October 28, 2021
Uranus is the seventh planet in the Solar System in order of distance from the Sun. It orbits around it at a distance of about 19.2 astronomical units (2.87 billion kilometers), with a period of revolution of 84.05 Earth years. It is the fourth most massive planet in the Solar System and the third largest in size. It is the first planet discovered in modern times with a telescope and not known since ancient times. Although it is visible to the naked eye, its planetary character is then not identified due to its very faint glow and its apparent movement in the sky very slowly. William Herschel observed it for the first time on March 13, 1781 and the confirmation that it was a planet and not a comet was made during the following months. Like Jupiter and Saturn, Uranus' atmosphere is composed mainly of hydrogen and helium with traces of hydrocarbons. However, like Neptune, it contains a higher proportion of "ice" in the physical sense, that is to say of volatile substances such as water, ammonia and methane, while the interior of the planet is mainly composed of ice and rocks, hence their name "ice giants". In addition, methane is the main responsible for the aquamarine hue of the planet. Its planetary atmosphere is the coldest in the Solar System, with a minimum temperature of 49 K (−224 ° C) at the tropopause, and exhibits a cloudy layered structure. Like the other giant planets, Uranus has a ring system and many natural satellites: it is known to have 13 narrow rings and 27 moons. Unique case in the Solar System, its axis of rotation is practically in its plane of revolution around the Sun - giving the impression that it "rolls" in its orbit, at least at a certain moment of its revolution - and its North poles and South are therefore found where most of the other planets have their equator. The planet is provided with a magnetosphere in the shape of a corkscrew due to this inclination of the axis. The distance of the planet from the Earth giving it a very small apparent size, its study is difficult with telescopes located on the Earth. Uranus is visited only once during the Voyager 2 mission, which overflows it on January 24, 1986. The images from the space probe then show a planet almost without relief in visible light, without the bands of clouds or the associated storms. to the other giant planets. The advent of the Hubble Space Telescope and large adaptive-optic ground-based telescopes then allows for additional detailed observations revealing seasonal change, increased weather activity, and winds of around 250 m / s as Uranus approaches. of its equinox in 2007. Its name comes from Ouranos, Greek deity of the sky (Uranus in Roman mythology), father of Cronos (Saturn) and grandfather of Zeus (Jupiter).