Uranus Is One of Nine

Uranus is one of nine planets in the same solar system as the Earth. It is the seventh planet from the Sun. It is approximately 2.87 billion kilometers from the Sun. This distance is about 19 times the distance of the Earth from the Sun. With the information that is currently available, scientists believe that Uranus is the third farthest planet from the Sun; however, additional research may reveal the presence of additional planets.

Uranus is the third largest planet in the solar system. Uranus has an equatorial diameter of 51,800 kilometers. Uranus' equatorial radius is over times bigger than Earth's. In addition, it is very massive. Uranus has a mass of 8.686e+25 kilograms. This makes Uranus approximately 15 times as massive as the Earth. Uranus is relatively dense as well, with a density of 1.29 grams per cubic centimeter. The volumetric mean radius of Uranus is 25,362 kilometers. (Williams, 2005). Uranus' mean density is 1,270 kilograms per cubic meter. (Williams, 2005).

It is common to compare other planets to earth. For example, Uranus' gravitational force is 8.97 meters per second squared, which is.905 Earth's gravitational force. (Williams, 2005). Uranus' acceleration is 8.69 meters per second squared, which is.889 Earth's acceleration. (Williams, 2005). Uranus' escape velocity is 21.3 kilometers per second, which is 1.903 Earth's escape velocity. (Williams, 2005). Uranus' bond albedo is.300, which is.980 of Earth's bond albedo. (Williams, 2005). Uranus' visual geometric albedo is.51, which is 1.390 times Earth's visual geometric albedo. (Williams, 2005). Uranus' visual magnitude is -7.19, as compared to -3.86 for Earth. (Williams, 2005). Uranus' solar irradiance is 3.71 Watts per meter squred, which is.0027 of Earth's solar irradiance. (Williams, 2005). The numbers only reveal part of the story; to people the greatest difference between Uranus and the Earth may be that Uranus is a gas planet, while Earth is not.

People like to say that Uranus was discovered in 1781 by William Herschel. However, Herschel was hardly the first person to notice the planet. "The earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri." (Arnett, 2004). The French astronomer Pierre Lemonnier "observed Uranus at least twelve times between 1750 and 1771, including on four consecutive nights." (Wikipedia Contributors, 2006). Although Herschel realized Uranus was not a star, he did not immediately realize it was a planet. Instead, he reported it as a comet on April 26, 1781. (Wikipedia Contributors, 2006). Furthermore, Uranus was the first planet to be discovered that was not known in ancient times. (Wikipedia Contributors, 2006). Herschel did not name the planet Uranus. In fact, he "named it 'the Georgium Sidus' (the Georgian Planet) in honor of his patron, the infamous (to Americans) King George III of England; others called it 'Herschel.'" (Arnett, 2004). Six years later, Herschel discovered Uranus' two largest moons, Titania and Oberon. Bode was the first to propose the name Uranus, but the name did not come into popular use until 1850. (Arnett, 2004).

The study of Uranus has been conducted primarily from Earth. Astronomers and other scientists have used telescopes and other observational devices to help determine the composition of Uranus' atmosphere, rotation, and ring structure. In addition to Earth-based observation, astronomers have photographs and other observations taken by the spacecraft Voyager 2 in January of 1986.

Voyager 2 was one of a pair of spacecraft launched to explore the planets of the outer solar system and the interplanetary environment:

Each Voyager had as its major objectives at each planet to: (1) investigate the circulation, dynamics, structure, and composition of the planet's atmosphere; (2) characterize the morphology, geology, and physical state of the satellites of the planet; (3) provide improved values for the mass, size, and shape of the planet, its satellites, and any rings; and (4) determine the magnetic field structure and characterize the composition and distribution of energetic trapped particles and plasma therein. (Bell, 2005).

Although Voyager 2 yielded tremendous information about Uranus, it is the only spacecraft to ever visit that planet.

A day on Uranus is shorter than a day on earth. "Each day on Uranus takes 17.9 Earth hours." (Enchanted Learning, 2006). However, a year on Uranus is much longer than a year on Earth. Uranus's orbit length is 84.01 Earth years. The fact that Uranus' has such a long orbit length and has only been known for a relatively short amount of time means that scientists simply have not had enough time to observe the impact of seasonal weather changes on the planet's atmospheric condition:

The seasonal changes on Earth are caused by our planet's rotational pole being slightly tilted. Consequently, the Earth's Southern and Northern hemispheres are alternately tipped toward or away from the Sun as the Earth moves around its orbit. Uranus is tilted completely over on its side, giving rise to extreme 20-year-long seasons and unusual weather. For nearly a quarter of the Uranian year, the sun shines directly over each pole, leaving the other half of the planet plunged into a long, dark, frigid winter. The Northern Hemisphere of Uranus is just now coming out of the grip of its decades-long winter. As the sunlight reaches some latitudes, it warms the atmosphere. This appears to be causing the atmosphere to come out of a frigid hibernation and stir back to life. (Savage, Neal, and Villard, 1999).

There is an additional component of Uranus weather that is interesting; Uranus appears to be warmer around its equator, despite the fact that it is on a tilted axis. Furthermore, Uranus has some meteorological components that are similar to Earth's; there is lightning on Uranus that is stronger than that on Earth, but not as strong as the lightning on Jupiter. (Space Today, 2006).

