What’s the most interesting fact you know about Uranus? The fact that its rotational axis is completely out of line with every other planet in the solar system? Or the fact that Uranus’ magnetosphere is asymmetrical, notably tilted relative to its rotational axis, and significantly offset from the center of the planet? Or the fact that it’s moons are all named after characters from Shakespeare or Alexander Pope?
All of those facts (with the exception of the literary references) have come from a very limited dataset. Some of the best data was collected during a Voyager 2 flyby in 1986. Since then, the only new data has come from Earth-based telescopes. While they’ve been steadily increasing in resolution, they have only been able to scratch the surface of what may be lurking in the system surrounding the closest Ice Giant. Hopefully that is about to change, as a team of scientists has published a white paper advocating for a visit from a new Flagship class spacecraft.
Composite image of Uranus’s atmosphere and rings at radio wavelengths, taken with the ALMA array in December 2017. The image shows thermal emission, or heat, from the rings of Uranus for the first time, enabling scientists to determine their temperature: a frigid 77 Kelvin (-320 F). Dark bands in Uranus’s atmosphere at these wavelengths show the presence of molecules that absorb radio waves, in particular hydrogen sulfide gas. Bright regions like the north polar spot (yellow spot at right, because Uranus is tipped on its side) contain very few of these molecules. (UC Berkeley image by Edward Molter and Imke de Pater)
During the late 1970s, scientists made a rather interesting discovery about the gas giants of the Solar System. Thanks to ongoing observations using improved optics, it was revealed that gas giants like Uranus – and not just Saturn – have ring systems about them. The main difference is, these ring systems are not easily visible from a distance using conventional optics and require exceptional timing to see light being reflected off of them.
Another way to study them is to observe their planet in infrared or radio wavelengths. This was recently demonstrated by a team of astronomers who conducted observations of Uranus using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Telescope (VLT). In addition to obtaining temperature readings from the rings, they confirmed what many scientists have suspected about them for some time.
Between 3 to 4 billion years ago, a body twice the size of Earth impacted Uranus, knocking the ice giant onto its side. Image Credit: Jacob A. Kegerreis/Durham University
Astronomers think they know how Uranus got flipped onto its side. According to detailed computer simulations, a body about twice the size of Earth slammed into Uranus between 3 to 4 billion years ago. The impact created an oddity in our Solar System: the only planet that rotates on its side.
A study explaining these findings was presented at the American Geophysical Union’s (AGU) Fall Meeting in Washington DC held between December 10th to 14th. It’s led by Jacob Kegerreis, a researcher at Durham University. It builds on previous studies pointing to an impact as the cause of Uranus’ unique orientation. Taken altogether, we’re getting a clearer picture of why Uranus rotates on its side compared to the other planets in our Solar System. The impact also explains why Uranus is unique in other ways. Continue reading “Something Twice the Size of Earth Slammed into Uranus and Knocked it Over on its Side”
Neptune, as seen by Voyager 2 in 1989. The deep blue color applied helped enhance the view of features in the atmosphere. Image Credit: NASA/JPL
Have you seen the outer ice giant planets for yourself?
This week is a good time to check the most difficult of the major planets off of your life list, as Neptune reaches opposition for 2018 on Friday, September 7th at at ~18:00 Universal Time (UT)/2:00 PM EDT. And while it may not look like much more than a gray-blue dot at the eyepiece, the outermost ice giant world has a fascinating tale to tell. Continue reading “Exploring the Ice Giants: Neptune and Uranus at Opposition for 2018”
A new study indicates that a massive impact may be why Uranus orbits on its side. Credit: NASA/JPL/Voyager mission
The gas/ice giant Uranus has long been a source of mystery to astronomers. In addition to presenting some thermal anomalies and a magnetic field that is off-center, the planet is also unique in that it is the only one in the Solar System to rotate on its side. With an axial tilt of 98°, the planet experiences radical seasons and a day-night cycle at the poles where a single day and night last 42 years each.
Thanks to a new study led by researchers from Durham University, the reason for these mysteries may finally have been found. With the help of NASA researchers and multiple scientific organizations, the team conducted simulations that indicated how Uranus may have suffered a massive impact in its past. Not only would this account for the planet’s extreme tilt and magnetic field, it would also explain why the planet’s outer atmosphere is so cold.
Uranus as seen by NASA's Voyager 2. Credit: NASA/JPL
If you’ve got really good eyesight and can find a place where the light pollution is non-existent, you might be able to see Uranus without a telescope. It’s only possible with the right conditions, and if you know exactly where to look. And for thousands of years, scholars and astronomers were doing just that. But given that it was just a tiny pinprick of light, they believed Uranus was a star.
It was not until the late 18th century that the first recorded observation that recognized Uranus as being a planet took place. This occurred on March 13th, 1781, when British astronomer Sir William Herschel observed the planet using a telescope of his own creation. From this point onwards, Uranus would be recognized as the seventh planet and the third gas giant of the Solar System.
Observations pre-18th Century:
The first recorded instance of Uranus being spotted in the night sky is believed to date back to Classical Antiquity. During the 2nd century BCE, Hipparchos – the Greek astronomer, mathematician and founder of trigonometry – apparently recorded the planet as a star in his star catalogue (completed in 129 BCE).
William Herschel’s telescope, through which the planet Uranus was first observed. Credit: Wikipedia Commons
This catalog was later incorporated into Ptolemy’s Almagest,which became the definitive source for Islamic astronomers and for scholars in Medieval Europe for over one-thousand years. During the 17th and 18th centuries, multiple recorded sightings were made by astronomers who also catalogued it as being a star.
This included English astronomer John Flamsteed, who in 1690 observed the star on six occasions and catalogued it as a star in the Taurus constellation (34 Tauri). During the mid-18th century, French astronomer Pierre Lemonnier made twelve recorded sightings, and also recorded it as being a star. It was not until March 13th, 1781, when William Herschel observed it from his garden house in Bath, that Uranus’ true nature began to be revealed.
Hershel’s Discovery:
On the evening in question – March 13th, 1781 – William Herschel was surveying the sky with his telescope, looking for binary stars. His first report on the object was recorded on April 26th, 1781. Initially, he described it as being a “Nebulous star or perhaps a comet”, but later settled on it being a comet since it appeared to have changed its position in the sky.
Portrait of Sir William Herschel, by Lewis Francis Abbot (1784). Credit: Wikipedia Commons/National Portrait Gallery
When he presented his discovery to the Royal Society, he maintained this theory, but also likened it to a planet. As was recorded in the Journal of the Royal Society and Royal Astronomical Society on the occasion of his presentation:
“The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.”
While Herschel would continue to maintain that what he observed was a comet, his “discovery” stimulated debate in the astronomical community about what Uranus was. In time, astronomers like Johann Elert Bode would conclude that it was a planet, based on its nearly-circular orbit. By 1783, Herschel himself acknowledged that it was a planet to the Royal Society.
Naming:
As he lived in England, Herschel originally wanted to name Uranus after his patron, King George III. Specifically, he wanted to call it Georgium Sidus (Latin for “George’s Star”), or the Georgian Planet. Although this was a popular name in Britain, the international astronomy community didn’t think much of it, and wanted to follow the historical precedent of naming the planets after ancient Greek and Roman gods.
Large floor mosaic from a Roman villa in Sassoferrato, Italy (ca. 200–250 CE). Aion (Uranus), the god of eternity, stands above Tellus (Gaia) and her four children (the seasons). Credit: Wikipedia Commons/Bibi Saint-Poi
Consistent with this, Bode proposed the name Uranus in a 1782 treatise. The Latin form of Ouranos, Uranus was the grandfather of Zeus (Jupiter in the Roman pantheon), the father of Cronos (Saturn), and the king of the Titans in Greek mythology. As it was discovered beyond the orbits of Jupiter and Saturn, the name seemed highly appropriate.
In the following century, Neptune would be discovered, the last of the eight official planets that are currently recognized by the IAU. And by the 20th century, astronomers would discovery Pluto and other minor planets within the Kuiper Belt. The process of discovery has been ongoing, and will likely continue for some time to come.
This is a composite image of Uranus by Voyager 2 and two different observations made by Hubble — one for the ring and one for the auroras. These auroras occurred in the planet’s southern latitudes near the planet’s south magnetic pole. Like Jupiter and Saturn, hydrogen atoms excited by blasts of the solar wind are the cause for the glowing white patches seen in both photos. Credit: NASA/ESA
Earth doesn’t have a corner on auroras. Venus, Mars, Jupiter, Saturn, Uranus and Neptune have their own distinctive versions. Jupiter’s are massive and powerful; Martian auroras patchy and weak.
Auroras are caused by streams of charged particles like electrons that originate with solar winds and in the case of Jupiter, volcanic gases spewed by the moon Io. Whether solar particles or volcanic sulfur, the material gets caught in powerful magnetic fields surrounding a planet and channeled into the upper atmosphere. There, the particles interact with atmospheric gases such as oxygen or nitrogen and spectacular bursts of light result. With Jupiter, Saturn and Uranus excited hydrogen is responsible for the show.
