Pluto (Dwarf Planet)

The largest known object in the kuiper-belt — discovered in 1930, called a planet for 76 years, demoted in 2006. The only solar system body named by an 11-year-old schoolgirl. The most thoroughly mythologised body in the solar system: every one of its five moons is named after an inhabitant of the underworld. Mythological counterpart: myth-hades. Crossing note: crossing-pluto-hades.

Key Properties

Discovery and Naming

Clyde Tombaugh discovered Pluto on February 18, 1930 at Lowell Observatory in Arizona, by comparing photographic plates taken weeks apart and spotting a moving point of light. He was 24 years old.

The name Pluto was proposed by Venetia Burney, an 11-year-old schoolgirl in Oxford, who suggested it to her grandfather over breakfast after reading about the discovery. Her grandfather passed it to the Oxford astronomy professor Herbert Hall Turner, who cabled it to Lowell Observatory. The name was adopted unanimously on March 24, 1930.

Two motivations converged: the Roman god of the underworld (dark, cold, far from the Sun) and the initials PL — honouring Percival Lowell, who had predicted a trans-Neptunian planet and founded the observatory, dying before it was found.

The Demotion

Pluto was classified as the ninth planet from 1930 until 2006. Its demotion followed from the discovery of eris-dwarf-planet in 2005 — an object at least as massive as Pluto, and one of many similar bodies being found in the outer solar system. The problem: if Pluto is a planet, so is Eris, and potentially dozens of other Kuiper Belt objects.

The IAU meeting in Prague in August 2006 defined planet for the first time, requiring three criteria:

1. Orbits the Sun
2. Has sufficient mass to be nearly round (hydrostatic equilibrium)
3. Has cleared the neighbourhood around its orbit

Pluto satisfies the first two but not the third — it shares its orbital neighbourhood with thousands of Kuiper Belt objects. It was reclassified as a dwarf planet alongside Eris, Ceres, Makemake, and Haumea.

The decision was controversial. Many planetary scientists objected to the definition (only ~4% of IAU members voted). The New Horizons mission, already en route, was launched to visit a "planet" and arrived at a dwarf planet.

New Horizons — July 14, 2015

The flyby at 14,000 km revealed a world of unexpected complexity:

Tombaugh Regio — the heart-shaped feature visible even from Earth approach, ~1,600 km across. Named after the discoverer. Its western lobe is Sputnik Planitia: a 1,000-km-wide basin of nitrogen ice, slowly convecting in polygonal cells driven by internal heat. The heart is geologically active.

Mountains — water ice mountains up to 3,500 m, comparable to the Rockies. Water ice is hard enough at −230°C to form rigid structures.

Atmosphere — a thin nitrogen atmosphere (surface pressure ~10 µbar, ~100,000× thinner than Earth's) with layers of photochemical haze. When Pluto reaches aphelion, the atmosphere partially collapses and freezes onto the surface.

Colour — reddish-brown from tholins: complex organic molecules formed when ultraviolet radiation hits nitrogen and methane. The same chemistry appears on Titan and in molecular clouds.

Possible subsurface ocean — the lack of expected surface cracking suggests a liquid water layer may exist beneath the ice, kept liquid by radiogenic heating.

The Moon System — An Underworld Pantheon

All five of Pluto's moons are named after figures from the Hadean mythology:

Charon is exceptional: at 1,212 km diameter, it is half the size of Pluto itself — the largest moon-to-planet ratio in the solar system. The two bodies orbit a common barycentre that lies *above Pluto's surface*, making them a true binary dwarf planet system. Charon's surface is grey water ice, contrasting with Pluto's reddish tholins — they are geologically distinct worlds.

Planet Nine

Pluto's orbital anomalies, and the clustered orbital alignments of distant Kuiper Belt objects, have led some astronomers to propose a Planet Nine: a body of ~5–10 Earth masses orbiting at ~400–800 AU. Pluto itself is not responsible — it is too small — but its discovery motivated the search of the outer solar system that produced the Planet Nine hypothesis. See sedna-dwarf-planet for another data point in that argument.