Key difference: A dwarf planet is “a celestial body in direct orbit of the Sun that is massive enough for its shape to be controlled by gravitation, but that unlike a planet has not cleared its orbital region of other objects.” A plutoid is a trans-Neptunian dwarf planet.

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The International Astronomical Union (IAU) defines a dwarf planet as “a celestial body in direct orbit of the Sun that is massive enough for its shape to be controlled by gravitation, but that unlike a planet has not cleared its orbital region of other objects.” The term dwarf planet was adopted by the IAU in 2006 as part of a three-way categorization of bodies orbiting the Sun.

Also see: Difference between asteroids and planets

As per the IAU's final Resolution 5A, planets and other bodies, except satellites, in our Solar System can be defined into three distinct categories:

1. A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
2. A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
3. All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies."

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The need for this three-way categorization arose as more and more trans-Neptunian objects were discovered. These trans-Neptunian objects, i.e. objects located further than Neptune, were equal to or even bigger than the size of Pluto. Furthermore, it was discovered that Pluto was actually roughly one-twentieth the mass of Mercury, or one-fifth as massive as Earth's Moon. In addition, it was discovered that Pluto has some unusual characteristics such as large orbital eccentricity and a high orbital inclination. Hence, it was completely different than the other planets.

According to the previous categorization, all of the newly discovered trans-Neptunian objects would also be classified as planets, even though, like Pluto, they did not fit the traditional definition of a planet. So, the new three-way categorization model was adopted. Under this model, Pluto was demoted to a dwarf planet, while other solar objects that were too big to be classified as asteroids but did not meet the requirements for a planet were also classified under dwarf planets.

Also see: Difference between dwarf planet and moons

The IAU defines a plutoid as “celestial bodies in orbit around the Sun at a semi-major axis greater than that of Neptune that has sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit. Satellites of plutoids are not plutoids themselves.”

Hence, a plutoid is an astronomical body orbiting beyond Neptune that is large enough to be rounded in shape. Essentially, it is a trans-Neptunian dwarf planet. A plutoid is also called an ice dwarf, and effectively refers to any and all objects that meet the requirement to be called a dwarf planet but lie specifically beyond the orbit of Neptune.

Currently, the IAU recognizes five dwarf planets in our Solar System: Ceres, Pluto, Haumea, Makemake, and Eris. However, only Ceres and Pluto have been observed in enough detail to demonstrate that they fit the definition. Hence, the other may or may not be reclassified as new information is available. It is estimated that there may be 200 dwarf planets in the Kuiper belt of the outer Solar System, and up to 10,000 in the region beyond.

Of these five dwarf planets, four of them: Pluto, Haumea, Makemake, and Eris are considered to be plutoids, as they are located in the Kuiper belt, which lies beyond the orbit of Neptune. Ceres, however, is located in the main asteroid belt between the orbits of Mars and Jupiter. Hence, while Ceres is a dwarf planet it is not termed as a plutoid, as it is located in the inner solar system.

Also see: Difference between planets and moons

Image Courtesy: factfile.org, met.reading.ac.uk