Planeta enano es el término creado por la Unión Astronómica Internacional (UAI) para definir a una nueva clase de cuerpos celestes, diferente de la de planeta y de la de cuerpo menor del sistema solar (o planeta menor). