Uranus: The Seventh Planet of the Solar System

Our solar system maintains a unique place in the cosmos due to its placement, composition, and overall structure. One area that deserves particular attention is the planet Uranus. As the seventh planet from the Sun, this Gas Giant plays a unique role and purpose in the overall function of our solar system, at large. This article provides an in-depth analysis of Uranus that focuses directly on its properties, composition, and current research that surrounds this fascinating object. After reading the material that follows, it is this author’s hope that a deeper understanding (and appreciation) of this remarkable planet will be gained by his readers.

Orbital Semimajor Axis: 19.19 Astronomical Units (2,871 Million Kilometers)

Orbital Eccentricity: 0.047

Perihelion: 18.29 Astronomical Units (2,736 Million Kilometers)

Aphelion: 20.10 Astronomical Units (3,006 Million Kilometers)

Mean/Average Orbital Speed: 6.80 Kilometers Per Second

Sidereal Orbital Period: 83.75 Years (Tropical)

Synodic Orbital Period: 369.66 Days (Solar)

Orbital Inclination to the Ecliptic: 0.77 Degrees

Greatest Angular Diameter (As Viewed From Earth): 4.1”

Overall Mass: 8.68 x 1025 Kilograms (14.54 of Earth’s Overall Mass, if Earth = 1)

Equatorial Radius: 25,559 Kilometers (4.01 of Earth’s Equatorial Radius, if Earth = 1)

Mean/Average Density: 1,271 Kilograms Per Meter Cubed (0.230 of Earth’s Average Density, if Earth = 1)

Surface Gravity: 8.87 Meters Per Second Squared (0.91 of Earth’s Surface Gravity, if Earth = 1)

Escape Speed/Velocity: 21.3 Kilometers Per Second

Sidereal Rotation Period: -0.72 Solar Days (Indicative of Retrograde Rotation)

Axial Tilt: 97.92 Degrees

Surface Magnetic Field: 0.74 of Earth’s Surface Magnetic Field, Assuming Earth = 1)

Magnetic Axis Tilt (Relative to Rotation Axis): 58.6 Degrees

Mean/Average Surface Temperature: 58 Kelvins (-355.27 Degrees Fahrenheit)

Number of Moons: 27 in Total

The planet Uranus was first discovered in 1781 by William Herschel. At first glance, Herschel believed that he had discovered a comet-like object. Upon later inspection, however, he was able to confirm that the dim object was, indeed, a planet. Herschel proposed that the planet should be named “Georgian Sidus” after King George III. After some debate, however, the name “Uranus” was chosen instead by astronomer Johann Bode; a name that derived from the Greek god, “Ouranos.” As the Greek deity of the sky, the name “Uranus” proved highly relevant and appropriate (and an acceptable name for the international community as well).

Although Herschel is credited with the discovery of Uranus, astronomers and observers were aware of the object during ancient times since it is visible to the naked eye. Due to its dimness and relatively slow orbit though, Uranus was believed to be a star in the night sky; escaping planet-classification until the Eighteenth Century. One of the earliest observations of Uranus was believed to have been recorded by Hipparchos around 128 BC, who incorporated the object into Ptolemy’s “Almagest.” John Flamsteed (1690) is also credited with observing the planet on six different occasions, recording it as 34 Tauri.

Uranus maintains a relatively rapid rotation rate, and completes a full cycle on its axis (on its side) every seventeen hours and fourteen minutes. Due to its tremendous distance from the Sun, however, it takes the planet nearly 84 Earth years to complete one full trip around the Sun. Since its discovery in 1781, the planet has completed two full cycles around the Sun (and will complete its third full orbit in 2033).

In addition to spinning on its side, Uranus also rotates around the Sun in a retrograde direction (opposite the direction that Earth and most of the other planets orbit). Combined with its strange orientation to the Sun, Uranus’s rotation “produces some extreme seasonal effects,” with each of its poles experiencing nearly 42 years of complete darkness or total light every cycle (McMillan, 341).

It remains unclear why Uranus rotates in this strange manner. However, it has been theorized by the scientific community that a catastrophic event in Uranus’s distant past may be to blame. Collisions with other planets, comets, or asteroids may have distorted Uranus’s tilt (relative to the Sun). At this time though, there is no physical evidence to substantiate such a claim.

As of 2019, it is believed that Uranus possesses thirteen known rings. Most of these rings are extremely narrow, making them difficult to spot from Earth. Scientists believe that the ring-system is a relatively new feature of the planet, and may have resulted from meteor or comet impacts on Uranus’s moons.

Uranus’s first nine rings were originally discovered in 1977, with Voyager 2 uncovering an additional two rings during its flyby in 1986. However, it wasn’t until the advent of the Hubble Space Telescope that two additional rings were found around Uranus, bringing the total number of rings to thirteen. As with most planetary ring systems, scientists hypothesize that the rings are composed of dust, rocky particles, and ice.

Uranus’s eleven interior rings stretch around the planet at a distance ranging from 24,000 to 32,000 miles from the planet, with each ring maintaining an average width of 1,000 to 1,500 miles.

