Astronomy

What kind of effects can maintain Uranus' rings' eccentricities?

What kind of effects can maintain Uranus' rings' eccentricities?

The recent paper Thermal Emission from the Uranian Ring System has been in the news recently. The introduction mentions:

The ε ring, the brightest and most massive of the narrow rings, was shown to maintain an appreciable eccentricity (e = 0.00794) and an azimuthally-varying width; the ring is five times wider and ∼2.5 times brighter in reflected sunlight at apoapsis than at periapsis (French et al. 19881; Karkoschka 2001a2). However, many fundamental parameters about the ring system remain unknown, including the filling factor, composition, thickness,

1French, R. G., Elliot, J. L., French, L. M., et al. 1988, Icarus, 73, 349, doi: 10.1016/0019-1035(88)90104-2

2Karkoschka, E. 2001a, Icarus, 151, 51, doi: 10.1006/icar.2001.6596

I've read in Wikipedia's Rings of Uranus that several rings are expected to be fairly young, and there may be interaction with some small moons, but I am surprised that the rings could maintain a significant eccentricity, and do not circularize or smear out due to collisions.

Question: What kind of dynamical effects can maintain Uranus' rings' eccentricities?

Uranus itself has a huge J2 and so any ring that isn't perfectly equatorial should have an eccentricity-dependent precession. The paper states that the apoapsis of the rings is five times wider than the periapsis, so there's definitely a population of different eccentricities and semi-major axes within the ε ring.

List of rings and their orbital parameters


A relevant paper is Papaloizou & Melita (2004) "Structuring eccentric-narrow planetary rings" which starts off promisingly with the following:

The nature of the dynamical mechanism that maintains the apse alignment of narrow-eccentric planetary rings is one of the most interesting and challenging problems of Celestial Mechanics.

According to the leading model (Goldreich and Tremaine 1979) the self-gravity of the ring counter-acts the differential precession induced by the oblateness of the central planet. Using this hypothesis, a prediction of the total mass of the ring can be made, which, in general, is not in good agreement with the inferred mass of the observed eccentric rings in the Uranus system

It then goes on to describe subsequent refinements taking account of additional effects including particle interactions and perturbations that bring the predictions more in line. The approach described is given in the introduction:

In this work we build, from first principles, a simple general continuum or fluid like model of a narrow-eccentric ring. The eccentric pattern in the ring can be described as being generated by a normal mode of oscillation of wave-number m = 1 which may be considered to be a standing wave. Dissipation can be allowed to occur due to inter-particle collisions leading to a viscosity which would lead to damping of the mode. However, this global m = 1 mode can also be perturbed by neighboring-shepherd satellites which can inject energy and angular momentum through resonances. In this way losses due to particle collisions may be balanced. It is that process that is the focus of this paper. Other possible mechanisms, such as mode excitation through self-excitation through viscous overstability, that could arise with an appropriate dependence of viscosity on physical state variables (see Papaloizou and Lin 1988, Longaretti & Rappaport 1995), are beyond the scope of this paper and accordingly not investigated here.

The specific case of the ε-ring of Uranus is described in section 10.2, where the main satellite forcing considered is due to the 47:49 second-order mean-motion resonance between the ring and Cordelia.

The paper goes into quite a bit of mathematical detail on the various processes involved.


Rings of Uranus

The rings of Uranus are intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink. William Herschel had also reported observing rings in 1789 modern astronomers are divided on whether he could have seen them, as they are very dark and faint. [1]

By 1977, nine distinct rings were identified. Two additional rings were discovered in 1986 in images taken by the Voyager 2 spacecraft, and two outer rings were found in 2003–2005 in Hubble Space Telescope photos. In the order of increasing distance from the planet the 13 known rings are designated 1986U2R/ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν and μ. Their radii range from about 38,000 km for the 1986U2R/ζ ring to about 98,000 km for the μ ring. Additional faint dust bands and incomplete arcs may exist between the main rings. The rings are extremely dark—the Bond albedo of the rings' particles does not exceed 2%. They are probably composed of water ice with the addition of some dark radiation-processed organics.

