Why are X-rays coming from Uranus?
SCIENTISTS HAVE MADE A BREAKTHROUGH IN A STUDY OF THE ICY GIANT When X-rays were detected from the ice giant for the first time, a team of astronomers began to look into their origin
When X-rays were detected from the ice giant for the first time, a team of astronomers began to look into their origin
Uranus has long been thought of as an oddball planet. It’s one of two ice giants in the outer Solar System, the other being Neptune, and it’s one of two planets to rotate from east to west, the other being Venus. Most strangely, its spin axis is titled by 98 degrees, which means it rotates on its side. And now scientists have discovered something else that’s just a little weird.
Data collected by NASA’s Chandra X-ray Observatory has allowed astronomers to detect X-rays from the planet for the very first time. For the most part there’s a simple explanation for these signals: researchers say one of the sources is reflected emissions from the Sun. But note the word ‘one’ in that statement, because there also appears to be other sources. What they could be is a mystery, and academics are keen to get to the bottom of them.
Arriving at this point has taken some time.
The scientists involved in the latest research have only recently studied observations taken as far back as 2002 by the Chandra X-ray Observatory: one in August 2002 and two in November 2017. They found that the first observation detected X-rays, while the second two detected a flare of X-rays. But why has it taken so many years to make such an exciting discovery about one of the Solar System’s most enigmatic planets?
“The signal from Uranus is really dim, so we had to do lots of tests to check that it was genuine,” says Dr William Dunn, a research fellow in the department of space and climate physics at University College London (UCL). “In fact, after a first cursory look at the data in 2017, I started drafting a paper titled No detection of X-rays from Uranus. Fortunately, I did a bit more digging into the data.”
Dunn was still at school when the first observations were taken. “But I guess that when those observers took the observations – without all our subsequent knowledge from Jupiter and Saturn – the X-rays from Uranus looked a lot like the background emission, and so it was assumed – very reasonably for the knowledge at the time – that it was a non-detection,” he says.
Since then astronomers have learned so much more about the outer planets – in particular Jupiter and Saturn – and it was the lessons which emerged from such studies that led a group headed by Dunn to re-examine the 2002 data in a new light. “In particular, it lets us throw away lots of background noise that we now know is not produced by planets,” Dunn continues. “Being able to filter that out of the observations was critical to revealing the planetary signal. Without being able to throw away the noise [X-rays with very high energies], it would be very difficult to detect the planet.”
By observing Jupiter and Saturn, astronomers have noted that the planets scatter X-ray light given off by the Sun. They do so because their atmospheres have lots of hydrogen in them, making them really good at reflecting X-rays from our Solar System’s star. Other objects also emit X-rays, from Venus to the moons of Jupiter.
“If a big solar flare goes off on the Sun, then the whole of Jupiter and Saturn brighten up with it, and the brightening lasts the same amount of time as the solar flare lasts, although we have to wait a couple of hours for the light to get from the Sun to Jupiter and Saturn and then back to us,” Dunn explains.
Uranus also has hydrogen in its atmosphere, albeit to a slightly lesser extent than on Jupiter
and Saturn. It’s this which has led astronomers to believe the Sun is involved in the detection of X-rays on Uranus, but whether or not it’s a main cause remains uncertain. “I don’t think we know that it’s a main cause,” Dunn says. “It’s certainly a cause, but we only have three exploratory observations, and there are hints that some of the emission came from elsewhere.”
As it stands there appears to be two possibilities for the detection of X-rays on Uranus, aside from them being reflected emissions from the Sun. The first explanation is that scientists are observing X-ray aurorae like we see on Jupiter and here on Earth, although they’re not sure what would trigger them on Uranus.
Aurorae manifest themselves as colourful light shows, and they’re linked to charged particles within the magnetosphere. On Earth and Jupiter, energetic electrons travel down the magnetic field lines to the poles and are slowed by the atmosphere. On Jupiter they’re also caused by positively charged atoms and molecules raining down on the poles.
“We know that Uranus has a really bizarre magnetic field: the field is tilted to the rotation axis by a huge amount, and it doesn’t even seem to go through the middle of the planet – it’s offset somehow to one side, and it doesn’t look like the bar magnet [dipole] situation we’re familiar with for Earth, Jupiter and Saturn. Instead it’s multipolar. This creates some really crazy interactions between the planet and the particles that stream out of the Sun,” explains Dunn.
