Space & the universe

A planet tipped on its side, and nobody knows why it's cold

Uranus is the seventh planet from the Sun, a pale cyan ice giant made mostly of water, ammonia and methane in a hot, dense fluid state rather than true ice. It has the coldest recorded temperature of any planet, and its axis is tipped over so far that it essentially rolls around the Sun on its side, giving each pole 42 years of continuous light followed by 42 years of darkness. William Herschel spotted it in 1781 while searching for stellar parallax, at first insisting he had found a comet, and it took astronomers decades to settle on naming it after the Greek sky god rather than after King George III. Only one spacecraft, Voyager 2, has ever visited, flying past in 1986 and gathering nearly everything we still know about the place, though a dedicated orbiter mission is now a stated priority and a Chinese probe is slated to fly by in the 2040s.

Inside, Uranus is thought to have a small rocky core wrapped in a thick slushy mantle of water, ammonia and methane, topped by a hydrogen-helium atmosphere, though scientists admit other internal arrangements could fit the data equally well. Extreme pressure may crush methane into diamond rain sinking through the mantle, an effect also suspected on the other giant planets. Oddly, Uranus gives off almost none of the leftover internal heat that its near-twin Neptune radiates in abundance, and nobody has settled on why; theories range from a lingering scar of the giant impact that knocked the planet onto its side to an insulating layer deep inside. Its atmosphere is comparatively bland and cloud-poor, though seasonal changes tied to its 84-year orbit bring bursts of storms, record wind speeds, and shifting bright polar collars as the decades pass. Its magnetic field is similarly strange, tilted way off from the rotation axis and offset from the planet's centre, producing a lopsided magnetosphere that a 2024 reanalysis suggests may have been distorted by unusually strong solar wind during the one flyby we've had.

Uranus carries a retinue of 29 known moons, named after Shakespeare and Pope characters, the largest five being Miranda, Ariel, Umbriel, Titania and Oberon, all icy-rocky bodies far less massive collectively than Neptune's single large moon Triton. Miranda in particular shows a bizarre jumbled surface with deep canyons, likely the result of past tidal heating from an old orbital resonance. Surrounding the planet are thirteen known dark, narrow rings, first properly detected in 1977 when a star's light flickered oddly during an occultation, with two additional faraway rings turning up decades later via Hubble. The rings and moons are darkened by charged particles trapped in the magnetosphere, and the whole system remains a subject of ongoing debate, from the origin of its tilt to the reason for its chilly interior.

Its rings are among the darkest objects known, reflecting only about 2% of the light that hits them avel possibly darkened by charged particles from the planet's lopsided magnetosphere.

Read the whole of Uranus on Wikipedia

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