The majority of Uranus's rings are opaque and only a few kilometres wide. The ring system contains little dust overall it consists mostly of large bodies 20 cm to 20 m in diameter. Some rings are optically thin: the broad and faint 1986U2R/ζ, μ and ν rings are made of small dust particles, while the narrow and faint λ ring also contains larger bodies. The relative lack of dust in the ring system may be due to aerodynamic drag from the extended Uranian exosphere.

The rings of Uranus are thought to be relatively young, and not more than 600 million years old. The Uranian ring system probably originated from the collisional fragmentation of several moons that once existed around the planet. After colliding, the moons probably broke up into many particles, which survived as narrow and optically dense rings only in strictly confined zones of maximum stability.

The mechanism that confines the narrow rings is not well understood. Initially it was assumed that every narrow ring had a pair of nearby shepherd moons corralling it into shape. In 1986 'Voyager 2' discovered only one such shepherd pair (Cordelia and Ophelia) around the brightest ring (ε).


Planetary Rings as a Model in Cosmogony

Since the beginning of the XVII th century, rings of dispersed matter play an important role in theories accounting for the origin of the solar system and proto-planetary nebulae. Planetary rings afford a good opportunity of studying some of the accretion mechanisms which operate during the early evolution of a proto-planetary nebula. Three new planetary rings have been recently discovered. Space and ground-based observations have completely renewed our image of ring systems. Simultaneously, a wealth of theoretical and numerical models have flourished. Collisions between ring’s particles and gravitational perturbations of nearby satellites should explain most of the ring’s structures. However, important questions are still unanswered. We do not understand why the rings are so dissimilar. We do not know the ring’s origin and their stability over billions of years. Most of the ring’s complex structures, the existence of arcs, and color and optical depth variations are not explained. Confinement mechanisms, which are so important in cosmogony, seem to be at work in planetary rings today.


Sudden Change & Upheaval

Upsetting the status quo, changing what has been accepted as stable or normal especially suddenly is often considered negative or bad. Is it always? No, and in general, most people recognize that change is a necessary part of growth. Uranus particularly is a symbol for sudden change for good or for bad, and often that viewpoint is subjective. Uranus is not a personal planet so in itself is considered neutral and although an instrument, symbol or force of change is not inherently good or bad, it is just sudden change. Saturn is often considered the planet of slow change.

Changes symbolized or brought upon by Uranus are often considered erupting or striking like a “bolt from the blue” and now that we know Uranus is blue it makes the cliché even more appropriate. Although the changes indicated by Uranus occur suddenly, when transiting, Uranus, Neptune and Pluto all seem to cast a shadow that foretells the influences about to occur. In the case of Uranus, it is an “unsettled” feeling that some will have just before a new Uranus transit. Uranus also seems to have a “eureka” influence that is also very sudden discussed below.

Uranian changes usually occur without any warning like an earthquake or volcanic eruption. Some Uranian changes are truly devastating in their effect. Uranus is often in play when people endure personal upheavals in their lives such as during a loss of job, a loved one, the end of a relationship, or even during a catastrophic event. However, Uranus also has a positive side in that it may be involved with sudden positive changes such as an unexpected pregnancy or birth, a sudden job promotion, raise or bonus, a lottery win or receiving another surprise windfall. Some upheavals are very good changes.

The sudden unpredictability of the change is the hallmark of Uranian change. Uranus may represent what happens to us, but also how we react to change.

We respond to life with the Sun with our personality, the Moon our emotions, Mercury our intellect, Venus our feelings, Mars with action or reaction, and with Uranus we respond with a desire to change the present situation. Similar to the flight or fight reaction of Mars, with Uranus we know we have to react with change but sometimes cannot tell when, what or where the change is needed. Sometimes, just any change is the solution, although the relationships between Uranus and other planets and signs or houses involved will provide many clues as to what is the right change.