Astronomers and scientists, alike, often refer to Uranus as an “ice giant” since it is believed that the planet’s iron core is surrounded by a mantle composed of icy materials. It is currently believed that Uranus is the second least dense planet in our solar system; second only to the planet Saturn. With a mean density of only 1.27 kilograms per meter³, it is only slightly denser than Saturn which maintains an overall mean density of 0.687 kilograms per meter³. As a result, an individual would experience only 89 percent of gravity’s force on Uranus.

Core

It is unknown what Uranus’s core is composed of (due to the lack of empirical observation). However, scientists theorize that the planet’s core may resemble the internal structure of both Jupiter and Saturn; a rocky and iron-based core that is approximately the size of Earth. Surrounding this core is a hypothetical “slush” of molecular hydrogen and ammonia that is enveloped by “thick layers of water clouds” in Uranus’s lower atmosphere (McMillan, 345).

Atmosphere

After careful analysis of the planet, scientists believe that Uranus’s atmosphere is composed primarily of hydrogen and helium with trace amounts of methane gas. Similar to its impact on Neptune, it is hypothesized that the presence of methane plays a vital role in Uranus’s coloration. As methane helps to absorb red light in the upper atmosphere, the planet gives way to a light shade of blue that is unique to the solar system. Spectroscopic studies indicate that the planet contains roughly 2-percent methane in its upper atmosphere, compared to 3-percent on Neptune. This helps to explain why Uranus is slightly lighter in coloration than Neptune.

Despite having large quantities of ammonia in its interior, Uranus is also unique within the realm of Jovian planets since this element is almost non-existent in its lower and upper atmospheres. This is unusual since ammonia plays a vital role in the chemical compositions of both Jupiter and Saturn, and is a common element to Gas Giants. To explain this discrepancy, scientists believe that planetary temperatures are to blame for the lack of ammonia in Uranus’s atmosphere. At 70 Kelvins, ammonia is known to freeze into ice crystals; thus, preventing the chemical from reaching the outer layers of Uranus’s atmosphere like the Gas Giants that precede the planet. As one might expect, these low temperatures are a direct result of Uranus’s tremendous distance from the Sun.

Internal Heat

Unlike most of the other gas-giants, Uranus maintains an internal temperature that is remarkably low (also known as a low thermal flux). Scientists are uncertain why this is the case since Neptune (a planet roughly the same size and same composition of Uranus) produces nearly 2.61 times the thermal energy of Uranus. In fact, Uranus maintains the lowest recorded temperature in our solar system at a reading of -224.2 degrees Celsius (or -371.5 degrees Fahrenheit).

In more recent years, scientists have speculated that Uranus’s low internal temperature may have been caused by a supermassive impact in its distant past. In this hypothesis, scientists believe that an impact from a comet, meteor, or asteroid may have dislodged much of the planet’s primordial heat, leaving it with a depleted core temperature. This theory appears logical, given the planet axial tilt (indicative of a large impact as well). Other theories for the planet’s low temperature include the hypothesis that Uranus possesses a barrier that may prevent heat from its core from reaching the surface of the planet. This would, in turn, greatly inhibit the upward movement of heat across the planet.

Although Uranus possesses twenty-seven moons, each of these natural satellites are extremely small. In fact, all of Uranus’s moons combined (in terms of mass) are less than half the mass of Triton (Neptune’s largest moon). The largest of Uranus’s moons are Miranda, Umbriel, Ariel, Oberon, and Titania. Scientists believe that most of these moons are composed primarily of rock and ice, as well as carbon dioxide and ammonia. Scientists also speculate that Titania and Oberon may possess oceans of liquid water between their cores and mantles; an indicator of possible lifeforms in the region.

Radiation Darkening

A unique feature of Uranus’s moons is their remarkably dark features. Practically invisible to the untrained eye, it is unclear why each of these moons lack reflective qualities. One theory suggests that their icy surfaces may be to blame for this issue, as the combination of ice and dust impedes the reflection of light. Another theory suggests that the “planetary environment in the vicinity of Uranus and Neptune contains more small ‘sooty’ particles than do the parts of the solar system that are closer to the Sun” (McMillan, 347). The most accepted hypothesis, however, lies with the theory of “radiation darkening.” This model argues that the effects of radiation and high-energy particles on the molecules of each moon’s surface has led to a series of chemical reactions that have slowly built up “a layer of dark, organic material” (McMillan, 347). If true, this would not only explain the darker coloration of Uranus’s moons, but also its darkened ring system as well.

Before Voyager 2 made its remarkable flyby around Uranus in 1986, little was known about Uranus’s magnetosphere due to its tremendous distance from the Sun. Prior to 1986, scientists believed that the planet’s magnetic field would correspond directly with solar winds. Direct observation by Voyager 2 quickly countered this hypothesis, however, as the spacecraft’s instruments recorded a magnetic field that was both peculiar and unique within the solar system. Voyager 2 discovered that the planet’s magnetic field does not originate from its geometric center due to its unusual rotation.

The composition of Uranus’s core and water-ammonia oceans (similar to Neptune) may also play a role in this unique feature, as the planet’s magnetic field has been observed to be highly asymmetrical. This indicates that Uranus’s magnetosphere is probably generated within the depths of its soup-like oceans, rather than at its core; a highly unique trait for planetary systems.

