Pluto, as seen by NASA's New Horizons spacecraft during its historic flyby of the dwarf planet in July 2015
NASA/JHUAPL/SwRI

Solar System Summer: Pluto

  • 22nd Jun 2023
  • Author: Elspeth Lewis

“I remember when Pluto was a planet” is a commonly heard phrase at the Space Centre. But just because it is no longer classed as a planet, doesn’t mean that Pluto isn’t worth learning about.

Discovery and naming of Pluto

In 1902, Percival Lowell suggested the existence of a ninth planet in the Solar System, and began searching for it in 1905. Unfortunately, Lowell would never see the discovery of his ninth planet, dying in 1915. In 1930, Clyde Tombaugh managed to differentiate the small white dot that was Pluto from the background star. Rather appropriately, he used the telescope at the Lowell Observatory to make his discovery.

Originally known simply as 'Planet X', the initial suggestion to name it Pluto came from 11-year-old Venetia Burney who suggested it over the breakfast table with her mother and grandfather. Her grandfather worked at the Bodleian Library at Oxford University. He thought that the name was so good that he suggested it to an astronomer at the university who cabled the idea to the Lowell Observatory.

In an interesting turn of history, 53 years earlier Venetia’s great uncle had named the moons of Mars.

When is a planet not a planet?

Pluto was considered the ninth planet in our Solar System until 2006 when it was reclassified as a dwarf planet. Pluto orbits in the Kuiper Belt, a region beyond Neptune which contains many other icy and rocky bodies. The discovery of Eris, another Kuiper Belt object, similar in size to Pluto, in 2005, led the International Astronomical Union (IAU) to set up a committee to define exactly what a planet was. The IAU decided that a planet was an object that:

  • orbits around a star
  • is nearly round in shape
  • has cleared the neighbourhood around its orbit.


It is this last point where Pluto fails due to the other Kuiper Belt objects orbiting with Pluto in its neck of the woods.

Pluto is in good company though, with four other official dwarf planets in our Solar System: Ceres, Eris, Haumea, and Eris.

Size comparison of the 5 dwarf planets and their moons
Jorisv (talk) CC by SA 3.0

Prominent features

  1. Model of the New Horizons Spacecraft
    NASA/APL

    Much of our understanding of Pluto comes from the New Horizons mission visiting this distant world in 2015. One of the most famous discoveries was a bright heart shaped region, named Tombaugh Regio, after Clyde Tombaugh. Tombaugh Regio is made of a large amount frozen nitrogen. During the daytime some of this nitrogen turns into gas, but then at night it turns back into a solid. This process drives westward winds of up to 20mph on the surface of the planet. The left half of this heart is a glacier called Sputnik Planitia made of frozen nitrogen, methane, and carbon monoxide.

  2. A Mountain Range within Pluto Heart
    NASA/Johns Hopkins University APL/SwRI

    Areas of this glacier are at least 4 kilometres thick. The surface of Sputnik Planitia has a series of indentations which are thought to have formed by ice fractioning and evaporating. Because there are very few impact craters on Sputnik Planitia, it is thought that these indentations have formed within the last 10 million years, which makes them relatively fresh considering the age of Pluto. There are craters on other areas of Pluto which are as large as 260 kilometres in diameter. Some of these craters show signs of erosion and refilling which suggests that Pluto may be tectonically active.

  3. Enhanced colour image showing highlighting the mountains in the Cthulhu region
    NASA/Johns Hopkins University APL/SwRI

    Next to Tombaugh Regio there is a region about the size of Alaska called Cthulhu. This region has a dark colour which is thought to be because the region is covered with tholins, which are molecules that are formed when methane is exposed to sunlight. Within this region there are bright sections, which scientist believe is a methane frost covering the peaks of a 420-kilometre-long mountain range. Unlike on Earth, mountains on Pluto are made from water ice.

Fact File

  1. Pluto in High Resolution
    NASA/Johns Hopkins University APL/SwRI

    Size: Pluto has a diameter of about 2380 kilometres which is five and a half times smaller than the Earth and about two thirds the width of Earth’s Moon.

    Gravity: The gravity on Pluto is about six percent of the gravity of the Earth.

    Age: Pluto formed early in the history of the Solar System, which makes it about 4.5 billion years old.