These composite images show Uranian auroras, which scientists caught glimpses of through the Hubble in 2011. In the left image, you can clearly see how the aurora stands high above the planet’s denser atmosphere. These photos combine Hubble pictures made in UV and visible light by Hubble with photos of Uranus’ disk from the Voyager 2 and a third image of the rings from the Gemini Observatory in Hawaii and Chile. The auroras are located close to the planet’s north magnetic pole, making these northern lights. Credit: NASA, ESA, and L. Lamy (Observatory of Paris, CNRS, CNES)
Auroras on Earth, Jupiter and Saturn have been well-studied but not so on the ice-giant planet Uranus. In 2011, the Hubble Space Telescope took the first-ever image of the auroras on Uranus. Then in 2012 and 2014 a team from the Paris Observatory took a second look at the auroras in ultraviolet light using the Space Telescope Imaging Spectrograph (STIS) installed on Hubble.
From left: Auroras on Earth (southern auroral oval is seen over Antarctica), Jupiter and Saturn. In each case, the rings of permanent aurora are centered on their planets’ magnetic poles which aren’t too far from the geographic poles, unlike topsy-turvy Uranus. Credit: NASA
Two powerful bursts of solar wind traveling from the sun to Uranus stoked the most intense auroras ever observed on the planet in those years. By watching the auroras over time, the team discovered that these powerful shimmering regions rotate with the planet. They also re-discovered Uranus’ long-lost magnetic poles, which were lost shortly after their discovery by Voyager 2 in 1986 due to uncertainties in measurements and the fact that the planet’s surface is practically featureless. Imagine trying to find the north and south poles of a cue ball. Yeah, something like that.
In both photos, the auroras look like glowing dots or patchy spots. Because Uranus’ magnetic field is inclined 59° to its spin axis (remember, this is the planet that rotates on its side!) , the auroral spots appear far from the planet’s north and south geographic poles. They almost look random but of course they’re not. In 2011, the spots lie close to the planet’s north magnetic pole, and in 2012 and 2014, near the south magnetic pole — just like auroras on Earth.
An auroral display can last for hours here on the home planet, but in the case of the 2011 Uranian lights, they pulsed for just minutes before fading away.
Want to know more? Read the team’s findings in detail here.
Uranus as seen by NASA's Voyager 2. Credit: NASA/JPL
Uranus is a most unusual planet. Aside from being the seventh planet of our Solar System and the third gas giant, it is also classified sometimes as an “ice giant” (along with Neptune). This is because of its peculiar chemical composition, where water and other volatiles (i.e. ammonia, methane, and other hydrocarbons) in its atmosphere are compressed to the point where they become solid.
In addition to that, it also has a very long orbital period. Basically, it takes Uranus a little over 84 Earth years to complete a single orbit of the Sun. What this means is that a single year on Uranus lasts almost as long as a century here on Earth. On top of that, because of it axial tilt, the planet also experiences extremes of night and day during the course of a year, and some pretty interesting seasonal changes.
Orbital Period:
Uranus orbits the Sun at an average distance (semi-major axis) of 2.875 billion km (1.786 billion mi), ranging from 2.742 billion km (1.7 mi) at perihelion to 3 billion km (1.86 billion mi) at aphelion. Another way to look at it would be to say that it orbits the Sun at an average distance of 19.2184 AU (over 19 times the distance between the Earth and the Sun), and ranges from 18.33 AU to 20.11 AU.
Images of Uranus taken over a four year period using the Hubble Space Telescope. Credit: NASA/ESA/HST
The difference between its minimum and maximum distance from the Sun is 269.3 million km (167.335 mi) or 1.8 AU, which is the most pronounced of any of the Solar Planets (with the possible exception of Pluto). And with an average orbital speed of 6.8 km/s (4.225 mi/s), Uranus has an orbital period equivalent to 84.0205 Earth years. This means that a single year on Uranus lasts as long as 30,688.5 Earth days.
However, since it takes 17 hours 14 minutes 24 seconds for Uranus to rotate once on its axis (a sidereal day). And because of its immense distance from the Sun, a single solar day on Uranus is about the same. This means that a single year on Uranus lasts 42,718 Uranian solar days. And like Venus, Uranus’ rotates in the direction opposite of its orbit around the Sun (a phenomena known as retrograde rotation).
Axial Tilt:
Another interesting thing about Uranus is the extreme inclination of its axis (97.7°). Whereas all of the Solar Planets are tilted on their axes to some degree, Uranus’s extreme tilt means that the planet’s axis of rotation is approximately parallel with the plane of the Solar System. The reason for this is unknown, but it has been theorized that during the formation of the Solar System, an Earth-sized protoplanet collided with Uranus and tilted it onto its side.
A consequence of this is that when Uranus is nearing its solstice, one pole faces the Sun continuously while the other faces away – leading to a very unusual day-night cycle across the planet. At the poles, one will experience 42 Earth years of day followed by 42 years of night.
This is similar to what is experienced in the Arctic Circle and Antarctica. During the winter season near the poles, a single night will last for more than 24 hours (aka. a “Polar Night”) while during the summer, a single day will last longer than 24 hours (a “Polar Day”, or “Midnight Sun”).