“Auroral emissions will relate to this somehow, but we have so little data from measurements in this environment – the only measurements we have are from when the Voyager spacecraft zipped past the planet in 1986 – that I would not like to yet guess at what process causes it. I think to truly answer this, we’ll need to send a spacecraft back to
“The signal is really dim, so we had to do lots of tests to check that it was genuine”
William Dunn
the planet to orbit it and see the evolution of the environment around the planet.”
The second possibility is that researchers are observing X-ray fluorescent glowing from the rings when electrons and protons which surround Uranus in nearby space smash into them. “There could be more going on, because everything we know right now is based on just these very short exploratory observations,” Dunn continues.
But what is prompting scientists to think the
Sun isn’t the primary cause of the X-rays? Quite simply, there appear to be more X-ray emissions from Uranus than would be expected solely from the planet’s atmosphere. “This was a surprise,” says Dunn, “And to explain why we have to consider what happens on other planets.”
He first draws attention to Jupiter, which has the strongest planetary magnetic field. “It spins faster, and its poor tiny moon Io is the most volcanic body in the Solar System, pumping a tonne of volcanic material into the space around Jupiter every second,” he says. “All of this creates crazy energetics and produces bright X-ray emissions, particularly bright flares from the aurorae.”
He then discusses Saturn, which was better observed when cutting-edge instruments were developed in the early 2000s. “People may have expected similar dynamic and bright auroral X-ray flares and things like that, but actually we’ve still not seen X-ray aurora from Saturn, and we’ve tried a few times now.”
Saturn does, however, have some cool stuff: water in its rings glows in X-rays when particles hit. “But mostly Saturn seems a bit X-ray boring. It seems to mostly just reflect the Sun’s emissions.”
With that in mind, the scientists expected Uranus to be like Saturn. “Just some scattered emission from the Sun and for it to be a bit boring,” Dunn laughs. But that is not happening. “Uranus is potentially throwing a spanner in the works here. If Uranus is making X-ray aurorae, then there are some crazy energetics going on for the ice
giants despite them being smaller than Saturn and rotating much slower.”
Dunn says that we already knew this to a degree. In 2010 Barry Mauk and Nicola Fox showed that Uranus has a higher intensity of near 100 kiloelectronvolt electrons than Jupiter or Saturn and near-identical intensities of megaelectronvolt electrons. “They showed there were lots of energetic particles smashing around in the space around Uranus,” Dunn continues. “I just don’t think we’d quite connected the dots as a community.”
Yet what if there is a mistake? What if the extra X-rays are not coming from Uranus at all, but somewhere else in the detector’s view? The scientists have certainly thought of that, and they have spent a lot of time – many months, in fact – checking the data to ensure that they haven’t missed something that would skew their conclusion and lead to another answer.
“In the end it turned out that it’s rarer than a one-in-a-million chance that the signal that we see from Uranus is randomly generated by the detector or the night sky,” Dunn says. “It could well be that we got unlucky and hit that one-in-a-million chance, but the energy of X-rays looks so much like they do from Jupiter and Saturn that it is even less likely than that.
“Not only would the detector or sky need to randomly generate the signal, it would also need to randomly produce photons of the same sorts of colours we see from Saturn and Jupiter. Again though, we probably need some more observations just to double-check that this wasn’t a one-in-amillion happening.”
And that is where we are today. From this point on there will be a proposal for more X-ray observations using Chandra or its sister X-ray observatory XMM-Newton. “That will involve the scientific community determining whether our case is more compelling than all of the other amazing science cases that they’ll receive – there’s lots of fantastic science that these instruments enable, and unfortunately there isn’t enough time for all of it,” Dunn laments.
One thing’s for sure, however: the scientists are going to need a much longer period of observation. “The three observations that the discovery is based on are all far too short to answer most of the questions being asked. What we’d ideally like is a few long observations so that we can see how the planet changes over a few rotations and collect enough data to answer those questions.”
Ultimately, a new spacecraft is needed to allow scientists to get up close and personal with the ice giants. So far only Voyager 2 has flown to Uranus, so there’s an over-reliance on telescopes. Given the distance between us and the outer planets, it’s far from ideal, although future technology such as the European Space Agency’s Advanced Telescope for High Energy Astrophysics (ATHENA), due to launch in 2031, may help.
“We need spacecraft orbiting these exotic and mysterious worlds to truly understand them, especially because it looks like ice giants are some of the most common planets in the universe,” concludes Dunn, whose team’s findings are detailed in a paper in the Journal of Geophysical
Research. “As it stands we have these two fantastic examples in our own Solar System, but we know so little about them.”