Uranus

Discovery and orbit

Uranus was discovered on 13 March 1781 by William Herschel in Bath, England. It was the first planet to be discovered with a telescope. Herschel initially reported the object as a possible comet or star before its planetary nature was confirmed by later observations and orbital calculations.

Uranus orbits the Sun once every approximately 84.02 Earth years (about 30,687 Earth days) at an average distance of roughly 2.9 billion kilometers, or about 19 astronomical units.

Atmosphere and climate

Uranus's atmosphere is dominated by hydrogen (about 83% by volume) and helium (about 15%), with methane making up about 2.3% as the third most abundant gas. Trace constituents include hydrogen deuteride and inferred traces of hydrogen sulfide, water, and ammonia at depth. Methane is strongly enriched relative to the Sun and absorbs red light, giving the planet its blue-green appearance.

The atmosphere is extremely cold. Temperatures at the tropopause range from about 49 to 57 K depending on latitude, and the 49 K (−224 °C) measurement is the coldest planetary atmospheric temperature recorded in the Solar System. Above the tropopause, temperatures rise through the stratosphere to roughly 800–850 K in a near-uniform thermosphere extending thousands of kilometers above the 1-bar level.

Clouds are organized in layers by pressure. A thin methane ice cloud deck sits near 1–2 bar, with deeper layers of hydrogen sulfide and ammonia clouds around 3–10 bar, ammonium hydrosulfide near 20–40 bar, and water clouds expected at roughly 50–100 bar. Higher up, ultraviolet sunlight breaks down methane to form a photochemical haze above the main clouds.

Moons

Uranus has 28 known moons according to NASA. Its five major moons are Miranda, Ariel, Umbriel, Titania, and Oberon. They are often called the literary moons because they are named mainly for characters from Shakespeare, with two names drawn from Alexander Pope.

Miranda is the smallest of the major moons and has a highly unusual, jumbled surface of craters, scarps, and deep rift valleys, suggesting a history of disruption and reformation. Titania is the largest Uranian moon, heavily cratered with long valleys and large trenches. Oberon, the second-largest, is heavily cratered and geologically inactive, with dark material in many crater floors and at least one tall mountain.

Voyager 2 flyby and future exploration

Voyager 2 made the only close visit to Uranus to date, with closest approach on 24 January 1986 at about 81,500 km above the cloud tops. The spacecraft returned more than 7,000 images and discovered 11 new moons and 2 new rings during an encounter window spanning late 1985 to February 1986. A planetary rotation period of 17 hours 14 minutes was derived from its radio and magnetic measurements.

Voyager 2's magnetometer revealed an unusual magnetic field: a dipole tilted about 59° from the rotation axis and offset from the planet's center by roughly one-third of Uranus's radius, producing a highly asymmetric, time-variable magnetosphere. Its electron radiation belts were second in intensity only to Jupiter's, yet very little cold plasma was detected, giving the appearance of a near 'vacuum magnetosphere.'

Modern re-analyses published in 2024 show that Uranus was struck by an unusual solar-wind event in the days before the flyby that strongly compressed its magnetosphere. The subsolar magnetopause was pushed inward from about 28 Uranus radii to roughly 17, a state estimated to occur only about 4–5% of the time. This compression likely intensified the radiation belts and depleted ambient plasma, meaning Voyager observed an atypical configuration; the average subsolar magnetopause is estimated nearer 22 Uranus radii.

The 2023 Planetary Science and Astrobiology Decadal Survey identifies a Uranus Orbiter and Probe as the highest-priority flagship mission for the coming decade. The concept would place an orbiter around Uranus for multi-year coverage and deliver an atmospheric entry probe, carrying magnetometers, plasma instruments, and auroral imagers to characterize the tilted, offset magnetic field and address the limitations of the single Voyager flyby.