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Solar System Series

Uranus

The sideways planet: the ice giant that rolls around the Sun

Author: Juan Carlos Euyoque Aguilera · October 7, 2026 · CC BY 4.0

Image of Uranus (NASA/JPL, public domain)
NASA/JPL illustration, public domain. Not to scale.
51,118 km
Diameter
-195 °C
Temperature
97.8°
Axial tilt
19.2 AU
Distance from the Sun

Uranus is the oddball of the solar system: it spins tilted at 97.8°, nearly lying on its side, so each pole spends about 21 straight years in sunlight and another 21 in darkness. Its methane atmosphere gives it a serene blue-green hue that hides winds of up to 900 km/h.

Only one spacecraft has visited it, Voyager 2, in January 1986. And yes — it has rings, discovered by accident in 1977. Here is what has been measured, what has been corrected, and what we still don't know.

Measured data [DATO]

DataValueStateSource
Diameter51,118 km[DATO] observedNASA Planetary Fact Sheet (NSSDCA)
Mass86.8 × 10²⁴ kg[DATO] observedNASA Planetary Fact Sheet (NSSDCA)
Mean density1,270 kg/m³[DATO] observedNASA Planetary Fact Sheet (NSSDCA)
Surface gravity8.7 m/s² (at the 1-bar level)[DATO] observedNASA Planetary Fact Sheet (NSSDCA)
Escape velocity21.3 km/s[DATO] observedNASA Planetary Fact Sheet (NSSDCA)
Mean distance from the Sun2,867 × 10⁶ km (19.2 AU)[DATO] observedNASA Planetary Fact Sheet (NSSDCA)
Orbital period30,589 days (83.7 years)[DATO] observedNASA Planetary Fact Sheet (NSSDCA)
Rotation period17.2 hours (retrograde)[DATO] observedNASA Planetary Fact Sheet (NSSDCA)
Mean temperature-195 °C (at the 1-bar level)[DATO] observedNASA Planetary Fact Sheet (NSSDCA)
Atmosphere / compositionHydrogen (~83%), helium (~15%), and methane (~2%); methane absorbs red light, giving it its blue-green color[DATO] observedNASA NSSDC — Uranus Fact Sheet; NASA StarChild (GSFC)
Notable featureAxial tilt of 97.8°: it rolls on its side around the Sun; each pole spends about 21 straight years in sunlight and another 21 in darkness[DATO] observedNASA Planetary Fact Sheet (NSSDCA); NASA
RingsYes — 13 narrow, dark rings; discovered on March 10, 1977 by Elliot, Dunham, and Mink during the occultation of the star SAO 158687[DATO] observedNature (Elliot, Dunham & Mink, 1977); Britannica
Known moons29 (NASA / Maryame El Moutamid's team (SwRI), Webb Space Telescope (NIRCam), August 2025)
28 in February 2024 -> 29 with S/2025 U1 (~10 km, between Ophelia and Bianca), detected 2-Feb-2025 and announced 19-Aug-2025. Formal IAU confirmation and official name pending (April 2026).		[DATO] observedNASA / Maryame El Moutamid's team (SwRI), Webb Space Telescope (NIRCam)

Truths [DATO]

  • [DATO] It spins on its side, with an axial tilt of 97.8°: it rolls around the Sun instead of spinning upright, producing the most extreme seasons in the solar system.
  • [DATO] Its atmosphere — hydrogen, helium, and ~2% methane — absorbs red light, giving it its characteristic blue-green color.
  • [DATO] A day lasts 17.2 hours and is retrograde; a year equals 83.7 Earth years.
  • [DATO] The only spacecraft to visit it is Voyager 2 (January 1986); it has 28 known moons and a system of 13 narrow, dark rings.

Marked myths [MITO]

[MITO] Only Saturn has rings; Uranus has none.

Uranus has 13 narrow, dark rings, discovered on March 10, 1977 by James Elliot, Edward Dunham, and Jessica Mink, who detected five occultations of the star SAO 158687 before and after Uranus blocked it (published in Nature).

Fuente: Idea común en cultura popular; descubrimiento documentado en Britannica y en Nature (Elliot, Dunham & Mink, 1977)

Open questions [PREGUNTA]

  • [PREGUNTA] What tilted it 97.8°? The leading hypothesis is a collision with an Earth-sized object in its past, but there is no direct proof.
  • [PREGUNTA] What is its interior really made of? We only have models: a hot mantle of water, ammonia, and methane over a rocky core.
  • [PREGUNTA] How do its 13 dark rings stay so narrow, and where did they come from?

The chain in brief

  1. [DATA] 1. The planet on its side: It spins tilted at 97.8°: it rolls around the Sun, and each pole spends about 21 straight years in sunlight and another 21 in darkness — the most extreme seasons in the solar system.
  2. [DISPUTED] 2. Did a collision tip it over?: The leading hypothesis is a crash with an Earth-sized object in its past, but there is no direct proof.
  3. [DATA] 3. Blue-green from methane: Its atmosphere — hydrogen, helium, and ~2% methane — absorbs red light and gives it its color; beneath that calm it hides winds of up to 900 km/h.
  4. [DATA] 4. Rings discovered by accident: It has 13 narrow, dark rings, discovered in 1977 during the occultation of a star: Saturn is not the only planet with rings.
  5. [MODEL] 5. One visit, and an interior known only from models: The only spacecraft to visit it is Voyager 2 (January 1986); of its interior we only have models: a hot mantle of water, ammonia, and methane over a rocky core.