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What Are Dwarf Planets?

Since Pluto's demotion from planethood in 2006, many of us are still wondering what are dwarf planets?

In the Kuiper belt in the outer solar system alone, it is estimated that there could be 200 dwarf planets, as well as anywhere up to 10,000 beyond that. And with so many celestial objects just waiting to be discovered, we can't help but wonder what are dwarf planets?

 The difference between a dwarf planet and a real planet is essentially nothing. Generally identical, dwarf planets and recognized planets differ according to criteria set up by the International Astronomical Union in that a planet orbits the sun, has achieved hydrostatic equilibrium (enough gravity to pull mass into round shape) and has cleared the neighborhood of other objects. Dwarf planets haven’t met the last criteria thus not becoming the gravitationally dominant object in its orbit.

Though dwarf planets have been around for an eons, we became aware of them in 2006 when Pluto, our former ninth planet, was relieved of its planet status and deemed a dwarf planet. Our solar system has five recognized and possible dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. Each have their own place in space and characteristics that make each unique.

The first dwarf planet was identified in 1801 and when Pluto was discovered some 130 years later, the solar system was finalized with nine planets, as well as the countless asteroids, comets, and more that inhabited our solar system. But as science is constantly evolving, so is the universe, always creating the need for more terms, exploration, and explanation.


Image via Space

Our oldest dwarf planet, Ceres is also the smallest. Discovered by Italian astronomer Giuseppe Piazzi in 1801, Ceres was identified along with other celestial objects between Mars and Jupiter. Originally named Cerere Ferdinandea for the goddess Ceres and King Ferdinand of Sicily, but after it wasn’t approved, the was shortened to Ceres and was originally classified as an asteroid. The orbit of Ceres is 4.6 Earth years and its distance from the sun is 2.8 times Earth’s distance, the closest dwarf planet to the sun. Its rotation period is close to one-third of Earth’s at nine hours and four minutes. The surface of Ceres was investigated by the NASA’s Dawn when the robotic spacecraft entered the dwarf planet’s orbit in March of 2015. Dawn discovered that Ceres had a cratered surface and that several distinct bright spots inside a crater that were related to a type of salt that are reflecting sunlight when photographed. 


Image via ABC News

Located in the Kuiper belt, a ring of bodies past Neptune, Pluto was discovered in 1930 by Clyde Tombaugh and was originally the ninth planet of our solar system. Though the farthest former planet from the sun, Pluto is actually the second closest dwarf planet to the sun. The Kuiper belt houses several other similar sized objects which lead to the doubt in Pluto’s planet status in 1992. Then in 2006, the International Astronomical Union was forced to define the term “planet” when Eris, an object larger than Pluto, was discovered the previous year. Pluto orbits the sun and is formed by its own gravity, two of the criteria needed to be considered a planet, but where Pluto fails is clearing neighborhood around its orbit and becoming the gravitationally dominant object. As a result, Pluto was reclassified as a dwarf planet, a new category for space objects that didn’t fit all the specifications to have planet status. Pluto has five moons: Nix, Kerberos, Hydra, Styx, and Charon, which is the largest. Its orbital period is 247.9 Earth years and its rotation period is six Earth days. Pluto is 39.5 times Earth’s distance away from the sun. The first and only attempt to explore Pluto to date is the New Horizons spacecraft which launched in 2006. It flew by Pluto in 2015 to accomplish goals to map the surface composition of Pluto and its largest moon, as well as analyze its neutral atmosphere and escape rate.


Image via Cool Space Facts

Located beyond Neptune’s orbit as the third dwarf planet closest to the sun, Haumea was discovered in 2004 and announced the next year. Haumea’s name is derived from the Hawaiian goddess of childbirth. Like earth, it is flattened at the poles and just meets the hydrostatic equilibrium criteria. Unlike earth, it has two moons, Hi’aka and Namaka. Haumea’s orbital period is 281.9 Earth years and its rotation period is very short at only 3.9 hours. Gemini and Keck telescopes detected spectra that showed crystalline features similar to that of Charon on Haumea. 


Image via NASA

Possibly the largest Kuiper belt object, Makemake is a dwarf planet whose 2003 discovery was announced in 2005, on the same day as Eris, both having been found by Michael E. Brown and his team. Its namesake comes from the creator of humanity and the god of fertility in the mythology of the Rapa Nui people. After Pluto, Makemake is the second visually brightest Kuiper belt object. Its distance from the sun is 45 times that of Earth’s distance and its orbital period is about 305 Earth years. Makemake has no moons and has a rotation period just shorter than Earth’s at 22.5 hours. Originally, Makemake was thought to have a similar atmosphere to Pluto, but it was then proven that it actually lacks a substantial atmosphere. 


Image via The Living Moon

Discovered in 2003, Eris is the largest dwarf planet by only 15 miles, with Pluto coming in second and is the farthest dwarf planet from the sun. Its discovery is responsible for the creation of a new category for astronomical objects, as well as stripping Pluto of its planethood. Eris, whose namesake is the Greek goddess, was discovered by a team led by Mike Brown at the Palomar Observatory. Totaling at 1,445 miles in diameter, Eris is often considered the “tenth planet.” It’s orbital period totals at 561.4 Earth years and its rotation period slightly longer than Earth’s at 25.9 hours. Eris has one moon, Dysomnia and observations of Dysomnia are what allowed scientists to figure out the mass of Eris.

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