Sometimes, change and upheaval occur deep within ourselves. We feel or recognize we are changing yet there appears to be no external forces for change at work. A thought occurs, or through analysis a solution is found, a new development unfolds, and voila insight or an invention is born. This too is Uranus.

As mentioned, Uranus is not considered positive or negative in itself. If you view change as good then it is if you view change as bad then it likely is too. However, Uranus is not malleable due to our perceptions. Changes indicated by Uranus will not usually be stopped. The pain of Uranian changes occurs when we fight it, try to stop it, repel it, or refuse to accept the inevitability of change. Uranus is a planetary energy where it may be best to just go with the flow of change if it cannot be redirected.

Uranus can be a great power of good or bad change and the aspects and relationships to Uranus may provide clues to transform one’s life or a situation. Uranian style changes are often intended to take us to a new level of understanding or insight.


Isolated lines

III.3.4 The Galatry profile

A more general approach was developed by Galatry 183 in order to extend the Dicke model [ Eq. (III.19) ] toward lower pressures (i.e., down to the Doppler regime). Assuming that radiators undergo only very small velocity changes per collision, and using the Brownian movement model, 176 leads to 183 :

where ν VC S is the (speed-independent) VC collision rate in this soft (labeled S) collision model. The correlation function [ Eq. (III.24) ] for the external degrees of freedom generalizes the diffusional limit [ Eq. (III.10) ] to lower collisional broadening. The rate ν V C S , sometimes called the dynamical friction coefficient, 181 is connected with the diffusion coefficient D for the active species in a buffer gas (and thus denoted νDiff) by the relation:

As discussed below ( Sections III.4.1 and III.5.2 ), this relation should be used with caution. Indeed, D, through Eq. (III.25) appears in a context different from that of the conventional mass diffusion coefficient in gas kinetic theory (e.g., Refs. [181,184]). Thus it does not have rigorously the same physical significance. This is the reason why some authors have introduced the terminology of “optical” diffusion coefficient Dopt, 185 to be distinguished from the mass diffusion coefficient. Note that C ext S ( t ) [ Eq. (III.24) ] consistently tends toward the diffusional limit C ext Diff ( t ) [ Eq. (III.10) ] for large values of ν VC S . Furthermore, VC and D collisions are assumed to be statistically uncorrelated—and denoted (VC+D) to signify that VC and D arise only in distinct collisions. Thus the Laplace transform of the product of Cint(t) [ Eq. (III.14) ] and C ext s ( t ) [ Eq. (III.24) ] leads to the Galatry profile (GP), expressed in dimensionless variables x, y, z [ Eq. (III.20) ] by:

IG(x, y, z) can also be expressed in terms of the confluent hypergeometric function. 56,181 56 181 The GP has the proper behavior for limiting values of s, y, z, that is: ID for s=y=z=0 (in the absence of both collisional shift, broadening and Dicke narrowing) and IV for z=0 (in the absence of the Dicke narrowing only).

In order to estimate the applicability conditions of the GP, the colliding molecules can be considered as rigid spheres. The mean persistence velocity ratio r v → , defined as the ratio of the mean value of the postcollisional radiator velocity over the precollisional one, can then be expressed in terms of the masses of the colliding molecules 186 :

with m ˜ = m / M ˜ , m ˜ p = m p / M ˜ , M ˜ = m + m p , mp being the mass of the perturber. For decreasing values of the mass ratio mp/m from infinity to zero, r v → varies between zero and unity. The case r v → ≈ 1 corresponds to mp/m≪1, that is, to the quasi persistence of the radiator velocity memory. Hence, the soft collision approximation underlying the GP model appears to be convenient for heavy radiators and light perturbers.