Meanwhile, near the time of the equinoxes, the Sun faces Uranus’ equator and gives it a period of day-night cycles that are similar to those seen on most of the other planets. Uranus reached its most recent equinox on December 7th, 2007. During the Voyager 2 probe’s historic flyby in 1986, Uranus’s south pole was pointed almost directly at the Sun.
Seasonal Change:
Uranus’ long orbital period and extreme axial tilt also lead to some extreme seasonal variations in terms of its weather. Determining the full extent of these changes is difficult because astronomers have yet to observe Uranus for a full Uranian year. However, data obtained from the mid-20th century onward has showed regular changes in terms of brightness, temperature and microwave radiation between the solstices and equinoxes.
These changes are believed to be related to visibility in the atmosphere, where the sunlit hemisphere is thought to experience a local thickening of methane clouds which produce strong hazes. Increases in cloud formation have also been observed, with very bright cloud features being spotted in 1999, 2004, and 2005. Changes in wind speed have also been noted that appeared to be related to seasonal increases in temperature.
Close up of Uranus Dark Spot, taken by the Hubble Telescope. Credit: NASA/ESA/HST
Uranus’ “Great Dark Spot” and its smaller dark spot are also thought to be related to seasonal changes. Much like Jupiter’s Great Red Spot, this feature is a giant cloud vortex that is created by winds – which in this case are estimated to reach speeds of up to 900 km/h (560 mph). In 2006, researchers at the Space Science Institute and the University of Wisconsin observed a storm that measured 1,700 by 3,000 kilometers (1,100 miles by 1,900 miles).
Interestingly enough, while Uranus’ polar regions receive more energy on average over the course of a year than the equatorial regions, the equatorial regions have been found to be hotter than the poles. The exact cause of this remains unknown, but is certainly believed to be due to something endogenic.
Yep, Uranus is a pretty weird place! On this planet, a single year lasts almost a century, and the seasons are characterized by extreme versions of Polar Nights and Midnight Suns. And of course, an average year brings all kinds of seasonal changes, complete with extreme winds, massive storms, and thickening methane clouds.
A pretty crescent moon will be the first thing you'll see appear in the sky tonight. Look southwest shortly after sunset to spot it. Source: Stellarium
Clear night ahead? Let’s see what’s up. We’ll start close to home with the Moon, zoom out to lonely Fomalhaut 25 light years away and then return to our own Solar System to track down the 7th planet. Even before the sky is dark, you can’t miss the 4-day-old crescent Moon reclining in the southwestern sky. Watch for it to wax to a half-moon by Thursday as it circles Earth at an average speed of 2,200 mph (3,600 km/hr). That fact that it orbits Earth means that the angle the Moon makes with the sun and our planet constantly varies, the reason for its ever-changing phase.
You’ll see two and possibly three lunar “seas” tonight (Nov. 15). Only a portion of Mare Tranquilliitatis (Seas of Tranquility) is exposed. The large crater Janssen, 118 miles wide and 1.8 miles deep, is visible in binoculars. Credit: Virtual Moon Atlas / Legrande and Chevalley
With the naked eye you’ll be able to make two prominent dark patches within the crescent — Mare Crisium (Sea of Crises) and Mare Fecunditatis (Sea of Fecundity). Each is a vast, lava-flooded plain peppered with thousands of craters , most of which require a telescope to see. Not so Janssen. This large, 118-mile-wide (190-km) ring will be easy to pick out in a pair of seven to 10 power binoculars. Janssen is named for 19th century French astronomer Pierre Janssen, who was the first to see the bright yellow line of helium in the sun’s spectrum while observing August 1868 total solar eclipse.
Piscis Austrinus, the Southern Fish, has but one bright star, 1st magnitude Fomalhaut. It shines all by its lonesome in the south around 7 p.m. local time at mid-month. The star is located only 25 light years from Earth. Source: Stellarium
English scientist Norman Lockyer also observed the line later in 1868 and concluded it represented a new solar element which he named “helium” after “helios”, the Greek word for sun. Helium on Earth wouldn’t be discovered for another 10 years, making this party-balloon gas the only element first discovered off-planet!
See the fish now? Greek mythology tells us that Piscis Austrinus is the “Great Fish”, the parent of the two fish in the zodiacal constellation of Pisces the Fish. Source: Stellarium
Directing your gaze south around 7 o’clock, you’ll see a single bright star low in the southern sky. This is Fomalhaut in the dim constellation of Piscis Austrinus, the Southern Fish. The Arabic name means “mouth of the fish”. If live under a dark, light-pollution-free sky, you’ll be able to make out a loop of faint stars vaguely fish-like in form. Aside from being the only first magnitude star among the seasonal fall constellations, Fomalhaut stands out in another way — the star is ringed by a planet-forming disk of dust and rock much as our own Solar System was more than 4 billion years ago.
