Strange things occurred long ago in the cold, distant realm of our Solar System’s two ice-giant planets, Uranus and Neptune–the duo of beautiful blue-banded denizens of the outer region of the planetary system that circles our Sun. Of the pair, the greenish-blue Uranus stands out in the crowd of our Solar System’s eight known major planets. This is because it is thought that Uranus was violently knocked over on its side when a doomed primordial world that was twice as large as Earth plunged into it billions of years ago. Uranus is also circled by a system of mysterious dark, slender rings that are invisible to all but the largest telescopes. For this reason, the Uranian moons were not discovered until 1977. However, in June 2019, a team of astronomers announced that the rings of Uranus are surprisingly bright in the heat images of the ice-giant taken by a pair of large telescopes in the high deserts of Chile.
This thermal glow provides astronomers with a new understanding of the nature of these exotic rings, which have been successfully observed simply because they reflect a small amount of light in the visible, or optical, range and in the near-infrared. The new images were obtained from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Telescope (VLT), and they allowed the team of astronomers to measure the temperature of the rings for the first time. The Uranian rings are frigid structures: a cool 77 Kelvin, or 77 degrees above absolute zero–the boiling temperature of liquid nitrogen and equivalent to 320 degrees below zero Fahrenheit.
The new observations also verify that Uranus’s brightest and densest ring, named the epsillon ring, is different from the other known ring systems circling other planets in our Solar System. This difference is especially pronounced for the beautiful and spectacular ring system of Saturn.
“Saturn’s mainly icy rings are broad, bright and have a range of particle sizes, from micron-sized dust in the innermost D ring to tens of meters in size in the main rings. The small end is missing in the main rings of Uranus, the brightest ring epsilon is composed of golf ball sized and larger rocks,” commented Dr. Imke de Pater in a June 20, 2019 University of California at Berkeley (UCB) Press Release. Dr. de Pater is a professor of astronomy at UCB.
In dramatic contrast, Jupiter’s rings are composed primarily of small micron-sized particles. Neptune’s rings are also primarily composed of dust, and even Uranus has broad sheets of dust situated between its slender main rings. One micron is a thousandth of a millimeter.
Wandering Primordial Worlds
In the dimly-lit and cold outer Solar System, in the distant realm of the giant planets, Uranus casts an eerie emerald-green glow upon its icy family of moons. The Uranian moon Miranda is especially interesting. It is a tiny moon-world that displays a chaotic icy surface that is unlike any other known world in our Solar System. Many astronomers think that the original Miranda was blasted to fragments billions of years ago and– after the wreck–the remnant frozen and mismatched fragments of the original moon collided and merged. The jumbled icy chunks, brought together again by the force of gravity, created a weird new moon with a chaotic and jumbled terrain.
Uranus was discovered on March 13, 1781 by the German-born English astronomer, William Herschel (1738-1822)–and its discovery was a complete accident. While surveying stars in the night sky, using a telescope that he had built himself, Herschel noticed that one of these “stars” seemed to be traveling to the beat of a different drum. After observing this strange “star” many more times, he realized that it was not a star at all, and that it was in orbit around our Sun. The strange “star” was the planet that we now call Uranus.
At last count, Uranus is orbited by 27 moons that are composed of rock, ice, or both. All of the Uranian moons are named after characters in William Shakespeare’s plays. The character Miranda is the heroine of A Midsummer Night’s Dream.
Uranus is the seventh major planet from our Star, and it is almost certain that Uranus–and its sister ice-giant Neptune–did not form where they are now, 19 and 30 astronomical units (AU) from the Sun. One AU is equivalent to the average separation between the Sun and Earth, which is about 93,000,000 miles. The accretionary process, that was responsible for the formation of the planets inhabiting our Solar System, was much slower farther from the Sun, where Uranus and Neptune are currently situated. This primordial protoplanetary accretion disk, that was made up of gas, dust, and ice, was too thin in this outer domain to allow planets of this large size to form as rapidly as they would in the warmer, denser region of the disk swirling closer to our Star.
Astronomers have had a difficult time creating a model that can explain how the duo of ice-giants attained their present hefty sizes if they were born where they are today. The protoplanetary accretion disk would have dissipated long before giant worlds had a chance to be born in this region of our Solar System. For this reason, many astronomers think that the cores of both Uranus and Neptune formed closer to the ancient Sun and later traveled to their current, remote locations long ago.
While it may seem peaceful now, we really live in a “cosmic shooting gallery”.Our early Solar System was a turbulent place where ancient objects, large and small, relentlessly blasted into one another–shattering each other into fragments after they collided. These continually growing primordial objects grew from pebble-size, to mountain-size, to moon-size, to planet-size in the crowded and violent disk environment. Sometimes planet-sized ancient bodies crashed into other planet-sized worlds, thus wreaking havoc. Gravitational influences–that resulted from the wanderings of these migrating worlds– shot some planets howling into other regions of our Solar System, or even out of our Solar System altogether.
Uranus orbits our Star on its side. The Uranian axis of rotation is approximately parallel with the plane of our Solar System, sporting an axial tilt of 97.77 (defined by prograde rotation). For this reason, Uranus experiences seasonal changes that are different from all of the other planets in our Sun’s family. Near the solstice, one pole faces our Star continuously, while the other pole’s face is continuously turned away. Only a very narrow region around the Uranian equator undergoes a rapid day-night cycle–but with the Sun suspended low over the horizon. In contrast, at the other side of Uranus’s orbit, the orientation of the poles towards our Sun is the opposite. Each pole gets approximately 42 years of ceaseless sunlight, followed by about 42 years of unending darkness. Approaching the time of the equinoxes, our Star faces the equator of Uranus giving a period of day-night cycles similar to those experienced on most of the other planets dwelling in our Solar System.
Uranus experienced its most recent equinox of December 7, 2007. One consequence of this axis orientation is that, averaged over the span of one Uranian year, the polar regions of Uranus are bestowed with a greater energy input from our Sun than its equatorial regions. Despite this, Uranus is hotter at its equator than at its poles–and the cause for this is unknown. The reason given for Uranus’s bizarre axial tilt is likewise unknown. However, the usual explanation given is that, about 4.5 billion years ago, an Earth-sized protoplanet collided with Uranus, thus skewing its orientation.
The Strange Rings Of A Distant Ice-Giant
Several theories have been proposed to explain the origin of Uranus’s rings: they could be erstwhile asteroids snared by the giant green planet’s gravity, the shattered remnants of wrecked moons that blasted into one another, the relic fragments of moons torn to pieces when they wandered too close to their parent-planet, or debris lingering from the ancient formation of our Solar System.
The new data were published in the July 2019 issue of The Astronomical Journal. De Pater and Molter led the ALMA observations, while Dr. Michael Roman and Dr. Leigh Fletcher from the University of Leicester in the United Kingdom led the VLT observations.
“The rings of Uranus are compositionally different from Saturn’s main ring in the sense that in optical and infrared the albedo is much lower; they are really dark, like charcoal. They are extremely narrow compared to the rings of Saturn. The widest, the epsilon ring, varies from 20 to 100 kilometers wide, whereas Saturn’s are 100’s or tens of thousands of kilometers wide,” Molter explained in the June 20, 2019 UCB Press Release.
The absence of dust-sized particles in Uranus’s main rings was first detected back in 1986, when Voyager 2 flew by the green ice-giant planet, and obtained revealing images of them. Alas, the spacecraft was unable to measure the temperature of the rings. Currently, astronomers have counted a total of 13 rings around the planet, with some bands of dust swirling between the rings. The Uranian rings differ in other ways from those of the gas-giant Saturn.
“It’s cool that we can even do this with the instruments we have. I was just trying to image the planet as best I could and I saw the rings. It was amazing,” Molter added.
Both the VLT and ALMA observations were designed to study the temperature structure of Uranus’ atmosphere, with VLT probing shorter wavelengths than ALMA.
The new research presents an intriguing opportunity for the upcoming James Webb Space Telescope, which will have the ability to provide greatly improved spectroscopic constraints on the rings of Uranus during the coming decade.
Dr. Fletcher commented in the June 20, 2019 UCB Press Release that “We were astonished to see the rings jump out clearly when we reduced the data for the first time.”