There are currently five official dwarf planets, and four being reviewed.
Pluto is by far the most famous. "Pluto" comes from the Latin god of the underworld. It was discovered on February 18th, 1930, by Clyde Tombaugh, an assistant at Lowell Observatory in Flagstaff, Arizona. This observatory is named for the person who predicted Pluto's existence in the early 1900's, Percival Lowell. From the orbits of Neptune and Uranus, he inferred that another celestial body, what he called "Planet X," was further out in space, slightly altering their obits. He died in 1916, but his work was carried on by the Lowell Observatory. Pluto was declared a dwarf planet in 2006,
when the IAU redefined what a planet is.
Pluto is located in the Kuiper Belt, an area of ice bodies past the orbit of Neptune. Pluto's elliptical orbit takes it from 30-49 Astronomical Units (AU) from the Sun (4.4-7.4 billion km). Its diameter is 2,302 kilometers. It has five moons, Charon, discovered in 1978; Nix and Hydra, discovered in 2005; and the provisionally named S/2011 134340 and S/2012 134340, discovered in 2011 and 2012 respectively. During its summer, Pluto's icy surface, which is comprised mainly of nitrogen ice with traces of carbon monoxide and methane, melts and forms a thin atmosphere, which freezes in the winter and falls to the surface as snow.
Ceres was the first of the current dwarf planets to be discovered. It is named after the Greek goddess of agriculture. It was discovered by the Sicilian Giuseppe Piazzi on January 1st, 1801. He found it by comparing data on a new star he thought he had found, but realized that it was not a fixed star, but a wandering star, a planet. He originally announced it as a comet, but by popular belief it became a planet. However, in the next few year other objects like it were discovered, it it was reclassified as an asteroid in 1850. Then, in 2006, it was declared a dwarf planet.
Ceres is located in the asteroid belt, the area between the orbits of Mars and Jupiter where rocky leftovers from the formation of the solar system are located. It is the biggest object in the asteroid belt; the next largest is half its size. In fact, Ceres has one-third of the mass of the entire asteroid belt. Its diameter is 950 kilometers, and it is approximately 2.8 AU from the Sun (419 million km).
Eris is the other dwarf planet classified as such is 2006. It was discovered in January 2005 by Mike Brown and his team at the Palomar Observatory in California. It is named for the Greek goddess of discord and fighting. Its moon, Dysnomia, is named for the daughter of Eris, the daemon of lawlessness.
Eris is possibly larger than Pluto, with an estimated diameter of 2,326 km. However, since there is an error bar because of the Pluto and Eris' extreme distance from us, it is not certain whether Pluto or Eris is larger. Eris is currently much farther away from the Sun than Pluto, at 96.6 AU (approximately 14.4 billion km). This is almost its maximum distance (aphelion) from the Sun, as its orbit is very eccentric.
Haumea is one of two dwarf planets added to the official list in 2008. It was first observed by Mike Brown and his team in 2004. It is named after a powerful Hawaiian sorceress who, according to legend, gave birth to most of the creatures on Earth.
Haumea is one of the most unique of the dwarf planets, as it is shaped like an egg. It is likely that this odd shape was caused by a collision with another celestial body, which would also account for Haumea's rapd rotation--it rotates fully once every 3.9 hours. It is probably comprised of a solid rock with a thin coating of ice. Haumea has two moons, Hi'iaka and Namaka, named after the mythological Haumea's daughters.
Makemake is the other of the two dwarf planets officially classified as such in 2008. It was discovered around Eastertime in 2005 by Mike Brown's team, and was therefore unofficially called "Easterbunny" until it was officially named after the Rapanui people Easter Island's god of fertility who supposedly created humans.
Makemake's temperature is astoundingly low; on Makemake, -406 degrees Fahrenheit (-243 degrees Celsi
us) is warm. While Earth takes 365 Earth days to orbit the Sun, Makemake takes 310 Earth years. As of 2009, Makemake is at a distance of 52 AU (4 billion miles) from the Sun.
There are also four other
Kuiper Belt Objects (KBOs) that are possible candidates for dwarf planets. These are Sedna, Orcus, 2007OR10, and Quaoar (KWA-o-war). They are not yet officially dwarf planets because the IAU
has not, or has not finished, reviewing them.
Sedna was discovered in
2004 by Mike Brown and his team, the same team that discovered Eris and
Makemake, and is named after the Inuit goddess of the sea. It
is located 90 Astronomical Units (AU) away from the Sun, and is currently moving
through the Kuiper Belt farther away, possibly to reach 900 AU away, or
83,700,000,000 miles. It takes even longer than
Makemake to orbit the Sun; its orbit length is 10,500 Earth years.
Orcus is another of the
KBOs that might be a dwarf planet. It is about the same size as Sedna, and is
named for Orcus, also known as Pluto, god of the underworld. It
was discovered in 2004, a month before Sedna was also discovered, and has the
same orbital period as Pluto.
Quaoar, named after a
Tongva Indian creation force, is about the size of Charon, one of Pluto’s
moons, and orbits 43 AU from the Sun. It was discovered in 2002,
and was one of the first of these possible dwarf planets to be discovered.
2007 OR10 has not been
given an official name yet. It was discovered in 2007, which is
what its name comes from. It is the third-farthest object from
the Sun, after Sedna and Eris.
Dwarf planets are a fascinating topic, and there will almost certainly be more discovered in years to come.
Bibliography
1. Aguilar, David A. 13 Planets: The Latest View of the Solar System. Washington, D.C.:
National Geographic, 2011.
2. Brown, Mike. How
I Killed Pluto and Why It Had It Coming. New York: Spiegel and Grau, 2010.
5. Daniels, Patricia. The New Solar System. Washington, D.C.: National Geographic, 2009.
6. Dwarfplanets.net. “Dwarf Planets.” Dwarfplanets.net. 2013.
Dwarfplanets.net. May 19, 2013. <http://dwarfplanets.net/>
7. Moche, Dinah L. Astronomy: A Self-Teaching Guide. Hoboken, NJ: John Wiley &
Sons, Inc., 2009.
13. Scott,
Elaine. When Is a Planet Not a Planet? New
York: Clarion Books, 2007.
