Uranus (astronomy)

Article

May 25, 2022

Uranus is the seventh planet in the solar system in order of distance from the Sun, the third by diameter and the fourth by mass. Its Unicode astronomical symbol is U + 26E2 () (occasionally ♅, stylization of William Herschel's initial H). It bears the name of the Greek god of the sky Uranus (Οὐρανός in ancient Greek), father of Cronus (Saturn), in turn father of Zeus (Jupiter). Although it is also visible to the naked eye, like the other five planets known since ancient times, until the 18th century it was not recognized as such and considered a star due to its low luminosity and its particularly slow orbit and was identified as something of other than a star only on March 13, 1781 by William Herschel. A curiosity about its discovery is that it came completely unexpected: the planets visible to the naked eye (up to Saturn) had been known for millennia and no one suspected the existence of other planets, until the discovery of Herschel, who noticed that a particular dim star seemed to move. From then on, no one was more sure of the true number of planets in our solar system. The chemical composition of Uranus is similar to that of Neptune but different from that of the larger gas giants (Jupiter and Saturn). For this reason, astronomers sometimes prefer to refer to Uranus and Neptune as a separate class, the "frozen giants". The planet's atmosphere, although similar to that of Jupiter and Saturn due to the abundant presence of hydrogen and helium, contains a high proportion of "ices", such as water, ammonia and methane, along with traces of hydrocarbons . Uranus is also the coldest atmosphere in the solar system, with a minimum temperature that can drop as low as 49 K (−224.2 ° C). It has a complex, well-layered cloud structure, in which water is thought to be in the lower layers and methane in the higher ones. The interior of the planet, on the other hand, would be composed mainly of ice and rocks.One of the most unusual characteristics of the planet is the orientation of its axis of rotation. All the other planets have their own axis almost perpendicular to the plane of the orbit, while that of Uranus is almost parallel. It then rotates exposing one of its poles to the Sun for half the period of revolution, resulting in extreme seasonal phases. Furthermore, since the axis is inclined by just over 90 °, the rotation is technically retrograde: Uranus rotates in the opposite direction to that of all the other planets in the solar system (except Venus) even if, given the exceptionality of the tilt, retrograde rotation is just a minor note. The period of its revolution around the Sun is about 84 Earth years. The orbit of Uranus deviates very little from the ecliptic (it has an inclination of 0.7 °). Like the other giant planets, Uranus possesses a system of planetary rings, a magnetosphere and numerous satellites; viewed from the Earth, due to the inclination of the planet, its rings can sometimes appear as a concentric system surrounding the planet, or, as in 2007 and 2008, appear cut-off. In 1986 the Voyager 2 probe showed Uranus as a planet with no distinctive markings on its surface, without the bands and storms typical of other gas giants. However, subsequent observations from Earth showed evidence of seasonal changes and increased climate activity as the planet approached the equinox.

Remark

The planet exhibits well-documented fluctuations in brightness, determined both by physical changes in the atmosphere, and by geometric and perspective factors. The brightness of Uranus is influenced by its distance from the Sun, by the distance from the Earth and by the particular view it offers to our planet: Uranus appears slightly more