SOLAR SYSTEM FAMILY-FACTS ABOUT URANUS, THE TILTED PLANET

 Our solar system is undoubtedly a unique place as each and every planet is totally different from the rest. Here we have Earth, which is in itself an alien world, and there is Jupiter, which could have transformed into a star if conditions were otherwise. Finally, there is Uranus who at some point in its past got kicked in the a** so hard that its rotational axis acquired a tilt of 97.77 degrees to the plane of the ecliptic. In absence of further collisions, this planet decided to settle as it is, so that we could feast our eyes upon a cyan coloured and a tilted planet.  

Discovery & Naming: Without the help of a telescope or a pair of binoculars, it is particularly difficult to isolate Uranus from the background stars. Moreover, due to its slow pace across the night sky, for a long time, it was mistaken as a star instead of a planet. On the night of 13th March 1781, although Sir William Herschel observed the star-like object, but he thought that he had observed a comet. Other astronomers made their independent observations and by 1783, everyone including Sir William Herschel accepted the newly discovered object as a planet. When Herschel was asked to provide a name for his discovery, he suggested Georgium Sidus(i.e., George's star or the Georgian planet) in honour of King George III. But the name did not gain much popularity among the astronomers and other alternatives were considered. Finally, it was decided to be named after the Greek God of the sky Ouranos(Latin-Uranus) so as not to sound different from Jupiter and Saturn, which were already named after the Greek Gods.

Basic Facts: The seventh planet from the Sun is also the third largest planet of our solar system. Although, its composition is similar to that of the gas giants, Uranus and for that matter, Neptune have been classified as ice giants. Ice giants are similar to gas giants, but they contain more ice than gas. The atmosphere takes a bluish hue and it is one of the coldest planetary atmospheres in the Solar System, with temperatures going as low as 49K(-224℃). 

The full disc of Uranus/Image Credits: NASA/JPL-Caltech, Public domain, via Wikimedia Commons

Planetary Characteristics:

• Orbital & Rotational Characteristics: Uranus orbits the Sun at an average distance of 19 AU, which is equal to 3 billion km, and has an orbital period of 84 Earth years. An orbital eccentricity of 0.046 implies that the perihelion and the aphelion distance differ by 1.8 AU which is also the highest among all the planets of our solar system. The planet as a whole undergoes differential rotation about its axis, as the interior rotates with a period of 17 hours, 14 minutes, whereas layers of the outer atmosphere rotates within 14 hours. One notable feature of this planet is its axial tilt of 97.77° and its axis is nearly parallel with the plane of the solar system. Therefore during solstice, one pole faces the sun continuously while the other faces away and consequently, each pole gets about 42 years of continuous sunlight and 42 years of darkness. This unique axial tilt might have been the result of a violent collision that occurred 3-4 billion years ago with a planetary mass larger than Earth.

• Internal Structure & Composition: With a mean radius of 25,362 km, Uranus is nearly four times wider than Earth which has a mean radius of 6371 km. Its mass is about 14.5 Earth masses and a density of 1.27g/cc makes it the second least dense planet in our solar system. Theoretical modelling indicates an interior differentiated into a small silicate or iron-nickel core, surrounded by an icy mantle and outer layers of hydrogen and helium gas. Almost 80% of the planetary mass is concentrated into various ices, such as that of water, methane and ammonia. It has also been proposed that deep inside extreme pressure and temperature break up the methane molecules, and as a result, the carbon atoms eventually condense into crystals of diamond. Hence, the liquid layers of water and ammonia experiences a rain of diamonds. Near the core, temperatures can easily exceed 5000K with pressures exceeding 800GPa.