The planet Fomalhaut b orbits Fomalhaut inside a circumstellar disk of dust and rock, taking about 1,700 years to orbit. Brilliant Fomalhaut, represented by the small, white dot, has been masked from view, so astronomers could photograph the much fainter disk. Credit: NASA / ESA / Hubble Space Telescope
Within that disk is a new planet, Fomalhaut b, with less than twice Jupiter’s mass and enshrouded either by a cloud of dusty debris or a ring system like Saturn. Fomalhaut b has the distinction of being the first extrasolar planet ever photographed in visible light. The plodding planet takes an estimated 1,700 years to make one loop around Fomalhaut, with its distance from its parent star varying from about 50 times Earth’s distance from the sun at closest to 300 times that distance at farthest.
Shoot a diagonal across the Square of Pegasus to 4th magnitude Delta Piscium. Point your binoculars here and slide east to 4th magnitude Epsilon and 2° south to the planet Uranus shines at magnitude +5.7 and can be glimpsed with the naked eye from a dark sky site. Time shown is around 7 p.m. local time. See detailed map below. Source: Stellarium
Next, we move on to one of the more remote planets in our own solar system, Uranus. The 7th planet from the sun, Uranus reached opposition — its closest to Earth and brightest appearance for the year — only a month ago. It’s well-placed for viewing in Pisces the Fish after nightfall high in the southeastern sky below the prominent sky asterism, the Great Square of Pegasus.
Wide-field binocular view of Uranus’ travels now through next April. I’ve labeled several stars near the planet with their magnitudes, which are similar in brightness to Uranus, so you’ll know to tell them apart from the planet. The others are naked eye stars in Pisces. Source: Chris Mariott’s SkyMap
A telescope will tease out its tiny, greenish disk, but almost any pair of binoculars will easily show the planet as a star-like point of light slowly marching westward against the starry backdrop in the coming weeks. Check in every few weeks to watch it move first west, in retrograde motion, and then turn back east around Christmas. For those with 8-inch and larger telescopes who love a challenge, use this Uranian Moon Finder to track the planet’s two brightest moons, Titania and Oberon, which glimmer weakly around 14th magnitude.
We’ve barely scratched the surface of the vacuum with these offerings; they’re just a few of the many highlights of mid-November nights that also include the annual Leonid meteor shower, which peaks Tuesday and Wednesday mornings (Nov. 17-18). So much to see!
Uranus as seen by NASA's Voyager 2. Credit: NASA/JPL
Uranus, which takes its name from the Greek God of the sky, is a gas giant and the seventh planet from our Sun. It is also the third largest planet in our Solar System, ranking behind Jupiter and Saturn. Like its fellow gas giants, it has many moons, a ring system, and is primarily composed of gases that are believed to surround a solid core.
Though it can be seen with the naked eye, the realization that Uranus is a planet was a relatively recent one. Though there are indications that it was spotted several times over the course of the past two thousands years, it was not until the 18th century that it was recognized for what it was. Since that time, the full-extent of the planet’s moons, ring system, and mysterious nature have come to be known.
Discovery and Naming:
Like the five classic planets – Mercury, Venus, Mars, Jupiter and Saturn – Uranus can be seen without the aid of a telescope. But due to its dimness and slow orbit, ancient astronomers believed it to be a star. The earliest known observation was performed by Hipparchos, who recorded it as a star in his star catalog in 128 BCE – observations which were later included in Ptolemy’s Almagest.
The earliest definite sighting of Uranus took place in 1690 when English astronomer John Flamsteed – the first Astronomer Royal – spotted it at least six times and cataloged it as a star (34 Tauri). The French astronomer Pierre Lemonnier also observed it at least twelve times between the years of 1750 and 1769.
A replica of the telescope which William Herschel used to observe Uranus. Credit: Wikipedia Commons
However, it was Sir William Herschel’s observation of Uranus on March 13th, 1781, that began the process of identifying it as a planet. At the time, he reported it as a comet sighting, but then engaged in a series of observations using a telescope of his own design to measure its position relative to the stars. When he reported on it to The Royal Society, he claimed it was a comet, but implicitly compared it to a planet.
Afterwards, several astronomers began to explore the possibility that Herschel’s “comet” was in fact a planet. These included Russian astronomer Anders Johan Lexell, who was the first to compute its nearly circular orbit, which led him to conclude it was a planet after all. Berlin astronomer Johann Elert Bode, a member of the “United Astronomical Society”, concurred with this after making similar observations of its orbit.
Soon, Uranus’ status as a planet became a scientific consensus, and by 1783, Herschel himself acknowledged this to the Royal Society. In recognition of his discovery, King George III of England gave Herschel an annual stipend of £200 on condition that he move to Windsor so that the Royal Family could look through his telescopes.
