Pluto

For seventy-six years, Pluto was the ninth planet of the solar system. Then, on August 24, 2006, the International Astronomical Union voted to redefine the word "planet," and Pluto became something else — a dwarf planet, the largest known member of the Kuiper Belt, a world that had not cleared the neighborhood of its orbit. The reclassification generated more public outrage than almost any scientific decision in living memory. Schoolchildren wrote letters of protest. Scientists argued bitterly. The state of New Mexico passed a resolution declaring Pluto a planet within its borders.

None of it changed what Pluto actually is: one of the most geologically complex and scientifically fascinating worlds in the solar system, sitting at the cold edge of everything we know, waiting to be understood.

It was discovered on February 18, 1930, by a twenty-four-year-old Kansas farm boy named Clyde Tombaugh, working at the Lowell Observatory in Flagstaff, Arizona, methodically comparing photographic plates of the sky looking for the movement that would betray a new world. He found it. The observatory received 1,200 suggestions for what to name it; an eleven-year-old girl in Oxford, England — Venetia Burney — suggested Pluto, after the Roman god of the underworld, and the name was adopted. The region of Pluto's surface that looks like a heart, imaged for the first time in 2015, was named Tombaugh Regio in his honor. He died in 1997, eight years before New Horizons launched to visit his discovery. A small portion of his ashes traveled with the spacecraft.

A World We Never Saw Clearly

For eighty-five years after its discovery, Pluto was a point of light in even the best telescopes — too small, too far, too faint to resolve into a disk. The Hubble Space Telescope produced blurry pixelated images that hinted at surface variations but revealed nothing of the world's true character. When NASA's New Horizons spacecraft — launched January 19, 2006, just months before the demotion vote — flew past Pluto on July 14, 2015, after nine and a half years of flight, the images it returned were a genuine scientific shock.

Nobody expected what they found. The solar system's most famous exile turned out to be a geologically active, visually stunning, scientifically bewildering world — not the dead, cratered ball most planetary scientists had assumed.

What Pluto Actually Looks Like

The most striking feature is immediately obvious in any New Horizons image: a vast, heart-shaped bright region sprawling across Pluto's equatorial zone, now named Tombaugh Regio. The left lobe of the heart — Sputnik Planitia — is a nitrogen ice plain roughly 1,000 kilometers across, so smooth it appears to have no impact craters at all, suggesting it is geologically young — perhaps only a few million years old at most in a solar system 4.5 billion years old. Convection cells in the nitrogen ice slowly churn the surface, erasing craters almost as fast as they form. Sputnik Planitia is almost certainly still active today.

Bordering the ice plains are mountains of water ice rising up to 3,500 meters — taller than the Alps, built from water ice so cold it behaves like rock at Pluto's temperatures. Other regions show methane ice mountains and vast fields of methane dunes — wind-blown features on a world with almost no atmosphere, driven by the sublimation of surface ices in the faint sunlight. The surface color varies dramatically: the nitrogen plains are brilliant white, the highlands are mottled reddish-brown, and some regions are almost black. The reddish coloring comes from tholins — complex organic molecules formed when ultraviolet light and cosmic rays interact with nitrogen and methane in the atmosphere and on the surface. Tholins are found across the outer solar system and are considered potential precursors to the organic chemistry of life.

Charon and the Five Moons

Pluto's largest moon, Charon, is so large relative to Pluto — roughly half its diameter and about one-eighth its mass — that the two are sometimes described as a binary dwarf planet system. They orbit their common center of gravity, which lies in the space between them rather than inside Pluto itself. Both are tidally locked to each other: the same side of Pluto always faces Charon, and the same side of Charon always faces Pluto. They orbit each other every 6.4 Earth days — the same as Pluto's day length.

Charon's surface is strikingly different from Pluto's — gray rather than reddish, covered with water ice rather than nitrogen, and marked by a vast canyon system stretching 1,600 kilometers across its face, suggesting the moon was once geologically active. Its north pole is capped with a reddish stain of tholins — material theorized to have escaped from Pluto's atmosphere and been captured by Charon's gravity, then frozen onto the pole during Charon's long winter nights.

The four smaller moons — Nix, Hydra, Kerberos, and Styx — were discovered by Hubble between 2005 and 2012. They are small, irregularly shaped, and spin chaotically — tumbling unpredictably rather than rotating in the orderly fashion of most moons. The entire system is thought to have formed from a giant impact early in solar system history, similar to the impact that created Earth's Moon.

The Atmosphere That Freezes and Thaws

Pluto's atmosphere is one of the strangest in the solar system — not because of what it contains but because of what it does. When Pluto is closer to the Sun in its highly elliptical 248-year orbit, surface ices sublimate directly from solid to gas, creating a thin nitrogen atmosphere that extends up to 1,600 kilometers above the surface — taller relative to the planet's size than Earth's atmosphere is relative to Earth. When Pluto swings far from the Sun, that atmosphere freezes and falls as snow onto the surface. The entire atmosphere collapses and rebuilds on a 248-year cycle. Pluto last reached its closest point to the Sun in 1989 and has been slowly receding ever since — its atmosphere, paradoxically, was at its thickest in the early 2000s even as the planet moved farther away, because the surface takes time to cool.

A Possible Ocean

Beneath Sputnik Planitia, models suggest, there may be a liquid water ocean — kept from freezing by the decay of radioactive elements in Pluto's rocky core, insulated by the thick nitrogen ice above. The evidence is circumstantial: the smoothness and youth of the surface, the apparent lack of expected craters, and the way Sputnik Planitia appears to have reoriented Pluto's rotation by its mass, as a subsurface ocean would allow. If confirmed, Pluto would join Europa, Enceladus, Titan, and Ganymede in a growing list of outer solar system worlds that may harbor liquid water — and with it, the possibility of conditions for life — in the cold dark far from the Sun.

The Only Visit

New Horizons flew past Pluto on July 14, 2015, at 14 kilometers per second, passing within 12,500 kilometers of the surface at closest approach. The flyby lasted only a matter of hours — New Horizons was moving too fast to slow down and enter orbit. It took sixteen months to transmit all the data it collected during those hours back to Earth across 4.8 billion kilometers of space. No spacecraft has been back. No mission to return to Pluto has been formally approved, though a Pluto orbiter concept has been studied extensively by NASA and the planetary science community considers it a high priority for the next decade.

Clyde Tombaugh found Pluto on a photographic plate in 1930 and spent the rest of his life knowing nothing more about it than that it existed. New Horizons showed us a world in a matter of hours that eighty-five years of observation from Earth had kept completely hidden. What it revealed — the heart on the surface, the mountains of ice, the collapsing atmosphere, the possible ocean — is still being analyzed, a decade after the flyby, by scientists who were not yet born when Tombaugh sat down at his blink comparator in Flagstaff and found a moving point of light.