‘Night vision’ helps to take the temperature of the rings of Uranus 
New discoveries

‘Night vision’ helps to take the temperature of the rings of Uranus 

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Astronomers have obtained new images of the rings of Uranus, a distant planet in our own solar system. The new images were made by two big observatories: ALMA, and the European Very Large Telescope (VLT). The astronomers were also able to take the temperature of the rings for the very first time: almost 200 degrees below zero. 

If your room is utterly dark, you can’t see anything. Your bed, chair and table are only visible if they reflect some light – either light from the sun, or light from a lamp. But if we would have night vision, like snakes, we could also see in the dark. That’s because every object has a certain temperature. And with every temperature comes a certain type of ‘invisible light’ – an infrared glow. 

The Very Large Telescope has cameras with night vision. It can spot dark objects in space at room temperature, like black clouds of dust. Not because the dust reflects visible light, but because it glows at infrared wavelengths. If an object is colder still, it produces radiation with even less energy, and it glows at millimeter wavelengths. That’s the type of radiation that ALMA can see. 

A team of astronomers used the VLT and ALMA (both in the north of Chile) to observe the distant planet Uranus. Their goal? To study the planet’s cold atmosphere at infrared and millimeter wavelengths. They were in for a surprise. The thin rings of the planet were glowing strongly at these wavelengths. In particular, the outermost ring of Uranus, called the Epsilon ring, appeared very bright. 

By measuring the glow of the rings, the team could calculate their temperature. They turn out to be really cool: a chilly 196 degrees below zero. That’s only 77 degrees above absolute zero – the lowest possible temperature in the universe. 

The new observations also revealed that the rings of Uranus are very ‘clean’. They contain almost no tiny dust particles. For instance, the Epsilon ring has only larger chunks of rock and ice – at least as large as golf balls. The rings of the planets Saturn and Jupiter are very different: they hold a lot of microscopic dust particles. No one knows why the rings of Uranus are so clean. 

Future observations by the James Webb Space Telescope may solve this riddle. Webb will be launched in 2021. It is much larger than Hubble. And like the VLT, it has sensitive cameras with infrared ‘night vision’. 

What? 

Uranus is the seventh planet in our solar system. It orbits the sun once every 84 years at an average distance of almost 3 billion kilometers. In 1977, astronomers discovered that the planet is surrounded by a number of dark, narrow rings. Nine years later, in 1986, NASA’s Voyager 2 spacecraft took the first close-up images if the rings. Since then, astronomers have tried to study them with the Hubble Space Telescope and with telescopes on the ground. 

Who? 

The new observations of Uranus were carried out by four astronomers. Edward Molter and Imke de Pater of the University of California at Berkeley used the European Very Large Telescope. Meanwhile, Michael Roman and Leigh Fletcher of the University of Leicester (United Kingdom) studied Uranus with ALMA. The new results have been published in The Astronomical Journal.

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