Dwarf Planets, Pluto
Surface of Pluto
surface of pluto Artists impression of Pluto's frozen surface

Dwarf Planet Eris
eris artist impression Artists impression of the distant dwarf planet Eris
Dwarf Planet Facts
  • The dwarf planets are Ceres, Pluto, Eris, Makemake and Haumea.
  • Ceres is in the asteroid belt, a region of rocky bodies between the orbits of Mars and Jupiter.
  • Pluto, Eris, Makemake and Haumea orbit in the Kuiper Belt, a region of icy bodies 2.8 billion miles (4.5 million km) from the sun. These objects are also classified as Plutoids.
  • The largest dwarf planet is Eris which is three quarters the size of our moon.
  • At its furthest point Eris orbits the sun at a distance of 14 billion km and takes 557 years to orbit the sun.
  • The smallest is Ceres which is around a quarter of the size of our moon.
  • Ceres was previously classified as an asteroid.
  • The dwarf planet Pluto has a very thin atmosphere consisting of nitrogen, methane and carbon dioxide.
  • The largest moon of Pluto, Charon, is half the size of the former planet.
  • Eris, Makemake and Haumea were discovered between 2003-2005.
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When is a planet not a planet?

In 2001 the International Astronomical Union (IAU) decided to modify the definition of what constitutes a planet. These definitions were again changed in 2003 and 2006. Now the definition of a planet is that it orbits the sun, is large enough to form a round shape and has cleared similar objects that orbit in its neighborhood.

These new definitions meant that poor old Pluto was now no longer considered a planet. So what is Pluto now? Well it is now considered a "dwarf planet" because even though it is large enough to have formed a round shape it has not cleared similar objects that orbit in its neighborhood. To further complicate matters yet another definition was introduced in 2008, "plutoids", which are dwarf planets which spend most of the time outside of Neptune's orbit.
Dwarf Planet Pluto
Planet pluto
In 2006 the International Astronomical Union made the controversial decision to reclassify Pluto as a dwarf planet. The new definitions of a planet included that it must have its own orbit around the sun, as Pluto orbits in the Kuiper Belt with other similar objects it could no longer continue to be seen as a planet.

The far flung former planet which is smaller than our moon orbits the sun at an average distance of almost 6 billion kilometers, with temperatures in its coldest areas just a few degrees above absolute zero. Unusually its orbit is highly elliptical sometimes bringing it closer to the sun than Neptune. As a result Pluto has seasons, during the periods that it is closer to the sun the planet heats up slightly, vaporizing the nitrogen based ice and snow on its surface creating a hazy atmosphere. The effects of solar wind strip most of this atmosphere away, as a result of this process Pluto has lost around one kilometer of ice from its surface since its formation. Light breezes blow the remainder of the atmosphere to the colder side of the planet where it once again freezes and falls to the surface as snow.

Pluto has three moons, the largest of these is Charon which is around half the size of the planet. Due to its large size in comparison to the planet the moon doesn’t actually orbit around Pluto, they essentially orbit around each other, as a result some astronomers consider Pluto and Charon to be twin planets. Their orbits are also locked meaning that the moon is only ever visible from one side of the planet.

Pluto was discovered in 1930 by the American astronomer Clyde Tombaugh, his ashes are on board the NASA spacecraft New Horizons which is speeding towards the planet due for arrival in July 2015. During its flyby many of Pluto’s secrets will be revealed along with a few surprises.

Future Missions to Dwarf Planets
Dawn
ion engine Launch: September 2007
Arrival: July 2011
Agency: NASA
Dawn will be the first spacecraft to orbit one object then after a period of time leave and continue to another body, this is thanks to its ion propulsion engine (pictured left). Although far less powerful than a conventional chemical rocket, ion engines are a lot more fuel efficient, allowing them to run for years.

Dawn will study two objects in the asteroid belt, the large asteroid Vesta and the dwarf planet Ceres. It is due to reach Vesta in July 2011, it will orbit the asteroid for a year then continue on to Ceres, it will study the dwarf planet over a period of 5 months between February and July 2015.

The goal of the mission is to characterize the conditions of the early solar system and the role of size and water in determining the evolution of the planets.

New Horizons
New Horizons Launch: January 2006
Arrival: July 2015
Agency: NASA
NASA launched New Horizons in January 2006 thus taking advantage of a once in a life time chance to use Jupiter as a gravity assist towards Pluto. It should arrive at Pluto in July 2015 thus becoming the first spacecraft ever to visit the planet. New Horizons will make a flyby of the dwarf planet at a distance of 6,000 miles (9,600 km) and its moon Charon at a distance of 17,000 miles (27,000 km).

Images
Dwarf Planet Ceres (Hubble Telescope)
dwarf planet ceres
MakeMake (artist impression)
makemake
Haumea (artist impression)
haumea
Pluto Statistics
Comparison with Earth
Diameter: 1,485 miles (2,390 km)
Average Distance from Sun: 3.6 billion miles (5.9 billion km)
Orbital Period: 248 years
Rotation Period: 6.39 days
Moons: 3

Maximum Surface Temperatures: -218C (-360F)
Minimum Surface Temperatures: -240C (-400F)

Gravity: 0.81 m/s2 (8% Earth's Gravity)
Density: 2.00 g/cm3 (40% Earth's Density)
Mass: 1.3 x 1022 kg (0.22% Earth's Mass)
Volume: 6.39 x 109 km3 (0.59% Earth's Volume)

Atmosphere: Nitrogen, methane, carbon dioxide.
Diameter: 7,926 miles (12,756 km)
Average Distance from Sun: 93 million miles (150 million km)
Orbital Period: 365.24 days
Rotation Period: 23.934 hours
Moons: 1

Maximum Surface Temperatures: 58C (136F)
Minimum Surface Temperatures: -88C (-126F)

Gravity: 9.766 m/s2
Density: 5.515 g/cm3
Mass: 5.9737 x 1024 kg
Volume: 1.0832 x 1012 km3

Atmosphere: 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide.
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