Uranus

Uranus is the seventh planet from the Sun and the first discovered with a telescope — by William Herschel in 1781. It is an ice giant, distinct from Jupiter and Saturn in that its bulk is dominated by water, methane, and ammonia ices rather than hydrogen and helium. Its most striking feature is its 97.8° axial tilt: Uranus orbits the Sun on its side, very likely the result of a giant impact early in its history. This produces extreme seasons in which each pole bakes in continuous sunlight for 21 Earth years before plunging into a 21-year night.

A small rocky core surrounded by a vast mantle of "icy" hot, dense fluid water, ammonia, and methane — under such pressure that diamond rain may be precipitating in its depths, a phenomenon recreated in laboratory shock experiments. Above the mantle lies a hydrogen-helium-methane atmosphere. Uranus is the coldest planetary atmosphere measured in the solar system, dipping to -224 °C — colder than Neptune despite being closer to the Sun.

The pale cyan colour comes from methane in the upper atmosphere absorbing red wavelengths. The disc looked nearly featureless when Voyager 2 flew past in 1986, but JWST and Hubble have since resolved bright polar caps, banded structures, and seasonal storm activity that intensifies as equinox approaches. Wind speeds reach about 900 km/h.

Only one spacecraft has ever visited: Voyager 2, which flew past in January 1986, returning the only close-up images we have, discovering 10 new moons and two new rings in a six-hour encounter. No follow-up mission has launched. The 2023 US Planetary Science Decadal Survey ranked a Uranus orbiter and probe as the highest-priority new flagship mission, with a possible launch in the early 2030s — but it has not yet been formally approved.