How It Works

Pluto: What we know about the dwarf planet so far


The astronomer Percival Lowell predicted the existence of a ninth planet in our solar system, beyond the orbit of Neptune. Lowell failed to find Planet X in his lifetime, but Clyde Tombaugh – using the Lowell Observatory in Arizona – confirmed his calculations. Shortly after Planet X’s discovery back in January 1930 it was named Pluto. In 1978, however, it was determined that Lowell’s theory based on the mass of Pluto and its effects on Uranus and Neptune were incorrect. Tombaugh’s discovery was just a very lucky coincidence.

Pluto takes a leisurely 248 years to orbit the Sun. Its highly elliptical orbit takes it to a maximum of 7.4 billion kilometres from the sun (at aphelion, or farthest from the Sun) to as close as 4.5 billion kilometres (at perihelion, or closest to the Sun). Twice in this orbit it is actually closer to the Sun than Neptune, as was the case from January 1979 to February 1999.

All the other planets orbit on the plane of the ecliptic, but Pluto’s orbit is at an inclination of 17 degrees to this plane. Pluto is also unusual because it rotates at an angle of 122 degrees to its own axis, in a clockwise direction. This retrograde motion means it is spinning in an opposite direction to its counter- clockwise orbit around the Sun.

So far, even the Hubble Space Telescope has only obtained grainy pictures of its surface, and it is not until the arrival of the New Horizons spacecraft this month that we should know more about this small, distant and very cold body.

Pluto stats

  • Diameter: 2,320 kilometres
  • Mass: 1.3 x 1022 kilograms
  • Density: 2 grams per cubic centimetre
  • Average surface temperature: -230 ̊C or -382 ̊F (44K)
  • Core temperature: Unknown
  • Average distance from the Sun: 5,913,520,000 kilometres (39.5 AU)
  • Surface gravity: 0.067g
  • Moons: 3

Pluto’s layers

Pluto layers
The layers of Pluto


This is about 1,700 kilometres in diameter. It is mainly composed of iron-nickel alloy and rock. At its centre might be hot radioactive material or ice.

Mantle 1

If Pluto has a hot radioactive core, then there could be a 180-kilometre thick liquid water ocean between the core and the outer mantel

Mantle 2

Composed of rock and water ice.


The rocky surface is covered by frozen nitrogen, methane and carbon monoxide.

Surface details

Using observations by the Hubble Space Telescope, and maps produced since the Eighties, it has been found that the surface of Pluto undergoes many large variations in brightness and colour.

From 1994 to 2003, the southern hemisphere darkened, while the northern hemisphere got brighter. It has a slightly less red colour than Mars, with an orange cast similar to Jupiter’s moon Io. It got redder from 2000 to 2002, and other colour variations of dark orange, charcoal black and white have been observed. These seasonal variations are regarded as being due to the orbital eccentricity and axial tilt of Pluto that are reflecting topographic features and the flux of the frozen surface of the planet with its rarefied atmosphere.

The most detailed view of the entire surface of the dwarf planet Pluto, as constructed from multiple NASA Hubble Space Telescope photographs taken from 2002 to 2003.

Pluto’s atmosphere

When Pluto’s elongated orbit takes it relatively close to the Sun, the frozen nitrogen, methane and carbon monoxide on its surface sublimates into a tenuous gaseous form. This creates winds and clouds, but the weak gravitational force of Pluto means that it can escape into space and interact with its moon, Charon.

In the process of sublimation an anti- greenhouse effect is created, which lowers the temperature of Pluto to -230°C against the expected -220°C, which is the temperature of Charon. In the lower atmosphere, a concentration of methane creates a temperature inversion that makes the upper atmosphere warmer by three to 15 degrees every kilometre upwards. On average, the upper atmosphere is 50°C warmer than the surface of Pluto.

When Pluto’s orbit takes it away from the Sun, the gaseous atmosphere freezes and falls to the surface.

Pluto’s closest moon

Pluto’s closest moon is Charon, which was discovered in 1978. It is 19,640 kilometres from Pluto, so from Earth they look like one planet. Charon has the same 6.4 day rate of rotation as Pluto so they always present the same face to each other. On Pluto, the surface facing Charon has more methane ice than the opposite face, which has more carbon monoxide and nitrogen ice.

Charon has a diameter of 1,210 kilometres, and has a grey surface with a bluer hue than Pluto. This indicates the surface could be covered in water ice rather than nitrogen ice. It is also speculated that methane has leaked from the grasp of its weak gravity to Pluto.

New Horizons Pluto Charon
NASA’s New Horizon’s spacecraft passing Charon on its way to Pluto

Why is pluto no longer considered a planet?

Pluto’s status as a planet was safe until the Nineties. This was when huge ‘hot Jupiter’ extra-solar planets were discovered, and objects were observed beyond the orbit of Neptune that rivalled the size of Pluto. Faced with the dilemma of defining a planet the International Astronomical Union (IAU) decided that it must be spherical, that it orbits the Sun and is clear of any planetary neighbours. Consequently, the IAU reclassified Pluto as a dwarf planet on the 24 August 2006.

Plutoids, as defined by the IAU, are dwarf planets that orbit the Sun beyond Neptune, are round, have not cleared the neighbourhood of other similar bodies, and are not satellites of another planetary body. There could be at least 70 trans- Neptunian objects (TNOs) that might be plutoids.

So far only a few have been found and named. Besides Pluto, Makemake, Haumea and Eris have been classified as plutoids. Mike Brown and his Caltech team at the Palomar Observatory discovered them all in 2005. Eris is virtually the same size as Pluto and might have been regarded as a planet before the new classification system came into effect.

What will NASA’s New Horizon’s mission tell us about Pluto?

What our video to find out…

Top 5 Pluto Facts

1) Finding Pluto

Clyde Tombaugh systematically photographed the sky and checked 1.5 million stars recorded by his photographic plates before he found Pluto.

2) Naming Pluto

Venetia Burney, an 11-year-old schoolgirl in Oxford, put forward the name Pluto. She picked it after the Roman god of the underworld. Her reward was a £5 note.

3) Nix and Hydra

The Hubble Space Telescope discovered these moons of Pluto in 2005. Nix orbits Pluto at a distance of 48,000 kilometres and Hydra, 65,000 kilometres.

4) Kuiper Belt

Pluto is part of a cluster of Kuiper Belt Objects (KBOs) that orbit beyond Neptune. It consists of icy and rocky objects that failed to form into planets.

5) Triton

It was thought that Pluto was a satellite of Neptune. This is no longer regarded as possible, but Pluto does have many characteristics similar to Neptune’s moon, Triton.

Discover more amazing facts about space in the latest issue of How It Works magazine. It’s available from all good retailers, or you can order it online from the ImagineShop. If you have a tablet or smartphone, you can also download the digital version onto your iOS or Android device. To make sure you never miss an issue of How It Works magazine, make sure you subscribe today!

Plus take a look at:

How many dwarf planets are in the Solar System?

New Horizons, the fastest-ever spacecraft

What is the Kuiper Belt?