Problems for evolution

The accepted evolutionary view of the origin of the solar system is usually called the Nebular Hypothesis. In this model, a giant cloud in space made up of mainly spinning, ionized gas with a magnetic field is believed to have pulled together by gravity into the sun, planets and other objects in our solar system. Computer simulations of this process do not start with initial conditions like those of real nebulas, and have other problems. One scientist summarized these by saying ‘The clouds are too hot, too magnetic, and they rotate too rapidly.𔄃 The contraction produces effects that tend to make the formation of planets impossible.10 One scientist described the Nebular Hypothesis as the theory with the ‘best fit’ to the observational evidence. However, he then stated that: ‘The argument is highly speculative and some of it borders on science fiction.󈧏

There is a particularly thorny problem for evolutionary solar system models. Everyone has watched accomplished skaters spin on ice. As skaters pull their arms in, their radius decreases and they spin faster. This effect is due to what physicists call the Law of Conservation of Angular Momentum. In the formation of our Sun from a nebula in space, the same effect would occur as the gases contracted into the centre to form the Sun. This would cause the sun to spin very rapidly as a result of this law. Actually, our sun spins very slowly while the planets move very rapidly around the sun.12 This pattern is directly opposite to the pattern predicted for the Nebular Hypothesis. Many scientists today no doubt assume that modern theories have solved this problem. But a well-known solar system scientist Dr Stuart Ross Taylor, has said in a recent book, ‘The ultimate origin of the solar system’s angular momentum remains obscure’.13

There is a competing evolutionary model for the origin of the solar system, called the Capture Theory. In this, a passing protostar, loosely held together, passes close to our Sun whose gravity pulls off a filament of the star’s material, which breaks up into segments that become six planets (not the current nine). Then two of these six collide and the asteroids, Venus, Earth, Mars, and our Moon represent either fragments of the collision or moons of the two planets that collided.14 The Capture Theory is considered unlikely by most astronomers and has unique problems of its own. Interestingly, today some catastrophes are being invoked to explain the solar system.

Dr Jonathan Henry, who teaches science at Clearwater Christian College, believes the solar system evidence to be consistent with a universal catastrophe, which he associates with God’s curse on all creation (Genesis 3). Dr Henry postulates that this was when (initially rapid) radioactive and other decay processes (including of planetary magnetic fields) began. This led to enormous interior heating and volcanism. By this approach, the asteroids would be the remnants of the disintegration of a planet from such internal overheating.15 This heating could have been a trigger of some processes in the Flood of Noah. It is also possible that some asteroids were created as they are and some are the result of collisions. A solar-system-wide bombardment event of some kind would explain widespread cratering within a young universe.

Christians should welcome the flow of new discoveries in space which, stripped of their evolutionary assumptions, continually highlight the incredible greatness and creativity of our God .


Astronomers Reveal ‘Cataclysmic’ Collision Shaped Uranus’ Evolution

New research shows that Uranus was hit by a massive object roughly twice the size of Earth that caused the planet to tilt and could explain its freezing temperatures.

Astronomers at Durham University led an international team of experts to investigate how Uranus came to be tilted on its side and what consequences a giant impact would have had on the planet’s evolution.

The team ran the first high-resolution computer simulations of different massive collisions with the ice giant to try to work out how the planet evolved.

The research confirms a previous study that said Uranus’ tilted position was caused by a collision with a massive object – most likely a young proto-planet made of rock and ice – during the formation of the solar system about 4 billion years ago.

The collision with Uranus of a massive object twice the size of Earth that caused the planet’s unusual spin, from a high-resolution simulation using over ten million particles, coloured by their internal energy. Jacob Kegerreis/Durham University

The simulations also suggested that debris from the impactor could form a thin shell near the edge of the planet’s ice layer and trap the heat emanating from Uranus’ core. The trapping of this internal heat could in part help explain Uranus’ extremely cold temperature of the planet’s outer atmosphere (-216 degrees Celsius, -357 degrees Fahrenheit), the researchers said.

The findings are published in The Astrophysical Journal.

Giant impact

Lead author Jacob Kegerreis, PhD researcher in Durham University’s Institute for Computational Cosmology, said: “Uranus spins on its side, with its axis pointing almost at right angles to those of all the other planets in the solar system. This was almost certainly caused by a giant impact, but we know very little about how this actually happened and how else such a violent event affected the planet.

