Why is Pluto no longer a planet?

In 2006 the International Astronomical Union decided that a planet must have cleared its orbital neighbourhood of other bodies. Pluto shares its region of the Kuiper Belt with many similar objects, so it was classified as a dwarf planet instead.

Pluto is about 2,377 km across, roughly two-thirds the diameter of Earth's Moon and less than a fifth of Earth's width. Its mass is only about 0.2% of Earth's.

What is the heart on Pluto?

The heart is a bright region called Tombaugh Regio. Its left half, Sputnik Planitia, is a huge basin filled with nitrogen ice that slowly churns, keeping its surface free of craters.

Does Pluto have an atmosphere?

Yes, a very thin one made mostly of nitrogen, with some methane and carbon monoxide. It contains layers of blue haze extending well over 100 km above the surface.

How long did it take New Horizons to reach Pluto?

About nine and a half years. It launched in January 2006 and made its closest approach on 14 July 2015.

The icy dwarf planet with a heart of nitrogen

Pluto

Dwarf planet · ≈ 39 AU

Pluto is a dwarf planet in the Kuiper Belt, the ring of icy bodies beyond Neptune. Discovered in 1930, it was counted as the ninth planet for 76 years until astronomers reclassified it in 2006. It is smaller than Earth's Moon, about 2,377 km across, and its eccentric orbit takes it between roughly 4.4 and 7.4 billion km from the Sun.

For decades Pluto was little more than a blurry dot. Then, on 14 July 2015, NASA's New Horizons spacecraft flew past and revealed a surprisingly active world: glaciers of nitrogen ice, mountains of water ice, a layered blue haze and a surface that in places is geologically young. It remains one of the most intriguing worlds in the outer Solar System.

Key facts

Diameter: ≈ 2,377 kmAbout two-thirds the width of the Moon
Mass: 1.30 × 10²² kg≈ 0.2% of Earth's; about a sixth of the Moon's
Surface gravity: 0.62 m/s²≈ 6% of Earth's
Distance from the Sun: 4.4–7.4 billion km≈ 30–49 AU
Orbital period: ≈ 248 years
Day length: ≈ 6.4 Earth daysRotates backwards relative to most planets
Surface temperature: ≈ −230°CAround 40 K
Moons: 5Charon, Styx, Nix, Kerberos, Hydra
Discovered: 18 February 1930Clyde Tombaugh, Lowell Observatory

Discovery and reclassification

Pluto was found on 18 February 1930 by Clyde Tombaugh, a young astronomer at Lowell Observatory in Arizona, who compared photographic plates taken nights apart to spot a faint point that moved. The name was suggested by Venetia Burney, an 11-year-old girl in Oxford, after the Roman god of the underworld. Its first two letters also honour Percival Lowell, who had founded the observatory and searched for a planet beyond Neptune.

From the 1990s onwards, astronomers found more and more icy objects beyond Neptune, including Eris, which is slightly smaller than Pluto but more massive. In 2006 the International Astronomical Union defined a planet as a body that orbits the Sun, is rounded by its own gravity and has cleared its orbital neighbourhood. Pluto shares its zone with many Kuiper Belt objects, so it became a dwarf planet. The decision is still debated by some scientists.

A long, tilted orbit

Pluto takes about 248 Earth years to orbit the Sun, so it has not completed a single orbit since its discovery. Its path is both eccentric and tilted by about 17 degrees to the plane of the planets. For part of each orbit it comes closer to the Sun than Neptune does; this last happened between 1979 and 1999.

Pluto and Neptune can never collide. Pluto is locked in a 3:2 resonance with Neptune, completing two orbits for every three of Neptune's, which keeps the two worlds far apart. Many other Kuiper Belt objects share this resonance and are nicknamed plutinos. Pluto rotates once every 6.4 Earth days, on its side and backwards, with an axis tilted about 120 degrees.

Tombaugh Regio and Sputnik Planitia

Pluto's most famous feature is a bright, heart-shaped region named Tombaugh Regio. Its western lobe is Sputnik Planitia, a vast basin roughly 1,000 km across filled with frozen nitrogen, with some methane and carbon monoxide ice. Its surface is divided into polygon-shaped cells tens of kilometres wide, where slowly churning convection brings warmer ice up from below.

