Equatorial Radius of Uranus

Nearly everything precise about Uranus's size dates from a single afternoon: Voyager 2's flyby on 24 January 1986 remains the only close visit, and the radius quoted here comes from its radio occultation, refined by watching the planet blot out stars. Uranus and Neptune form their own class — "ice giants", four Earth-diameters across, built mostly of water, methane and ammonia around rocky cores, with hydrogen envelopes far shallower than Jupiter's or Saturn's. Between them and the hydrogen giants sits a factor of three in radius that the solar system left unfilled.

That gap matters beyond the solar system: statistically, planets of Uranus's size are among the most common the exoplanet surveys find, which makes the one pair we can visit disproportionately important, and the case for a dedicated orbiter — ranked top priority in the 2023 US planetary decadal survey — rests partly on numbers like this one being forty years old.