Each planet in the Solar System has its own idiosyncrasies, but Uranus is truly one of a kind.
Not only is it tilted sideways so its axis of rotation is practically parallel to its orbital plane, it smells terrible, it leaks everywhere, its magnetic field is a total mess, and it has rings like no other planetary ring in the System. Solar.
But wait, there’s more. About 20 years ago, astronomers used their instruments to capture X-ray emissions from Saturn, Uranus, and Neptune. Unlike all the planets before him, Uranus didn’t have a glimmer in sight.
Now, for the first time, we have detected X-rays emanating from the strangest ball in the Solar System, and it is not very clear where they come from or what they mean.
Observations and discoveries about Uranus, and Neptune, for that matter, are quite difficult to make, compared to the rest of the Solar System. These two planets are really far away, and few probes have ever ventured into their icy neighborhood.
Generally, we rely on telescopes close to home to observe them – telescopes that are optimized for looking at things much further away than Uranus or Neptune, so the details can be a bit fuzzy around the edges.
The new discovery is based on observations taken using the Chandra X-ray Observatory, a space telescope in orbit around Earth. The first set of observations was taken in 2002, then another two sets in 2017. When a team of astrophysicists led by William Dunn of University College London in the UK finally set out to analyze the 2002 observational data, they found clear evidence of X- rays of Uranus.
That Uranus should emit X-rays is not so surprising; X radiation emitted from many bodies in the Solar System has been detected, including comets, Venus, Earth, Mars, Saturn, Pluto, Jupiter, and even some of Jupiter’s moons. Nor is it surprising that we have not detected them so far, given the difficulties of studying the distant planet.
The funny thing is that we do not know the complete picture of how Uranus emits X-rays.
There are a few options. Most of the X-radiation in the Solar System comes from the Sun (obviously), which is known to scatter when it hits the clouds of Jupiter and Saturn. This is probably happening on Uranus as well, although the team’s calculations point to more X-ray photons than this process could account for.
Based on other objects in the Solar System, we have some clues as to what the potential source of this excess could be. The rings of Saturn are one example, and they are known to fluoresce in X-rays generated by energetic particles that interact with oxygen atoms in the rings.
Although the rings of Uranus are less conspicuous than those of Saturn, studies of the radiation belt have found a greater intensity of energetic electrons around Uranus. If these were interacting with atoms in the rings, they could be producing a similar X-ray fluorescence.
Another process that produces X-rays in the Solar System is the aurora. These occur when energetic particles interact with a planetary atmosphere. On Earth, this produces an impressive display of dancing green light in the sky, but it is also known to occur on other planets; Jupiter, Mars, Saturn, and even comets can have auroras.
In most cases, a magnetic field plays a role in the generation of auroras; particles are accelerated along magnetic field lines before settling in the atmosphere.
A similar process may be taking place on Uranus, generating auroras in the upper atmosphere. However, if it is, because Uranus’ magnetic field is an off-axis mess, these auroras could be far more complex than any we’ve observed in the Solar System.
Longer Chandra observations in the future could help scientists map the locations of X-ray emissions on Uranus, helping to figure out what is causing them. However, with our current generation of instruments it is not possible to make more detailed observations that can characterize fluctuations in emission.
Upcoming observatories, such as ESA’s Athena or NASA’s Lynx, will be able to better tell us what’s going on. That could help us not only better understand Uranus’s atmosphere and magnetic field, but also gain a deeper understanding of X-ray sources throughout the Universe.
The team’s research has been published in JGR space physics.