In honor of his new patron, William Herschel decided to name his discovery Georgium Sidus (“George’s Star” or “Georges Planet”). Outside of Britain, this name was not popular, and alternatives were soon proposed. These included French astronomer Jerome Lalande proposing to call it Hershel in honor of its discovery, and Swedish astronomer Erik Prosperin proposing the name Neptune.
Images of Uranus captured by the Hubble Space Telescope. Image credit: NASA/ESA/Hubble
Johann Elert Bode proposed the name Uranus, the Latinized version of the Greek god of the sky, Ouranos. This name seemed appropriate, given that Saturn was named after the mythical father of Jupiter, so this new planet should be named after the mythical father of Saturn. Ultimately, Bode’s suggestion became the most widely used and became universal by 1850.
Uranus’ Size, Mass and Orbit:
With a mean radius of approximately 25,360 km, a volume of 6.833×1013 km3, and a mass of 8.68 × 1025 kg, Uranus is approximately 4 times the sizes of Earth and 63 times its volume. However, as a gas giant, its density (1.27 g/cm3) is significantly lower; hence, it is only 14.5 as massive as Earth. Its low density also means that while it is the third largest of the gas giants, it is the least massive (falling behind Neptune by 2.6 Earth masses).
The variation of Uranus’ distance from the Sun is also greater than that any other planet (not including dwarf planets or plutoids). Essentially, the gas giant’s distance from the Sun varies from 18.28 AU (2,735,118,100 km) at perihelion to 20.09 AU (3,006,224,700 km) at aphelion. At an average distance of 3 billion km from the Sun, it takes Uranus roughly 84 years (or 30,687 days) to complete a single orbit of the Sun.
The rotational period of the interior of Uranus is 17 hours, 14 minutes. As with all giant planets, its upper atmosphere experiences strong winds in the direction of rotation. At some latitudes, such as about 60 degrees south, visible features of the atmosphere move much faster, making a full rotation in as little as 14 hours.
Diameter comparison of Uranus and Earth. Approximate scale is 90 km/px. Credit: NASA
One unique feature of Uranus is that it rotates on its side. Whereas all of the Solar System’s planets are tilted on their axes to some degree, Uranus has the most extreme axial tilt of 98°. This leads to the radical seasons that the planet experiences, not to mention an unusual day-night cycle at the poles. At the equator, Uranus experiences normal days and nights; but at the poles, each experience 42 Earth years of day followed by 42 years of night.
Uranus’ Composition:
The standard model of Uranus’s structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the center, an icy mantle in the middle and an outer envelope of gaseous hydrogen and helium. Much like Jupiter and Saturn, hydrogen and helium account for the majority of the atmosphere – approximately 83% and 15% – but only a small portion of the planet’s overall mass (0.5 to 1.5 Earth masses).
The third most abundant element is methane ice (CH4), which accounts for 2.3% of its composition and which accounts for the planet’s aquamarine or cyan coloring. Trace amounts of various hydrocarbons are also found in the stratosphere of Uranus, which are thought to be produced from methane and ultraviolent radiation-induced photolysis. They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), and diacetylene (C2HC2H).
In addition, spectroscopy has uncovered carbon monoxide and carbon dioxide in Uranus’ upper atmosphere, as well as the presence icy clouds of water vapor and other volatiles, such as ammonia and hydrogen sulfide. Because of this, Uranus and Neptune are considered a distinct class of giant planet – known as “Ice Giants” – since they are composed mainly of heavier volatile substances.
The ice mantle is not in fact composed of ice in the conventional sense, but of a hot and dense fluid consisting of water, ammonia and other volatiles. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.
Diagram of the interior of Uranus. Credit: Public Domain
The core of Uranus is relatively small, with a mass of only 0.55 Earth masses and a radius that is less than 20% of the planet’s overall size. The mantle comprises its bulk, with around 13.4 Earth masses, and the upper atmosphere is relatively insubstantial, weighing about 0.5 Earth masses and extending for the last 20% of Uranus’s radius.
Uranus’s core density is estimated to be 9 g/cm3, with a pressure in the center of 8 million bars (800 GPa) and a temperature of about 5000 K (which is comparable to the surface of the Sun).
Uranus’ Atmosphere:
As with Earth, the atmosphere of Uranus is broken into layers, depending upon temperature and pressure. Like the other gas giants, the planet doesn’t have a firm surface, and scientists define the surface as the region where the atmospheric pressure exceeds one bar (the pressure found on Earth at sea level). Anything accessible to remote-sensing capability – which extends down to roughly 300 km below the 1 bar level – is also considered to be the atmosphere.
Using these references points, Uranus’ atmosphere can be divided into three layers. The first is the troposphere, between altitudes of -300 km below the surface and 50 km above it, where pressures range from 100 to 0.1 bar (10 MPa to 10 kPa). The second layer is the stratosphere, which reaches between 50 and 4000 km and experiences pressures between 0.1 and 10-10 bar (10 kPa to 10 µPa).
