Pluto — Dwarf Planet

Key facts

Mass

1.30 × 10²² kg

0.177 Moon masses

Radius

1,188 km

19% of Earth's radius

Surface gravity

0.616 m/s²

6% of Earth's gravity

Day length

6.4 days

Year

245 Earth years

89,582 Earth days

Distance from Sun

39.2 AU

Moons

5

Charon, Styx, Nix, Kerberos & Hydra

Science

Physical

Mass: 1.30 × 10²² kg · 0.177 Moon masses
Radius: 1,188 km · 19% of Earth's radius
Density: 1,854 kg/m³ · 1.9× water; Computed from mass and radius
Surface gravity: 0.616 m/s² · 6% of Earth's gravity; Computed from mass and radius
Escape velocity: 1.21 km/s · 11% of Earth's escape velocity; Computed from mass and radius
Axial tilt: 122.53 °

Orbital

Distance from the Sun: 35.6 AU; Computed from the position at the selected time
Semi-major axis: 39.2 AU; Computed from the cached state vector
Eccentricity: 0.249; Computed from the cached state vector
Inclination: 23.47 °; Computed from the cached state vector
Orbital period: 245.26 years; Computed from the cached state vector
Perihelion: 29.4 AU; Computed from the orbital elements
Aphelion: 48.9 AU; Computed from the orbital elements
Mean orbital velocity: 4.76 km/s; Computed from the orbital elements

Rotation

Rotation period: 6.4 days

Atmosphere

Composition: N₂ with CH₄ and CO
Surface pressure: 1 Pa

Temperature

Minimum: 33 K (-240 °C)
Mean: 44 K (-229 °C)
Maximum: 55 K (-218 °C)

Composition

Bulk composition: Rock and water-ice interior; nitrogen-ice surface

Radiation & temperature

Insolation: 0.000651 S⊕ · 1/1,535 of the sunlight Earth receives; Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
Bond albedo: 0.72 · reflects 72% of incoming sunlight
Equilibrium temperature: 32 K (-241 °C); Computed from albedo + distance from the Sun (airless black body)
Measured mean temperature: 44 K (-229 °C)
Greenhouse warming: +12 K; Measured temperature minus the airless equilibrium — the greenhouse effect

Discovery & history

Discovered: February 18, 1930
By: Clyde Tombaugh
Method: Photographic plates
Name: The Roman god of the underworld — suggested by 11-year-old Venetia Burney. Its initials, PL, also honour Percival Lowell.

Pluto was found by 24-year-old Clyde Tombaugh at the Lowell Observatory, who painstakingly compared photographic plates with a blink comparator until, on 18 February 1930, a faint dot shifted between exposures. Hailed as the ninth planet, Pluto turned out to be tiny — and the first of many icy worlds in the Kuiper belt. The 2005 discovery of the more massive Eris led the IAU to reclassify Pluto as a 'dwarf planet' in 2006. New Horizons revealed its true face in 2015.