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There are many criteria that go into explaining what is a dwarf planet. The International Astronomical Union(IAU) defines a dwarf planet like this ‘… is a celestial body orbiting a star that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetismals and is not a satellite. More explicitly, it has to have sufficient mass to overcome its compressive strength and achieve hydrostatic equilibrium.’. The term was adopted, in 2006, as part of a three pronged way to categorize bodies orbiting the Sun. There seemed to be a need for this because of an increase in discoveries of trans-Neptunian objects(TNOs) that were similar in size to Pluto and the discovery of Eris which is larger than Pluto.
The new classification systems says that bodies that are large enough to have cleared their orbit of planetismals are defined as planets. On the other end of the spectrum, those bodies that are not massive enough to be rounded by their own gravity are defined as small solar system bodies. Dwarf planets are in between. There is some controversy about the system that is mainly based on a lack of observation of a few of the dwarf planets.
There are currently five dwarf planets: Pluto, Eris, Makemake, Haumea, and Ceres. Only Ceres and Pluto have been observed enough to indisputably fit into the category. The IAU decided that unnamed TNOs with an absolute magnitude brighter than +1 (and a mathematically delimited minimum diameter of 838 km) are to be named as dwarf planets. It is possible that there at least another 40 known objects in the solar system are dwarf planets. Once the Kuiper belt and the scattered disc are fully explored, there may be as many as 2,250 dwarf planets. That does not even take into account the Oort cloud.
Understanding what is a dwarf planet according to the IAU is easy enough, but making the solar system fit into a three tiered classification system will prove increasingly difficult as our understanding of the universe increases and we are able to see farther and farther into space.
We’ve also recorded an entire episode of Astronomy Cast all about Dwarf Planets. Listen here, Episode 194: Dwarf Planets.