• Atmosphere: As there is no solid surface to land on for a hypothetical explorer, with increasing depth the gaseous atmospheric layers gradually merges with the fluid interior. Nevertheless, the atmosphere has been divided to three distinct layers-the troposphere, stratosphere and the thermosphere. These regions are some of the coldest places in our solar system and temperatures can go as low as 49 K. The outer atmosphere is primarily composed of hydrogen and helium, with an abundance of methane and traces of ammonia, water, and some hydrocarbons. The methane gives Uranus its aquamarine or cyan color. Another interesting feature of the Uranian atmosphere is the presence of a corona which extends as far as 50,000 km from the planet's surface. Uranus undergoes extreme seasonal variations due of its axial tilt and an absence of internal heating mechanism further intensifies the effect. Just like the gas giants, Uranus have also got cloud bands and vortices, where wind speeds can become as high as 900km/h. The cloud bands are however, not as extreme and prominent like those found in the gas giants. Near the poles, the winds blow in a direction parallel to the direction of planetary rotation, whereas at the equatorial regions the direction is reversed. 

 Aurorae and the thin ring system/Image Credits: ESA/Hubble, CC BY 4.0, via Wikimedia Commons

• Magnetosphere & Magnetic Field: Uranus has a peculiar magnetic field and thereby an equally weird magnetosphere. In general, for all the planets their magnetic poles are aligned with the axis of rotation, but for Uranus, the magnetic axis is tilted 60 degrees from the rotational axis. Again, as the magnetic field does not originate from the planet's geometric centre, the magnetosphere is asymmetric and the magnetic field strength varies from place to place. Scientists speculate that this tilted magnetic field could be the result of the circulation of a fluid water-ammonia ocean instead of the typical magnetic field generated inside the core of the terrestrial or the gas giant planets. However, the magnetosphere is strong enough to deflect solar wind plasma, and the magneto-tail extends for millions of kilometers beyond the planet. Uranus also generate aurorae, although they are not as significant compared to Jupiter or Saturn. 

• Ring System: Uranus has a ring system which s not quite extensive like Saturn, but is similar to that of Jupiter. The set of thirteen distinct rings extends from 38,000-98,000 km from the planet's centre. These rings are comparatively young, and probably formed from the collision of a large number of moons that existed around the planet at some point in the past. The innermost rings are relatively thin and dark and might be composed of icy or organic materials, while the outer two sets are more dusty, with one being red and the other, blue in colour.  

The rings of Uranus/Image Credits: Hubble Space Telescope - NASA Marshall Space Flight Center, Public domain, via Wikimedia Commons

• Natural Satellites: Uranus has 27 known moons, of which the major ones are believed to have originated from the accretion of gas and dust in the sub-nebula that surrounded the planet. The moons are divided into three major groups viz., the thirteen inner moons, the five major moons which include Miranda, Ariel, Titania, Umbriel, and Oberon, and finally the outer irregular moons. Not much is known about these distant worlds as they have only been explored by the Voyager 2 spacecraft. Nevertheless, the moons have been found to be composed of ice(ammonia, water, carbon dioxide) and rock in different proportions. Their surfaces are heavily cratered and are relatively darker in appearance. Miranda(diameter=472km) orbits closest to the planet at an average distance of 129,000 km from the latter's surface in about 34 hours, while Ariel(diameter=1156 km), the second closest orbits at a distance of 190,000 km in about 2.5 Earth days. Umbriel(diameter=1168km), Titania(diameter=1576km) and Oberon(diameter=1522km) takes about 4.1, 8.7, 13.5 Earth days to orbit Uranus at average distances of 266000, 435900 and 535500 km respectively. Some of these worlds are devoid of any significant atmospheres, many have a differentiated interior, while others possess surface features which are a product of past geological activity. 

Life & Biological Considerations: Life as we know it may have no chance at all in surviving the harsh conditions out there. Even if life exists, it has to be completely different from ours. 

Earth, Uranus, and a different shade of blue/Image Credits: Orange-kun (old version user: Brian0918), Public domain, via Wikimedia Commons

Explorations: So far Uranus has only been explored by the Voyager 2 spacecraft, launched by NASA on 1977, when the spacecraft entered the Uranian system on 1986. As it made the closest approach to this distant planet, the Voyager craft discovered 10 additional new moons, two more rings and made detailed observations of the planet's atmosphere, magnetic field, and surface features of the moons. 

Some Quick Facts:

References: 

1. https://en.wikipedia.org/wiki/Uranus
   
2. https://solarsystem.nasa.gov/planets/uranus/overview/