URANUS

With an equatorial diameter of 51 118  km (at  the  1  bar atmospheric pressure level),Uranus is the seventh farthest and the third largest of the planets  in  the  solar  system,  slightly  larger  than  the more  distant  Neptune.  However, because of its lower density, Uranus ranks fourth among solar system planets in mass at 14.54 terrestrial masses. Its characteristic blue–green appearance is due to a layer of clouds of methane ice in its upper atmosphere that is not present in the atmospheres of Jupiter and Saturn. This cloud layer and the overlying methane gas preferentially absorb the red portion of sunlight, resulting in the distinctive color. Prior to the flyby of Uranus by Voyager 2 in 1989, little was known about the rotational or other physical characteristics of Uranus.

Uranus has a ring system and many satellites. These rings were first detected from observations of stellar occultations as the narrow rings passed in front of distant stars, causing the observed light to blink out temporarily as viewed from Earth. Nine narrow rings were detected in this manner. Voyager 2 detected a tenth narrow ring, a broad diffuse ring closer to the planet, and tenuous dust rings scattered among the rest. The Uranian system has a unique configuration among the other planets of the solar system because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. As such, south and north poles lie where most other planets have their equators. Uranus has 27 known natural satellites. It is interesting to note that the names for these satellites are chosen from characters from the works of Shakespeare and Alexander Pope. The five most significant satellites are Miranda, Ariel, Umbriel, Titania and Oberon.

Uranus is the first of the non-naked-eye planets to be discovered. Observations of the closer planets (Mercury through Saturn) date back to antiquity, but Uranus was discovered by William Herschel in March of 1781 using his home-made telescope in the back yard of his home in Bath, England.

Like its larger counterparts (Jupiter, Saturn and Neptune), Uranus is composed primarily of the light elements hydrogen and helium. Even the somewhat heavier compounds, confirmed methane (CH4) and suspected ammonia (NH3) and water (H2O), are composed largely of hydrogen. This dominance of light elements is reflected in the low densities of the gas giant planets and in the absence of distinct liquid or solid surfaces in the interiors of these planets.  However, in contrast to Jupiter and Saturn, methane,  ammonia, water  and  the heavier  core materials  are  thought  to  constitute  a  larger  total mass than  the  overlying  hydrogen  and  helium.