Weather patterns on Uranus are very similar to Jupiter and Saturn. Storms rotate around the planet in separate bands, and are known to generate wind speeds in excess of 900 kilometers per hour. Various dark spots in Uranus’s clouds have also been recorded by scientists in recent years. These features are believed to be vortexes and are often large enough to engulf much of the United States (for comparative purposes).

Studying weather patterns on Uranus is often described as “difficult” by the scientific community for a number of reasons. Unlike Earth, where storms can be observed from space via satellite imagery, storm systems are much more difficult to spot on Uranus due to its thick upper atmosphere. This problem is made worse by the fact that Uranus’s cold surface temperatures causes storm systems to form at low-lying levels (close to the planet’s mantle). For these reasons, little is known about Uranus’s storm systems, aside from their tremendous strength and size.

Scientists believe that Uranus formed out of the "presolar nebula" that contained both gas and dust in the early stages of our solar system's development. The gases, which were primarily hydrogen and helium, are believed to have aided in the development of our Sun, while dust particles likely helped form the first protoplanets.

As these planets continued to grow in size, scientists hypothesize that some of the planets (such as Uranus) may have had enough gravitational pull (due to their size) to collect parts of the nebula's gases. Following the "Nice Model" of planetary migration, it is believed that planets such as Uranus formed relatively close to the Sun, but moved outwards shortly after their formation. After drifting in the vastness of space, astronomers theorize that the Sun’s gravitational pull would have eventually “captured” Uranus; thus, locking it into orbit over a period of time.

To date, Voyager 2 remains the only spacecraft to have visited Uranus. Some scientists in 2009 were optimistic about sending the Cassini spacecraft to Uranus after its mission to Saturn. However, this option was scrubbed in favor of crashing the spacecraft on Saturn to gain a better view of the planet. Although no missions are currently in the works for a mission to Uranus, the Planetary Science Decadal Survey in 2011 hinted at the possibility of such a mission between 2020-2023. Using the Pioneer Venus Multiprobe, a probe could easily descend throughout the first layers of Uranus’s atmosphere. Support for these missions, however, have been difficult to muster in the scientific community due to the extensive time it would take for probes to reach Uranus (nearly twelve to thirteen years with current technology).

Fun Fact #1: Due to its orbital patterns, each of Uranus’s poles receive 42 years of direct sunlight or darkness, depending on its location around the Sun.

Fun Fact #2: Only one spacecraft has approached Uranus (Voyager 2). The spacecraft swept past the planet at a distance of 81,500 kilometers, returning numerous up-close images of not only the planet, but also its moons and ring-system.

Fun Fact #3: The largest moon orbiting Uranus is known as Titania. It is the eighth largest moon in the solar system. So far, 27 moons have been discovered around Uranus with the majority being named after characters from William Shakespeare and Alexander Pope.

Fun Fact #4: Wind speeds on Uranus can reach up to 900 kilometers per hour (or approximately 560 miles per hour).

Fun Fact #5: Uranus gets is pale blue color from its upper atmosphere, which is composed primarily of water, ammonia, and methane-based ice crystals.

Fun Fact #6: The planet Uranus plays a key role in astrology, and is the ruling planet of Aquarius.

Fun Fact #7: Shortly after the discovery of Uranus, German chemist, Martin Heinrich Klaproth, discovered a new chemical element in 1789; an element he later named Uranium, in honor of the discovery of Uranus only a few years prior.

Fun Fact #8: Uranus is approximately four times wider than Earth, in terms of size. This concept can be illustrated with the difference between an apple and basketball.

Fun Fact #9: Currently, scientists do not believe that Uranus is capable of supporting any form of life on its surface, due to its extreme conditions.

Quote #1: “Although Uranus and Neptune are superficially twin planets, they are different enough to remind us – as do Venus and Earth – that we still have a lot to learn about the mix of natural laws and historical accidents that formed the planets and fashioned their destinies.” – Timothy Ferris

Quote #2: “There are amateurs who have seen that one of Uranus’s poles is brighter than the other, or who have seen cloud formations on the planet. For all we know, interesting things are happening there all the time.” – Heidi Hammel

Quote #3: “Even after years of observing, a new picture of Uranus from Keck Observatory can stop me in my tracks and make me say, ‘Wow!’” – Heidi Hammel

In closing, the planet Uranus is a remarkable component to our solar system and offers many exciting opportunities for scientific expeditions in the near (and distant) future. Further exploration of the planet may offer important clues to the formation of both our solar system and universe, at large. Only time will tell what new facts and figures can be learned about this fascinating planet, its origins, and role that it plays in the functionality of our solar system.

Slawson, Larry. "Neptune: The Eighth Planet of the Solar System." HubPages. 2020.

Steve McMillan and Eric Chaisson, Astronomy Today. San Francisco, California: Pearson Education, 2008.

“Uranus.” NASA Science. NASA. Accessed January 10, 2020. https://solarsystem.nasa.gov/planets/uranus/overview/.