Astronomers detect X-rays from Uranus

Using NASA’s Chandra X-ray Observatory, astronomers detected X-rays from Uranus, revealing a previously unknown dimension of this majestic ice giant.

The new discovery, published in JGR: Physics, means that X-ray emissions have been detected on all planets in the solar system except Neptune. In addition, the study may yield new insights into more distant X-ray emitting objects, including black holes, supernovae, quasars, and neutron stars. The new article was led by astronomer William Dunn of University College London.

Composed primarily of hydrogen and helium, Uranus exhibits two sets of rings, both in orbit above its equator. The planet is somewhat strange, as it rotates sideways in relation to the plane of the solar system (no other planet does that). NASA’s Voyager 2 spacecraft visited Uranus very briefly in 1986, so in addition, astronomers have relied on telescopes, like Chandra and Hubble, to study the seventh planet from the Sun.

Dunn – along with physicist Affelia Wibisono, a doctoral student at UCL and co-author of the study – discovered the evidence for Uranus X-ray emissions in the Chandra data collected in 2002 and 2017. The 2002 data was collected by Chandra Advanced CCD Imaging Spectrometer, while data for 2017 came from Chandra’s High-Resolution Camera, in addition to optical observations. The observed signals are very weak, but they are there.
With the X-rays confirmed in Uranus, the challenge now is to determine the cause.

“There are three main ways for a planet to produce X-rays: fluorescence, scattering of X-rays from the sun, and auroral emissions,” explained Wibisono in an article he wrote for the Chandra website.

Uranus, like many other objects in the solar system – including comets, moons, and even Pluto – is probably dispersing the X-rays received by the Sun. But this does not appear to be the exclusive source of X-rays in Uranus. As Wibisono points out, “our calculations suggest that Uranus was producing more X-rays than it should have been if the planet was just scattering the X-rays from the Sun.”

The authors proposed two different theories to explain the emissions. One possibility is that Uranus’ rings are emitting this radiation, similar to what is happening with the rings around Saturn. This process, known as fluorescence, occurs when charged particles, such as electrons and protons, collide with the rings, making them shine in X-rays.

Another possibility is that they are being produced by the Uranus auroras, as NASA explains in a statement.

On Earth, we can see colorful light shows in the sky called auroras, which happen when high energy particles interact with the atmosphere. X-rays are emitted at Earth’s auroras, produced by energetic electrons after they travel along the lines of the planet’s magnetic field to their poles and are slowed down by the atmosphere. Jupiter also has auroras. The X-rays of auroras on Jupiter come from two sources: electrons traveling along the lines of the magnetic field, as on Earth, and positively charged atoms and molecules raining down on the polar regions of Jupiter.

The problem is that the cause of the auroras in Uranus is not yet fully understood, so much of it remains an assumption. Other “observations of Uranus by Chandra and other X-ray telescopes are necessary before we can give a definitive answer”, admits Wibisono.

Uranus represents a fascinating object for studying various aspects of distant planets, and this is because of its unusual axis of rotation and unstable magnetic field. With its strange inclination, astronomers can see Uranus at an irregular angle and, due to its magnetic field, which is also strangely inclined, astronomers may eventually find a connection with the planet’s complex and variable auroras. There is still a lot to learn about this strange and wonderful ice giant.

Source: Nasa / NASA’s Chandra X-ray Observatory

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