Temperature profile of the Uranian troposphere and lower stratosphere. Cloud and haze layers are also indicated. Credit: Wikipedia/Ruslik0
The troposphere is the densest layer in Uranus’ atmosphere. Here, the temperature ranges from 320 K (46.85 °C/116 °F) at the base (-300 km) to 53 K (-220 °C/-364 °F) at 50 km, with the upper region being the coldest in the solar system. The tropopause region is responsible for the vast majority of Uranus’s thermal infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.
Within the troposphere are layers of clouds – water clouds at the lowest pressures, with ammonium hydrosulfide clouds above them. Ammonia and hydrogen sulfide clouds come next. Finally, thin methane clouds lay on the top.
In the stratosphere, temperatures range from 53 K (-220 °C/-364 °F) at the upper level to between 800 and 850 K (527 – 577 °C/980 – 1070 °F) at the base of the thermosphere, thanks largely to heating caused by solar radiation. The stratosphere contains ethane smog, which may contribute to the planet’s dull appearance. Acetylene and methane are also present, and these hazes help warm the stratosphere.
The outermost layer, the thermosphere and corona, extend from 4,000 km to as high as 50,000 km from the surface. This region has a uniform temperature of 800-850 (577 °C/1,070 °F), although scientists are unsure as to the reason. Because the distance to Uranus from the Sun is so great, the amount of heat coming from it is insufficient to generate such high temperatures.
Like Jupiter and Saturn, Uranus’s weather follows a similar pattern where systems are broken up into bands that rotate around the planet, which are driven by internal heat rising to the upper atmosphere. As a result, winds on Uranus can reach up to 900 km/h (560 mph), creating massive storms like the one spotted by the Hubble Space Telescope in 2012. Similar to Jupiter’s Great Red Spot, this “Dark Spot” was a giant cloud vortex that measured 1,700 kilometers by 3,000 kilometers (1,100 miles by 1,900 miles).
Uranus’ Moons:
Uranus has 27 known satellites, which are divided into the categories of larger moons, inner moons, and irregular moons (similar to other gas giants). The largest moons of Uranus are, in order of size, Miranda, Ariel, Umbriel, Oberon and Titania. These moons range in diameter and mass from 472 km and 6.7 × 1019 kg for Miranda to 1578 km and 3.5 × 1021 kg for Titania. Each of these moons is particularly dark, with low bond and geometric albedos. Ariel is the brightest while Umbriel is the darkest.
A montage of Uranus’s moons. Image credit: NASA
All of the large moons of Uranus are believed to have formed in the accretion disc, which existed around Uranus for some time after its formation, or resulted from the large impact suffered by Uranus early in its history. Each one is comprised of roughly equal amounts of rock and ice, except for Miranda which is made primarily of ice.
The ice component may include ammonia and carbon dioxide, while the rocky material is believed to be composed of carbonaceous material, including organic compounds (similar to asteroids and comets). Their compositions are believed to be differentiated, with an icy mantle surrounding a rocky core.
In the case of Titania and Oberon, it is believed that liquid water oceans may exist at the core/mantle boundary. Their surfaces are also heavily cratered; but in each case, endogenic resurfacing has led to a degree of renewal of their features. Ariel appears to have the youngest surface with the fewest impact craters while Umbriel appears to be the the oldest and most cratered.
The major moons of Uranus have no discernible atmosphere. Also, because of their orbit around Uranus, they experience extreme seasonal cycles. Because Uranus orbits the Sun almost on its side, and the large moons all orbit around Uranus’ equatorial plane, the northern and southern hemispheres experience prolonged periods of daytime and nighttime (42 years at a time).
As of 2008, Uranus is known to possess 13 inner moons whose orbits lie inside that of Miranda. They are, in order of distance from the planet: Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, Perdita, Puck and Mab. Consistent with the naming of the Uranus’ larger moons, all are named after characters from Shakespearean plays.
Uranus and its system of Moons. Credit: NASA/JPL
All inner moons are intimately connected to Uranus’ ring system, which probably resulted from the fragmentation of one or several small inner moons. Puck, at 162 km, is the largest of the inner moons of Uranus – and the only one imaged by Voyager 2 in any detail – while Puck and Mab are the two outermost inner satellites of Uranus.
All inner moons are dark objects. They are made of water ice contaminated with a dark material, which is probably organic materials processed by Uranus’ radiation. The system is also chaotic and apparently unstable. Computer simulations estimate that collisions may occur, particularly between Desdemona and Cressida or Juliet within the next 100 million years.
As of 2005, Uranus is also known to have nine irregular moons, which orbit it at a distance much greater than that of Oberon. All the irregular moons are probably captured objects that were trapped by Uranus soon after its formation. They are, in order of distance from Uranus: Francisco, Caliban, Stephano, Trincutio, Sycorax, Margaret, Prospero, Setebos, and Ferdinard (once again, named for characters in Shakespearean plays).