“We ran more than 50 different impact scenarios using a high-powered super computer to see if we could recreate the conditions that shaped the planet’s evolution.

“Our findings confirm that the most likely outcome was that the young Uranus was involved in a cataclysmic collision with an object twice the mass of Earth, if not larger, knocking it on to its side and setting in process the events that helped create the planet we see today.”

A 2004 infrared composite image of the two hemispheres of Uranus obtained with Keck Telescope adaptive optics. Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory.

There has been a question mark over how Uranus managed to retain its atmosphere when a violent collision might have been expected to send it hurtling into space.

According to the simulations, this can most likely be explained by the impact object striking a grazing blow on the planet. The collision was strong enough to affect Uranus’ tilt, but the planet was able to retain the majority of its atmosphere.

Rings and moons

The research could also help explain the formation of Uranus’ rings and moons, with the simulations suggesting the impact could jettison rock and ice into orbit around the planet. This rock and ice could have then clumped together to form the planet’s inner satellites and perhaps altered the rotation of any pre-existing moons already orbiting Uranus.

The simulations show that the impact could have created molten ice and lopsided lumps of rock inside the planet. This could help explain Uranus’ tilted and off-centre magnetic field.

Uranus is similar to the most common type of exoplanets – planets found outside of our solar system – and the researchers hope their findings will help explain how these planets evolved and understand more about their chemical composition.

Co-author Dr Luis Teodoro, of the BAERI/NASA Ames Research Center, said: “All the evidence points to giant impacts being frequent during planet formation, and with this kind of research we are now gaining more insight into their effect on potentially habitable exoplanets.”

The research was funded by the Science and Technology Facilities Council, The Royal Society, NASA and Los Alamos National Laboratory.

Publication: J. A. Kegerreis, et al., “Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion,” ApJ, 2018 doi:10.3847/1538-4357/aac725


Asteroid Found with Rings! First-of-Its-Kind Discovery Stuns Astronomers (Video, Images)

Scientists have made a stunning discovery in the outer realm of the solar system — an asteroid with its own set of rings that orbits the sun between Saturn and Uranus. The space rock is the first non-planetary object ever found to have its own ring system, researchers say.

The pair of space rock rings encircle the asteroid Chariklo. They were most likely formed after a collision scattered debris around the asteroid, according to a new study unveiled today (March 27). The asteroid rings also suggests the presence of a still-undiscovered moon around Chariklo that's keeping them stable, researchers said.

"We weren&rsquot looking for a ring and didn&rsquot think small bodies like Chariklo had them at all, so the discovery — and the amazing amount of detail we saw in the system — came as a complete surprise!" study leader Felipe Braga-Ribas, of the National Observatory in Brazil said in a statement today. [Asteroid with Rings: Artist Views of Space Rock Chariklo (Photos)]

Astronomers used seven telescopes, but just one revealed the pair of rings orbiting the rocky Chariklo. The asteroid's 155-mile diameter (250 kilometers) is dwarfed by the giant gas planets, the only other bodies known to have rings.

"This discovery shows that size is not important in order to have — or not have — rings," Felipe Braga-Ribas, of the National Observatory in Brazil, told Space.com by email.

An asteroid among giants

On June 3, 2013, Braga-Ribas led a team of astronomers in observing Chariklo as it passed in front of a distant star — a process known as an occultation. As the asteroid traveled, it blocked light from the star, enabling scientists to learn more about it. [The Strangest Asteroids in the Solar System]

The astronomers were surprised to discover that a few seconds before and after the main occultation, the light dimmed slightly, indicating that something circled the rocky asteroid. By comparing the data gathered from seven different telescopes, the team was able to identify the shape, size and orientation of the rings.

The system consists of a dense, 4-mile-wide (7 km) ring near the planet, and a smaller 2-mile-wide (3 km) ring farther out.

