Dwarf Planets
eris artist impression
Artists impression of the distant dwarf planet Eris and its moon Dysnomia
Dwarf Planet Facts
  • The dwarf planets of our solar system 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 billion km) from the sun. These objects are also classified as Plutoids.
  • The largest dwarf planet is Pluto, it has a diameter around half that of the planet Mercury
  • Eris is the second largest of the dwarf planets, it is around two thirds 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 make one orbit.
  • The smallest dwarf planet Ceres was previously classified as an asteroid, it is around a quarter of the size of our moon.
  • A cluster of bright spots on the surface of Ceres are believed to be light reflecting of deposits of ice.
  • 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.

What is a Dwarf Planet?

makemake
The dwarf planet Makemake orbits in the Kuiper Belt
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 no longer considered the ninth planet in the solar system. 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

surface of pluto
Artists impression of Pluto's frozen surface
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 (3.7 billion miles), 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.
charon moon
The largest moon of Pluto, Charon.
Pluto has more moons than previously thought, Charon is the largest of its five moons with a diameter around half that of the planet it orbits. Due to the moon's comparatively large size some astronomers consider Pluto and Charon to be twin planets. Pluto and Charon are in locked orbit around one another 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 sped past the planet at 50,000 km per hour in July 2015. During its flyby many of Pluto’s secrets were revealed along with a few surprises.
The biggest surprise is that images from the craft showed few craters on the surface of Pluto and it's largest moon Charon, meaning that both were recently geologically active and perhaps still are. In other cold parts of the solar system we see geological activity on moons which orbit large planets such as Jupiter or Saturn, this activity is thought to be a result of internal heat generated by the huge gravitational forces exerted on them by the large bodies they orbit. But with Pluto and Charon there is no massive planet nearby so what's creating the heat? The main theory at the moment is that perhaps both Pluto and Charon have internal water oceans which are slowly freezing which in turn releases energy toward the surface. This energy has caused the formation of huge mountains ranges on Pluto and deep canyons across Charon's surface.

Missions to Dwarf Planets
Dawn
ion engine Launch: September 2007
Arrival: March 2015
Agency: NASA
Summary: Dawn completed the first part of its mission which was to study the large asteroid Vesta. It successfully entered into orbit around the asteroid in July 2011 before leaving in September 2012. In March 2015 the craft successfully entered into orbit around Ceres and began analyzing and imaging the dwarf planet's surface in late April. Dawn has been the first craft to successfully enter into orbit around two separate bodies, this is thanks to the crafts ion propulsion engine (pictured left), which is more fuel efficient than a conventional chemical rocket allowing it to make more maneuvers and run for longer. The goal of the mission is to characterize the early solar system and determine how size and the presence of water influenced the evolution of planets.
New Horizons
New Horizons Launch: January 2006
Arrival: July 2015
Agency: NASA
Summary: 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 the Pluto system. It arrived at Pluto in July 2015 thus becoming the first spacecraft ever to visit the planet. The craft made a very close flyby of the dwarf planet at a distance of just under 8,000 miles (12,500 km). During the approach and flyby striking images and data were also gathered of Charon and the other moon's of Pluto. Due to the huge distances involved and the enormous amount of data gathered by New Horizons it will take almost a year and a half for it to transmit all the data it acquired during the flyby.

dwarf planet pluto
Pluto Statistics

Diameter: 1,473 miles (2,370 km)
Average Distance from Sun: 3.6 billion miles (5.9 billion km)
Orbital Period: 248 years
Rotation Period: 6.39 days
Moons: 5

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

Mass: 0.22% Earth's Mass
Density: 40% Earth's Density

Atmosphere: Nitrogen, methane, carbon dioxide.

Discovery Date: 1930

dwarf planet ceres
Ceres Statistics

Diameter: 590 miles (950 km)
Average Distance from Sun: 257 million miles (414 million km)
Orbital Period: 4.6 years
Rotation Period: 9 hours
Moons: 0

Maximum Surface Temperatures: -37C (-35F)
Minimum Surface Temperatures: -143C (-225F)

Mass: 0.015% Earth's Mass
Density: 38% Earth's Density

Atmosphere: Very small amounts of water vapor.

Discovery Date: 1801

dwarf planet eris
Eris Statistics

Diameter: 1,442 miles (2,320 km)
Average Distance from Sun: 6.3 billion miles (10.2 billion km)
Orbital Period: 557 years
Rotation Period: 26 hours
Moons: 1

Maximum Surface Temperatures: -217C (-359F)
Minimum Surface Temperatures: -240C (-405F)

Mass: 0.28% Earth's Mass
Density: 46% Earth's Density

Atmosphere: Tenuous atmosphere consisting of methane.

Discovery Date: 2005

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