No impact craters have been found on Sputnik Planitia, suggesting its surface is younger than about 10 million years, extremely young by Solar System standards. Around its edges, nitrogen glaciers flow into the basin, and mountains of water ice, which behaves like rock at Pluto's temperatures, rise several kilometres high. Elsewhere, dark reddish regions are coloured by tholins, complex organic molecules made when sunlight breaks up methane.

A hidden ocean?

Several clues suggest Pluto may have a liquid water ocean beneath its icy shell. Sputnik Planitia's position, close to the line facing directly away from Charon, could be explained by a dense subsurface ocean beneath it. Possible ice volcanoes such as Wright Mons also hint at internal activity, though the ocean has not been confirmed.

A thin, hazy atmosphere

Pluto has a very thin atmosphere, mostly nitrogen with small amounts of methane and carbon monoxide. Surface pressure is around 100,000 times lower than at sea level on Earth. The atmosphere is fed by ice that turns to gas when sunlight warms the surface, and it may partly freeze back onto the ground as Pluto moves farther from the Sun.

Looking back at Pluto against the Sun, New Horizons saw a blue haze extending well over 100 km above the surface, arranged in about 20 distinct layers. The haze particles form when ultraviolet sunlight breaks apart methane and nitrogen, and the tiny particles scatter blue light, much as dust and molecules do in Earth's sky.

Charon and the smaller moons

Charon, discovered in 1978 by James Christy, is about 1,212 km across, just over half Pluto's diameter. The pair are so close in size that they orbit a point in the space between them, and both are tidally locked: each always shows the same face to the other. Some astronomers describe them as a double dwarf planet.

Charon's surface is mostly grey water ice, with a dark reddish cap at its north pole nicknamed Mordor Macula, thought to be made from gas escaping Pluto that is captured and chemically altered. Pluto has four much smaller moons: Nix and Hydra, found in 2005, Kerberos in 2011 and Styx in 2012. They are irregular in shape and tumble as they orbit.

The New Horizons flyby

New Horizons launched on 19 January 2006 and, after a gravity assist from Jupiter, flew past Pluto on 14 July 2015 at about 12,500 km from the surface. It was too fast and too light on fuel to stop, so it gathered as much data as possible in a few hours. Because its radio link is so weak at that distance, sending all the data back to Earth took more than a year.

The spacecraft carried a small portion of Clyde Tombaugh's ashes. After Pluto it continued deeper into the Kuiper Belt and flew past the small object Arrokoth on 1 January 2019. Its Pluto images and measurements are still being studied, and they have reshaped how scientists think about small, cold worlds far from the Sun. See the New Horizons spacecraft guide for more about the mission itself.

Surprising facts about Pluto

Pluto is smaller than seven moons in the Solar System, including Earth's Moon.
Since its discovery in 1930, Pluto has not yet completed a single orbit of the Sun; it will do so in the 2170s.
Sunlight takes between about four and seven hours to reach Pluto, depending on where it is in its orbit, compared with about eight minutes for Earth.
Water ice is less dense than nitrogen ice, so some hills of water ice at the edge of Sputnik Planitia are thought to float like icebergs.
Charon is so large relative to Pluto that the centre of their orbit lies outside Pluto, in the space between the two.

Frequently asked questions

Why is Pluto no longer a planet?: In 2006 the International Astronomical Union decided that a planet must have cleared its orbital neighbourhood of other bodies. Pluto shares its region of the Kuiper Belt with many similar objects, so it was classified as a dwarf planet instead.
How big is Pluto?: Pluto is about 2,377 km across, roughly two-thirds the diameter of Earth's Moon and less than a fifth of Earth's width. Its mass is only about 0.2% of Earth's.
What is the heart on Pluto?: The heart is a bright region called Tombaugh Regio. Its left half, Sputnik Planitia, is a huge basin filled with nitrogen ice that slowly churns, keeping its surface free of craters.
Does Pluto have an atmosphere?: Yes, a very thin one made mostly of nitrogen, with some methane and carbon monoxide. It contains layers of blue haze extending well over 100 km above the surface.
How long did it take New Horizons to reach Pluto?: About nine and a half years. It launched in January 2006 and made its closest approach on 14 July 2015.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources