Every planet in the solar system has its idiosyncrasies, but Uranus is truly unique.
Not only does it lean sideways, so its axis of rotation is virtually parallel to its orbital plane, it smells terrible, it escapes everywhere, its magnetic field is a total mess and it has rings unlike any other planetary ring in the Solar System.
But wait, there are more. About 20 years ago, astronomers rotated their instruments to capture X-ray emissions from Saturn, Uranus and Neptune. Unlike all previous planets, Uranus had a flash to see.
Now, for the first time, we have detected X-rays emanating from the strangest ball in the Solar System, and it is not clear where they come from or what they mean.
Observations and discoveries about Uranus and Neptune, in this sense, are quite difficult to make, compared to the rest of the solar system. These two planets are far apart, and few probes have ever ventured into their icy neighborhood.
We generally rely on telescopes close to home to make a sketch, telescopes optimized to look at things much farther than Uranus or Neptune, so the details can be a little fuzzy at the edges.
The new discovery is based on observations made by Chandra’s X-ray Observatory, a space telescope orbiting the Earth. The first set of observations was made in 2002, then two more in 2017. When a team of astrophysicists led by William Dunn of University College London in the United Kingdom finally set about analyzing the 2002 observation data, they found clear evidence of Uranus rays.
The 2017 observation. (NASA / CXO / University College London / W. Dunn et al; WM Keck Observatory)
That Uranus emits X-rays is not so surprising; X-rays have been detected emitting from many bodies in the solar system, including comets, Venus, Earth, Mars, Saturn, Pluto, Jupiter, and even some of Jupiter’s moons. Nor is it strange that we have not detected them so far, given the difficulties involved in studying the distant planet.
The weird part is that we don’t know the full picture of how Uranus emits X-rays.
There are some options. Most of the X-ray radiation from the solar system comes from the Sun (obviously), which is known to disperse when it hits the clouds of Jupiter and Saturn. This is likely to happen in Uranus as well, although the team’s calculations point to more X-ray photons than this process could entail.
Based on other objects in the solar system, we have some clues as to what could be the potential source of this excess. Saturn’s rings are one such example, known for the X-ray fluorescence generated by energy particles that interact with the oxygen atoms in the rings.
Although Uranus’ rings are less showy than those of Saturn, studies of the radiation belt have found a higher intensity of energy electrons around Uranus. If these interacted with the atoms in the rings, they could produce a similar X-ray fluorescence.
Another process that produces X-rays in the solar system is the aurora. They occur when energy particles interact with a planetary atmosphere. On Earth, this produces an impressive display of green light dancing in the sky, but they are known to occur on other planets as well; Jupiter, Mars, Saturn and even comets can have auroras.
In most cases, a magnetic field plays a role in the generation of auroras; the particles accelerate along the lines of the magnetic field before being deposited in the atmosphere.
It is possible that a process similar to Uranus occurs, which generates auroras in the upper atmosphere. If it is, however, because Uranus’ magnetic field is an off-axis mess, these auroras could be much more complex than we have ever observed in the solar system.
Chandra’s longer observations in the future could help scientists map the location of X-ray emissions in Uranus, which would help find out what is causing them. However, it is not possible to make more detailed observations that may characterize the fluctuations in the issue with our current generation of instruments.
Upcoming observatories, such as ESA’s Athena or NASA’s Lynx, will be able to better tell us what’s going on. This could not only help us to better understand the atmosphere and magnetic field of Uranus, but also to better understand the X-ray sources of the Universe.
The team’s research has been published in JGR Space Physics.