From the surface of the asteroid, "they would be two spectacular sharp and really bright rings, crossing all the sky," Braga-Ribas said. "They would be noticeably close, as they are at about 1/1,000 of the moon's distance from us," he added.

He went on to say that the larger, inner ring would block the view of the outer ring from the ground. The rings are similar to those around Saturn, in that both are very dense, bright and possibly formed by rock and water ice. But their scales are quite different.

"The whole Chariklo system would fit about 12 times in the Cassini Division," Braga-Ribas said, referring to the largest gap in Saturn's rings.

Particles orbiting Chariklo also travel more slowly — only tens of meters per second, compared with tens of kilometers per second in the rings of Saturn.

While Saturn is the most well-known ringed body in the solar system, Jupiter, Neptune and Uranus also have their own, fainter rings. These gas giants significantly dwarf the smaller asteroid. [See Saturn's Glorious Rings Up Close (Photos)]

Astronomers utilized seven telescopes, most of which were located in South America. Of them, only the European Southern Observatory's La Silla telescope in Chile was able to capture the small gap between the rings.

"This was possible due to the use of the 'Lucky Imager,' a fast and sensible camera that obtained a sequence of images like a video at a rate of 10 images per second," Braga-Ribas said. "As the stellar occultation by both rings lasted for 0.6 seconds in total, it was able to 'see' the rings in detail."

The other telescopes had exposure times greater than 0.7 seconds, so they were only able to observe a single gap in the light.

What's so special about this asteroid to make it have rings?
"Chariklo seems to be nothing special, otherwise," Joseph Burns, of Cornell University, told Space.com by email. Burns was not a member of Braga-Ribas' team, but he studies planetary rings and the small bodies of the solar system. He authored a perspective article that appeared alongside the new findings.

Chariklo may not be the only nonplanetary body to have rings, Braga-Ribas said. "Rings may be a much more common property than we thought," he said.

The research and Burns' accompanying article were published online today (March 26) in the journal Nature.

Chariklo's 'toy ring'

Chariklo is the largest of the centaurs, several bodies in the outer solar system whose orbits cross — and are changed by — the outer planets. The centaurs share characteristics with both asteroids and comets, and are thought to come from the Kuiper Belt region beyond Pluto. Rocky Chariklo appears to be more asteroid than comet in composition, according to the paper.

This placement may help to explain the presence of Chariklo's rings and their absence in the asteroid belt that lies between Mars and Jupiter. The rocky inner planets and the asteroid belt lie closer to the sun, and experience stronger forces from the solar wind, which can more efficiently blow small particles away from objects they might otherwise orbit, Braga-Ribas said.

Collisions in the fast-moving asteroid belt are also violent processes due to their faster orbital speeds. Crashes between the nearby rocky bodies may wind up hurling any potential ring material away too quickly. The collision that likely created Chariklo's rings would have had to have been a slow-moving impact. The asteroid's small size means it has very little gravity, allowing fast-moving objects to easily escape from its orbit the asteroid would only have been able to hold on to slower-traveling objects.

The presence of a ring system answers questions about why the asteroid has brightened since observations in 2008. Originally viewed edge-on, the rings have become visible over the last five years as their inclination changed.

The age of the rings remains another mystery. Over the course of a few million years, the small pieces of a ring system should spread out. Because they are still contained as a ring, the authors concluded that either the system is very young, or the asteroid hosts a small moon that shepherds and confines the particles in their orbit. The moon would be about as massive as both rings combined, and would easily escape detection given Chariklo's great distance.

"Shepherds are the preferred — and basically only — explanation," Burns said. "But Saturn's and Uranus' rings have many gaps where we should see shepherds and we don't. Something is missing in our understanding. Maybe studying Chariklo's toy rings will bring us ideas."

If a missing moon circles the asteroid, keeping the rings in line, then the system could have lasted since the dawn of the solar system, Braga-Ribas said, adding that the disturbance of the gas giant that moved Chariklo to its present-day orbit would require a very close pass to disturb the ring system, indicating that they could have survived the migration.

