Pluto, the Kuiper Belt’s most famous dwarf planet


  • Though once considered our solar system’s ninth planet, Pluto was reclassified as a dwarf planet in 2006.

  • Pluto is a Kuiper Belt object, meaning it lies within a thick disc of worlds beyond Neptune.

  • NASA’s New Horizons is the only spacecraft to have visited Pluto.

It’s hard not to have a soft spot for Pluto. Despite its cold, rough exterior, the tiny world still wears its heart on its surface.

Since 2006 much of the conversation around Pluto has centered around its reclassification as a dwarf planet. While the controversy around this will probably always continue in one form or another, truthfully, Pluto is so much more than how we classify it. The underworld-inspired Kuiper Belt object is fascinating in its own right.

Why do we study Pluto?

Pluto is the largest of the Kuiper Belt objects (KBOs) — a collection of ice-rock bodies found beyond the reaches of Neptune’s orbit. We don’t know exactly what created the Kuiper Belt, but the objects within it are thought to be leftovers from the formation of the solar system. Despite the many unsolved mysteries that lie within this remote region, only one mission — NASA’s New Horizons — has ever investigated an object within this vast space.

Exploring Pluto brings us closer to understanding the Kuiper Belt, an almost alien region within our own solar system. What can Pluto’s moon system teach us about other KBOs with similar satellites? How does Pluto’s unusual orbit distinguish it from other dwarf planets? Without more exploration, it’s difficult to discern these answers and so many others.

High-resolution enhanced-color global MVIC portrait of Pluto
Pluto up-close Originally captured by NASA’s New Horizons spacecraft, this color-enhanced image shows Pluto's surface features, most notably its "heart."Image: NASA / JHUAPL / SwRI

Pluto Facts

Surface temperature: -233 to -223 Celsius (-387 to -369 Fahrenheit)

Average distance from Sun: 39.5 AU; 5.9 billion kilometers (3.7 billion miles)

Diameter: 2,370 kilometers (1,473 miles)

Volume: 6.4 billion cubic kilometers (1.5 billion cubic miles)

Gravity: 0.62 m/s²; about 1/15 the gravity of Earth

Solar day: About 153 hours

Solar year: 248 Earth years

Atmosphere: Very thin; mainly made up of nitrogen, methane and carbon monoxide

What are Pluto’s moons?

Many Kuiper Belt objects have moons, and Pluto has five of its own — Charon, Styx, Nix, Kerberos, and Hydra. Pluto is tidally locked with its largest moon, Charon, which is about half the dwarf planet’s diameter. The two worlds are cosmic tango partners, always showing the same face to one another. Their dance is careful and synchronized as Charon orbits Pluto every 6.4 Earth days.

Geography of Pluto

There’s a lot we don’t know about Pluto, but one thing’s for sure: it’s very, very cold. Much too chilly to sustain life, but possibly suitable for a subterranean ocean at some point in the past.

Today, most of the dwarf planet is enveloped in nitrogen, methane and carbon monoxide frost. Its frigid mountains are a bit unusual — some of them appear to be just 100 million years old, which is very young in the context of our solar system, a staggering 4.6 billion years old.

In 2015, New Horizons spotted a heart-shaped feature on Pluto’s surface, poking out from a sea of reddish tar and ice. It’s thought that this nitrogen-rich region, called Tombaugh Regio, may create winds that shape surface features. In a way, Pluto’s “beating heart” helps keep the icy world going.