which is the coldest in our solar system. Scientists determined the rings' temperature thanks to these thermal images.

The findings were described in a study published last month in The Astronomical Journal. To capture the images, researchers used the Atacama Large Millimeter Array and the Very Large Telescope in Chile to measure the temperature structure of Uranus' atmosphere. They were surprised to find that they'd picked up thermal readings of the planet's rings.

"It's cool that we can even do this with the instruments we have," Edward Molter, a graduate student at the University of California at Berkeley and the study's lead author, said in a press release. "I was just trying to image the planet as best I could and I saw the rings. It was amazing."

This composite image shows heat from the rings of Uranus for the first time, enabling scientists to determine their temperature: a frigid 77 Kelvin (-320 Fahrenheit). UC Berkeley image by Edward Molter and Imke de Pater

Molter and his coauthor Imke de Pater, a professor of astronomy, made the above composite image, which shows the rings' thermal glow at radio wavelengths. The dark bands in the image capture molecules that absorb radio waves; in Uranus' case, that's probably hydrogen sulfide. The yellow spot is the planet's north pole, where those molecules are sparse.

"We were astonished to see the rings jump out clearly when we reduced the data for the first time," said Leigh Fletcher, who led the telescope observations.

The study confirmed that Uranus' epsilon ring - the brightest, widest, and densest of the planet's rings - is unique among other rings in our solar system.

Saturn's ice rings, bright and wide enough to see with a standard telescope, are made of particles of varying sizes, from dust with a width of one-thousandth of a millimeter to house-sized chunks of ice. Jupiter's and Neptune's rings are mostly made up of those tiny dust particles.

These images from the Atacama Large Millimeter Array and the Very Large Telescope captured the Uranus ring system at different wavelengths. The planet itself is masked since it is very bright compared with the rings. Edward Molter, Imke de Pater, Michael Roman and Leigh Fletcher

Uranus' epsilon ring, however, contains only rocks at least the size of golf balls.

"We already know that the epsilon ring is a bit weird, because we don't see the smaller stuff," Molter said. "Something has been sweeping the smaller stuff out, or it's all glomming together. We just don't know. This is a step toward understanding their composition and whether all of the rings came from the same source material, or are different for each ring."

Astronomers first identified Uranus' rings in 1977. It took so long to notice them because they're much thinner and darker than Saturn's rings. They reflect only tiny amounts of light in the visible range, with more reflection in the infrared and near-infrared ranges.

"They are really dark, like charcoal," Molter said.

Voyager 2 snapped the first up-close images of Uranus in 1986. At the time, astronomers thought the planet had only nine rings. The spacecraft showed them two more. NASA/JPL-Caltech

After Voyager 2 flew by Uranus and snapped the first up-close photos of the planet in 1986, scientists noticed the absence of tiny dust particles in its rings.

The reasons for this unique ring makeup are still unknown/ Uranus' rings could have come from asteroids that fell into orbit around Uranus, the remnants of moons that crashed into each other or got torn apart by the planet's gravity, or leftover debris from the formation of the solar system.

NASA's James Webb Space Telescope, set to launch in 2021, should be able to observe the mysterious rings in greater detail.

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