What Is A Dwarf Planet?
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A dwarf planet is a body that orbits the Sun and is round under its own gravity but hasn't cleared its orbital neighborhood of other similar-sized objects. The International Astronomical Union officially recognizes five dwarf planets: Pluto, Eris, Haumea, Makemake, and Ceres. More candidates await formal classification.
A dwarf planet is a category created in 2006 to handle objects like Pluto: round, orbiting the Sun, but not big enough to dominate their orbital neighborhoods. There are five officially recognized dwarf planets, with several more candidates being studied. The category resolved the awkward question of what to call objects that don't quite qualify as planets but are clearly more than asteroids.
What makes something a dwarf planet?
Three criteria from the International Astronomical Union. According to NASA, a dwarf planet must orbit the Sun, be round under its own gravity, and not have cleared its orbital neighborhood. The third criterion is what separates dwarf planets from full planets: the eight major planets dominate their orbital regions, while dwarf planets share their orbital neighborhoods with similar-sized objects. The category was created in 2006 specifically to handle objects like Pluto, which meets the first two criteria but fails the third.
How many dwarf planets are there?
Five officially recognized, with more candidates. The IAU has formally classified Pluto, Eris, Haumea, Makemake, and Ceres as dwarf planets. Pluto and Eris are similar in size and the largest known. Ceres is unique among them as the only dwarf planet in the asteroid belt; the others are in the Kuiper Belt. Several additional objects are dwarf planet candidates awaiting formal classification, including Sedna, Quaoar, Orcus, and Gonggong. The actual number of dwarf planets in our solar system could be over 100.
Why isn't Pluto a planet?
Because it doesn't clear its orbital neighborhood. Pluto shares the Kuiper Belt with thousands of similar icy objects, including Eris which has roughly the same mass. If Pluto counted as a planet, dozens or hundreds of similar objects would also have to count. The IAU's 2006 decision was specifically designed to handle this problem, creating the dwarf planet category instead of dramatically expanding the planet list. The decision remains controversial, with some scientists arguing the orbital-clearing rule isn't scientifically meaningful.
Are there dwarf planets we haven't found yet?
Very likely. The outer solar system, beyond Neptune, contains an enormous population of icy objects, many of which are probably large enough to be round and qualify as dwarf planets. The Kuiper Belt extends out to about 50 AU, but the further-out Oort Cloud may contain even more potential dwarf planets. Surveys of the outer solar system are ongoing. The total population of dwarf planets in our solar system may eventually be in the hundreds, though confirming round shape and orbital details takes detailed observation.
A dwarf planet is a body that orbits the Sun and is round under its own gravity but shares its orbital neighborhood with similar-sized objects. Five are officially recognized: Pluto, Eris, Haumea, Makemake, and Ceres. The category was created in 2006 specifically to handle objects like Pluto. Many more dwarf planets probably exist in the outer solar system, with the total possibly in the hundreds when fully surveyed.
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