Studying the stability of Chariklo's rings can tell astronomers about the environment required to form and maintain them — a process that can be used to understand the dynamics of the early stages of the solar system.

On a wider scale, the tiny ringed asteroid can also help scientists to understand more about how galaxies form.

"The shepherd mechanism seems to be universal from the giant planets to the small minor planet," Braga-Ribas said. "This mechanism may be acting in other kinds of debris discs, such as proto-planetary nebulae and galaxies."


X-ray emissions from Uranus are detected for the first time

Astronomers have spotted X-ray emissions from the planet Uranus for the first time. The international team, led by William Dunn at Mullard Space Science Laboratory in the UK, discovered the signals through new analysis of data from NASA’s Chandra X-ray Observatory. The observations could provide important guidance for upcoming X-ray studies of Uranus and Neptune.

X-ray emissions have been detected from most planets in the solar system and can originate from a variety of processes including the scattering of X-ray photons from the Sun collisions between plasmas and planetary rings and aurorae generated as solar winds interact with polar atmospheres. Until recently, however, evidence for X-ray emissions were notably absent from the solar system’s two ice giants: Uranus and Neptune.

Through new analysis of data gathered by the Chandra X-ray Observatory, Dunn’s team have identified three clear X-ray signals originating from Uranus: first in 2002, and then on two consecutive days in 2017. These observations are particularly interesting because of the planet’s unique orientation. Unlike other planets in the solar system, Uranus’s rotational axis lies parallel to its orbital plane and the planet’s magnetic field has a significant tilt relative to its axis of rotation. Indeed, the magnetic field misses the planet’s centre by roughly a third of its radius.

Complex relationship

This unusual configuration creates a complex relationship between Uranus’ magnetosphere and the solar wind. The resulting effects have already been probed at other wavelengths: during its 1986 flyby, Voyager 2 picked up patchy clusters of auroral emissions around both magnetic poles. Three decades later, the Hubble Space Telescope detected far more complex and time-variable emissions in the Uranian aurora. These results, combined with the known mechanisms for X-ray emissions on other planets, enabled Dunn and colleagues to present several theories for their X-ray observations.

Things we don’t know about Uranus (and Neptune)

The strengths of all three signals detected by Chandra were stronger than would be expected, had they originated from solar X-ray scattering. According to Dunn’s team, this could mean that Uranus is more reflective to incident X-rays than Jupiter and Saturn – but may also hint at additional mechanisms on the planet itself. These could include particle collisions in the aurora or a glow in Uranus’ two icy rings, triggered by collisions with surrounding protons and electrons.

Further observations will be required to constrain these potential mechanisms, and to pin down the locations of X-ray sources on the Uranian surface. Dunn’s team hope this could be achieved through deeper observations with Chandra. However, future observations will be greatly improved by upcoming missions including ESA’s ATHENA X-ray Observatory, and NASA’s Lynx X-ray Observatory – both planned for launch in the 2030s. The team’s results could one day provide valuable guidance for these future observations.


Research & exploration

NASA's Voyager 2 was the first and as yet, only spacecraft to visit Uranus. Although there isn't a spacecraft on its way to Uranus at the moment, astronomers regularly check in with the planet using the Hubble and Keck telescopes.

In 2011, the Planetary Science Decadal Survey recommended that NASA consider a mission to the icy planet. And in 2017, NASA suggested a number of potential future missions to Uranus in support of the forthcoming Planetary Science Decadal Survey, including flybys, orbiters and even a spacecraft to dive into Uranus' atmosphere. Scientists are still discussing the idea. In 2019, NASA's Goddard Space Flight Center suggested one possible design could involve an atmospheric probe, similar to the one used in Jupiter during the Galileo mission.

In 2018, an ambitious group of early-career scientists and engineers created a theoretical full mission design that would cost $1 billion, and take advantage of a planetary alignment that would happen in the 2030s. At that relatively low cost, the mission would perform minimal science, but could still include items such as a magnetometer, a methane detector and a camera.