Although composed of the same elements as the atmospheres of other planets in the solar system, Uranus has a fairly unique atmosphere. "The atmosphere of Uranus is composed of 83% hydrogen, 15% helium, 2% methane and small amounts of acetylene and other hydrocarbons." (Hamilton, 2003). Like Jupiter and Saturn, Uranus' atmosphere is arranged into latitudinal bands. At the mid-latititudes, the winds blow at tremendously high-velocities, ranging from 40 to 160 meters per second. (Hamilton, 2003). There are also high atmospheric winds at the equator, where radio science experiments have found winds of about 100 meters per second. (Hamilton, 2003). The existence of these winds and clouds was hinted at by Voyager. However, "recent observations with HST show larger and more pronounced streaks." (Arnett, 2004). The atmospheric pressure is 1.2 bars. In addition, Uranus is a very cold planet. The mean cloud temperature is -193 degrees Celsius.

Uranus looks different than the other planets in the solar system. The first distinction is that Uranus appears blue-green. The blue-green tint is due to the methane in Uranus' upper atmosphere, which gives Uranus a blue-green color, because the methane and high-altitude photochemical smog absorbs red-light. Furthermore, "there may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer." (Arnett, 2004).

Uranus' color is not its only unique facet; Uranus is also tipped on its side. The tip results in 97.86 degree tilt in Uranus' axis. "Its unusual position is thought to be the result of a collision with a planet-sized body early in the solar system's history. (Hamilton, 2003). Therefore, Uranus looks different than the other planets in the solar system.

In addition, the tilt also provides some controversy for scientists studying the planet Uranus. "Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic." (Arnett, 2004). The result is that "Uranus' polar regions receive more energy input from the Sun than do its equatorial regions." (Arnett, 2004). In fact, it appears that, at least at the time that Voyager 2 passed Uranus, Uranus' south pole was directed towards the sun. However:

there's an ongoing battle over which of Uranus' poles is its north pole! Either its axial inclination is a bit over 90 degrees and its rotation is direct, or it's a bit less than 90 degrees and the rotation is retrograde. The problem is that you need to draw a dividing line *somewhere*, because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180). (Arnett, 2004).

Uranus' tilt gives it unique properties outside of its appearance. "Voyager 2 gound that one of the most striking influences of this sideways position is its effect on the tail of the magnetic field, which is itself tilted 60 degrees from the planet's axis of rotation. The magnetotail was shown to be twisted by the planet's rotation into a long corkscrew shape behind the planet." (Hamilton, 2003). While the magnetic fields of other planets appear to be created by electrical currents deep in the cores of the planets, Uranus is not believed to have such a core. In addition, the ocean that was once believed to cover Uranus' surface is no longer believed to exist. Therefore, it is uncertain what causes Uranus' magnetic fields. Furthermore, "Uranus' magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60 degrees with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus." (Arnett, 2004).

Like the other gas planets, Uranus has rings." (Arnett, 2004). Nine of Uranus' rings were discovered in 1977. However, Uranus' rings differ from those found surrounding both Jupiter and Saturn. "The outermost epsilon ring is composed mostly of ice boulders several feet across. A very tenuous distribution of fine dust also seems to be spread throughout the ring system." (Hamilton, 2003). In addition, there are several narrow or incomplete rings that surround Uranus. Uranus' rings "are composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust. Uranus' rings were discovered after Saturn's. The discovery of Uranus' rings "was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone." (Arnett, 2004).

There are 11 known rings, all very faint; the brightest is known as the Epsilon ring." (Arnett, 2004). By far the largest ring is 1986U2R, which is 2,500 kilometers wide, and approximately 38,000 kilometers from Uranus' center. Ring 6 is 1-3 kilometers wide and approximately 41,840 kilometers from the center of Uranus. Ring 5 is 2-3 kilometers wide and is approximately 42,230 kilometers from the center. Ring 4 is also 2-3 kilometers wide, and is located just past ring 5. Alpha is 7-12 kilometers wide and is 44,720 kilometers from the center. Beta is 7-12 kilometers wide and is 45,670 kilometers from the center. Eta is 0-2 kilometers wide and is 47,190 kilometers from the center. Gamma is 1-4 kilometers wide and is 47,630 kilometers from the center. Delta is 1-4 kilometers wide and is located 48,290 kilometers from the center. 1986U1R is 1-2 kilometers wide and is located 48,290 kilometers from the center. Epsilon, the most visible of the rings from the surface of the Earth, is 20-100 kilometers wide and is located approximately 51,140 kilometers from the center of Uranus. Epsilon is the probably the most visible because it is the outer ring and is much larger than most of the other rings.

Uranus also has shepherd satellites for some of its rings. The moons Cordelia and Ophelia are on either side of Uranus' epsilon ring. In fact, Uranus has a very high number of moons and other satellites. In fact, Uranus has 27 moons, 21 that have been named and six unnamed ones. Unlike the moons of other planets, which have been named after figures in classical mythology, "Uranus' moons take their names from the writings of Shakespeare and Pope." (Arnett, 2004). Most of the moons have circular orbits, although the outer moons have more elliptical orbits.