  2. Atmospheric Haze about Pluto
    NASA/JHUAPL/SwRI

    Distance: Pluto has a highly elliptical orbit which means that its distance from the Sun can range from between 4-7 billion kilometres. This equates to between 30 and 49.3 times further from our star than Earth. The average distance however is 5.9 billion kilometres or 39 times further away than Earth. Because of this orbit Pluto sometimes comes closer to the Sun than Neptune and the last time this happened was between 1979 and 1999.

  3. Internal Structure Of Pluto
    Jcpag2012, CC BY-SA 4.0

    Composition/ Structure: Pluto is believed to have a rocky core, with a mantle of water ice with other types of ice on its surface.

    Atmosphere: Pluto has a thin atmosphere that is mainly made from nitrogen. This atmosphere is thicker when Pluto is closest to the Sun as more nitrogen is able to turn into gas from the frozen surface.

    Temperature: Although the temperature of Pluto is between -226°C and -240°C, beneath parts of the icy surface of there may be liquid oceans.

  4. On approach in July 2015, the cameras on NASA's New Horizons spacecraft captured Pluto rotating over the course of a full "Pluto day"
    Credit: NASA/JHUAPL/SwRI

    Rotation: One day on Pluto lasts 153 hours which is 6.4 Earth days. This length of time is the same amount of time it takes one of Pluto’s moons Charon to orbit it. This means that Charon never rises or sets but would appear to stay above the same place on Pluto’s surface.

    Speed: Pluto takes 248 years to orbit the Sun. Therefore, Pluto has not yet completed a full orbit of the Sun since being discovered. Pluto travels around our star at a speed of 4.7km/s which is about six times slower than Earth.
  5. Family Portrait of Pluto Moons
    NASA/Johns Hopkins University APL/SwRI

    Moons: Pluto has 5 moons, all named after mythological creatures related to the underworld. Charon is the largest and closest to Pluto and is named after the creature who ferries souls into the underworld. Styx is named after the river separating the world of the living from the world of the dead. Nix is named after the goddess of darkness and night who was Charon’s mother. Kerberos and Hydra are named after monsters who guarded the underworld, with Kerberos being a three-headed dog and Hydra a nine-headed serpent. Charon is about half the size of Pluto and because of this Pluto and Charon are sometimes referred to as a binary planet system. The same side of Charon always faces Pluto.

  6. An illustration of the New Horizons spacecraft flying past Pluto and its moon Charon.
    NASA/Johns Hopkins University APL/SwRI

    Discovered by: Clyde Tombaugh in 1930.

    Missions: Pluto has only been visited by New Horizons in 2015. Some of the ashes of Clyde Tombaugh were onboard the space craft.

Full image credits / references

(Banner image) Pluto, as seen by NASA's New Horizons spacecraft during its historic flyby of the dwarf planet in July 2015. Credit: NASA/JHUAPL/SwRI

(1a) Clyde W. Tombaugh with his homemade 9-inch telescope. Credit: Public Domain

(1b) Clyde Tombaugh at the guide scope of the 13-inch astrograph he would use to discover Pluto. Credit: Lowell Observatory

(2) Size comparison of the 5 dwarf planets and their moons. Credit: Jorisv (talk) CC by SA 3.0 (https://commons.wikimedia.org/wiki/File:Dwarfplanets5.png)

(3a) Model of the New Horizons Spacecraft. Credit: NASA/APL

(3b) A Mountain Range within Pluto Heart. Credit: NASA/Johns Hopkins University APL/SwRI

(3c) Enhanced colour image showing highlighting the mountains in the Cthulhu region. Credit: NASA/Johns Hopkins University APL/SwRI

(4a) Pluto in High Resolution. Credit: NASA/Johns Hopkins University APL/SwRI

(4b) Atmospheric Haze about Pluto seen on the final approach of New Horizons. Credit: NASA/JHUAPL/SwRI

(4c) Internal Structure Of Pluto. Credit: Jcpag2012, CC BY-SA 4.0 (https://en.m.wikipedia.org/wiki/File:Internal_Structure_of_Pluto.jpg)

(4d) On approach in July 2015, the cameras on NASA's New Horizons spacecraft captured Pluto rotating over the course of a full "Pluto day". Credit: NASA/Johns Hopkins University APL/SwRI

(4e) Family Portrait of Pluto Moons. Credit: NASA/Johns Hopkins University APL/SwRI

(4f) An illustration of the New Horizons spacecraft flying past Pluto and its moon Charon. Credit: NASA/Johns Hopkins University APL/SwRI