Pluto and Charon

Core Concepts

Pluto's place: the Kuiper Belt and the resonance

Pluto orbits the Sun at an average distance of about 39 AU (astronomical units — 39 times Earth's distance; a commonly taught reference value), on an orbit that is more eccentric and inclined than any planet's. Its orbit actually crosses inside Neptune's, but the two never collide because of a 2:3 orbital resonance: Pluto completes two orbits for every three of Neptune's, so their positions repeat in a stable pattern that keeps them apart. Pluto is the largest known object in the Kuiper Belt (diameter roughly 2,377 km — a commonly cited reference value), a region populated by countless icy bodies, several of which — Eris, Haumea, Makemake, and Ceres in the asteroid belt — are also classified as dwarf planets. Seeing Pluto as the "king of the Kuiper Belt" rather than a demoted planet is the modern, and more accurate, framing.

The 2006 classification: what is a planet, anyway?

In 2006 the IAU defined three conditions for a planet: (1) it orbits the Sun; (2) it has enough mass to be rounded by its own gravity (hydrostatic equilibrium); and (3) it has cleared its neighborhood of other objects. Pluto satisfies (1) and (2) but not (3): its orbit is crowded with other Kuiper Belt bodies. It was therefore placed in the new category of dwarf planet — a body that meets (1) and (2) but has not cleared its neighborhood. The decision was controversial and remains debated, but the underlying science is not: Pluto is the prototype of a large, distinct population of worlds, and the definition simply made that population visible. A useful way to think about it: "dwarf planet" does not mean "failed planet"; it names a different, equally real class of world.

Pluto's surface: the heart, the glaciers, and the mountains

Before 2015, the best images of Pluto were blurry pixels; New Horizons returned maps of a stunningly varied world. The most famous feature is Tombaugh Regio, the bright, heart-shaped region, whose left lobe — Sputnik Planitia — is a vast plain of nitrogen ice hundreds of kilometers across, flowing like a glacier. At Pluto's temperatures (about 40 K, −233 °C), nitrogen, methane, and carbon monoxide are ices soft enough to flow, and New Horizons imaged glacier-like surfaces with cell-like convection patterns. Surrounding the nitrogen plain are mountains of water ice — at Pluto's temperatures, water ice is as hard as rock, and these peaks may be "icebergs" of water ice floating in the denser nitrogen ice. The surface shows almost no impact craters in places, implying it is being continuously renewed — by glacial flow, by seasonal sublimation and re-deposition of ices, and possibly by cryovolcanoes (candidate features like Wright Mons suggest ice volcanoes that erupted water-ice lavas). The reddish-brown colors come from tholins — complex organic molecules made when sunlight processes methane in the atmosphere, the same material that colors Titan and Triton.

Pluto's atmosphere: thin, seasonal, and escaping

Pluto has a thin atmosphere of nitrogen with traces of methane and carbon monoxide, detected from Earth by stellar occultations (watching a star blink out behind Pluto's limb) and studied directly by New Horizons. Because Pluto's orbit is eccentric, its distance from the Sun varies a lot; as Pluto moves closer, more surface ice sublimates and the atmosphere thickens; farther out, gas condenses back onto the surface. The atmosphere is also escaping into space — nitrogen molecules are light enough, and Pluto's gravity weak enough, that the gas streams away, possibly carrying haze particles with it. This atmospheric escape is one reason astronomers care about Pluto's climate: it connects surface ices, atmosphere, and the loss of material to space.

Charon: the other half of a double planet

Charon was discovered in 1978 by James Christy as a "bump" on Pluto's image, and it is enormous relative to its partner: roughly 1,212 km in diameter versus Pluto's ~2,377 km (commonly cited reference values), about half of Pluto's size. Because the two are so comparable, the barycenter (center of mass) of the Pluto–Charon system lies outside Pluto's surface, so Pluto and Charon both orbit that external point — the definition of a binary system, and the reason Pluto and Charon are often called a double planet. The two are mutually tidally locked: each always shows the same face to the other, so a viewer on Pluto's near side would see Charon frozen in the sky forever. New Horizons found Charon to be a world in its own right: a giant equatorial canyon system suggesting ancient expansion, smooth plains hinting at cryovolcanic resurfacing, and a striking dark red polar cap — likely tholins produced from methane that escaped Pluto's atmosphere and became trapped at Charon's cold poles. Charon has no significant atmosphere.

The rest of the family

Pluto has four additional, much smaller moons discovered by the Hubble Space Telescope between 2005 and 2012: Nix, Hydra, Kerberos, and Styx. Their small sizes and chaotic rotations are consistent with formation from a giant impact early in Pluto's history — the same kind of impact thought to have formed Earth's Moon — with debris coalescing into Charon and the smaller moons. The Pluto system is thus a miniature collision laboratory, and its architecture supports the idea that the outer solar system was a violent, crowded place in its youth.

Common Confusions