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The International Astronomical Union officially recognizes five dwarf planets in our Solar System. They are Ceres, Haumea, Makemake, Eris and Pluto. Ceres is found in the outer Solar System and the other four are found in the asteroid belt. The largest is Pluto and the smallest is Ceres.
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The order of the dwarf planets from closest to the Sun outwards is Ceres, Pluto, Haumea, Makemake and then Eres, which is the furthest from the Sun.
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Ceres is the earliest known and the smallest of the dwarf planets. It was discovered in 1801 by Sicilian astronomer Giuseppe Piazzi. It is only 590 miles in diameter and has a mass of just 0.015 percent of the Earth. It is classified as both a dwarf planet and an asteroid. Ceres is the largest resident of the main asteroid belt between Jupiter and Mars. It is also the first dwarf planet to be visited by a spacecraft.
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There have only been two visits by space probes to dwarf planets. In 2015 NASA's Dawn and New Horizons missions reached Ceres and Pluto.
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Pluto, Haumea, Eris and Makemake are also known as plutoids, unlike the asteroidal dwarf planetoid Ceres. A plutoid is a dwarf planet with an orbit outside that of Neptune.
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Plutoids are sometimes also referred to as ice dwarfs due to their cold surface temperatures and diminutive size.
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A Pluto-sized world, astronomers discovered Eris in 2003. It takes icy Eris 557 Earth years to complete a single orbit around the sun. All the asteroids in the asteroid belt would fit inside Eris. However, like Pluto, Eris is still smaller than the Earth's moon.
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Scientists believe there may be dozens or possibly even more than one hundred dwarf planets awaiting discovery in the Solar System. Estimates are even up to 200 dwarf planets that could be found when the entire region known as the Kuiper belt is explored and that number may exceed 10,000 when objects scattered outside the Kuiper belt are considered.
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The most well-known dwarf planet Pluto has been somewhat controversial. Pluto was classified as the ninth planet of the Sun for 76 years. It was discovered on February 18, 1930 by Clyde W. Tombaugh.
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After 1992, Pluto's planethood was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, when Eris was discovered at 27% more massive than Pluto, it led the International Astronomical Union to define the term planet formally for the first time the following year. This definition excluded Pluto and it was reclassified as a dwarf planet.
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