Pluto

Dwarf planet · Dwarf planet (Kuiper belt) · orbits Sun

Pluto is the best-known world of the Kuiper belt: a dwarf planet of nitrogen glaciers, water-ice mountains and blue haze. New Horizons’ 2015 flyby revealed Sputnik Planitia, the bright half of Pluto’s famous “heart” - a thousand-kilometre plain of frozen nitrogen that is still slowly churning today.

Open the 3D Solar System

Quick facts

Sputnik Planitia’s nitrogen ice slowly overturns in lava-lamp-like cells, keeping parts of the surface younger than 10 million years.
Its largest moon Charon is over half Pluto’s diameter - the pair orbit a point in the space between them.
One Pluto year lasts 248 Earth years; it has not completed a single orbit since its discovery in 1930.

Why Pluto is a dwarf planet

In 2006 the IAU defined a planet as a body that orbits the Sun, is round, and has cleared its orbital neighbourhood. Pluto passes the first two tests but honestly fails the third - it shares its zone with thousands of Kuiper belt objects and even crosses Neptune’s orbit. The reclassification was less a demotion than a recognition that Pluto is the brightest member of a vast new family.

Physical characteristics

Diameter	2,377 km
Mass	1.300e+22 kg
Surface gravity	0.62 m/s²
Density	1.85 g/cm³
Rotation period	-153.3 h (retrograde)
Axial tilt	122.5°
Mean temperature	-230 °C
Temperature range	-240 °C to -218 °C
Geometric albedo	0.52
Known moons	5

Atmosphere and composition

Atmosphere	A thin nitrogen atmosphere with methane and carbon monoxide, ~1/100,000 of Earth’s surface pressure - it may partially freeze out onto the surface as Pluto recedes from the Sun.
Composition	A rocky core beneath a water-ice mantle; surface ices of nitrogen, methane and carbon monoxide.

Interior structure

Rocky core modelled

Dense rock making up most of Pluto’s mass.

Possible liquid ocean modelled

Sputnik Planitia’s position hints at liquid water under the ice - still debated.

Water-ice mantle modelled

At Pluto’s temperatures, water ice is as rigid as rock.

Volatile ice surface modelled

Nitrogen and methane ices that sublime into the thin atmosphere.

How we know: New Horizons imaged the surface and measured the shape; the layering - and especially the ocean - is inference from that single 2015 flyby.

Discovery

Discovered by	Clyde Tombaugh
Year	1930
How	Blink comparison of photographic plates at Lowell Observatory

Missions

New Horizons NASA · Flyby probe · 2006 · active

New Horizons gave humanity its first close look at Pluto in July 2015, revealing nitrogen-ice glaciers and towering water-ice mountains. It then flew past the Kuiper belt object Arrokoth in 2019 - the most distant world ever explored - and continues outward on an extended Kuiper belt mission.