Uranus’s irregular moons range in size from about 150 km (Sycorax) to 18 km (Trinculo). With the exception of Margaret, all circle Uranus in retrograde orbits (meaning they orbit the planet in the opposite direction of its spin).
Uranus’ Ring System:
Like Saturn and Jupiter, Uranus has a ring system. However, these rings are composed of extremely dark particles which vary in size from micrometers to a fraction of a meter – hence why they are not nearly as discernible as Saturn’s. Thirteen distinct rings are presently known, the brightest being the epsilon ring. And with the exception of two very narrow ones, these rings usually measure a few kilometers in width.
Uranus viewed in the infrared spectrum, revealing internal heating and its ring system. Credit: Lawrence Sromovsky (Univ. Wisconsin-Madison)/Keck Observatory
The rings are probably quite young, and are not believed to have formed with Uranus. The matter in the rings may once have been part of a moon (or moons) that was shattered by high-speed impacts. From numerous pieces of debris that formed as a result of those impacts, only a few particles survived, in stable zones corresponding to the locations of the present rings.
The earliest known observations of the ring system took place on March 10th, 1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink using the Kuiper Airborne Observatory. During an occultation of the star SAO 158687 (also known as HD 128598), they discerned five rings existing within a system around the planet, and observed four more later.
The rings were directly imaged when Voyager 2 passed Uranus in 1986, and the probe was able to detect two additional faint rings – bringing the number of observed rings to 11. In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings, bringing the total to 13. The largest is located twice as far from Uranus as the previously known rings, hence why they are called the “outer” ring system.
In April 2006, images of the new rings from the Keck Observatory yielded the colors of the outer rings: the outermost is blue and the other one red. In contrast, Uranus’s inner rings appear grey. One hypothesis concerning the outer ring’s blue color is that it is composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light.
Exploration:
Uranus has only been visited once by any spacecraft: NASA’s Voyager 2 space probe, which flew past the planet in 1986. On January 24th, 1986, Voyager 2 passed within 81,500 km of the surface of the planet, sending back the only close up pictures ever taken of Uranus. Voyager 2 then continued on to make a close encounter with Neptune in 1989.
These two pictures of Uranus — one in true color (left) and the other in false color — were compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. Credit: NASA/JPL
The possibility of sending the Cassini spacecraft from Saturn to Uranus was evaluated during a mission extension planning phase in 2009. However, this never came to fruition, as it would have taken about twenty years for Cassini to get to the Uranian system after departing Saturn.
In terms of future missions, multiple proposals have been made. For instance, a Uranus orbiter and probe was recommended by the 2013–2022 Planetary Science Decadal Survey published in 2011. This proposal envisaged a launch taking place between 2020–2023 and a 13-year cruise to Uranus. A New Frontiers Uranus Orbiter has been evaluated and was recommended in the study, The Case for a Uranus Orbiter. However, this mission is considered to be lower-priority than future missions to Mars and the Jovian System.
Scientists from the Mullard Space Science Laboratory in the United Kingdom have proposed a joint NASA-ESA mission to Uranus known as Uranus Pathfinder. This mission would involve launching a medium-class mission by 2022, and estimates place its cost at €470 million (~$525 million USD).
Another mission to Uranus, called Herschel Orbital Reconnaissance of the Uranian System (HORUS), was designed by the Applied Physics Laboratory of Johns Hopkins University. The proposal is for a nuclear-powered orbiter carrying a set of instruments, including an imaging camera, spectrometers and a magnetometer. The mission would launch in April 2021 and arrive at Uranus 17 years later.
Uranus, as imaged by the Hubble Space Telescope. Image credit: NASA/Hubble
In 2009, a team of planetary scientists from NASA’s Jet Propulsion Laboratory advanced possible designs for a solar-powered Uranus orbiter. The most favorable launch window for such a probe would be in August 2018, with arrival at Uranus in September 2030. The science package may include magnetometers, particle detectors and, possibly, an imaging camera.
Suffice it to say, Uranus is a hard target when it comes to exploration, and its distance has made the process of observing it recognizing it for what it was problematic in the past. And in the future, with most of our mission focused on exploring Mars, Europa, and Near-Earth Asteroids, the prospect of a mission to this region of the Solar System doesn’t seem very likely.
But budget environments change, as do scientific priorities. And with interest in the Kuiper Belt exploding thanks to the discovery of many Trans-Neptunian Objects in recent years, it is entirely possible that scientists will demand that a mission to the out solar system be mounted. If and when one occurs, it may be possible to have the probe swing by Uranus on its way out, gathering information and pictures to help advance our understanding of this “Ice Giant”.
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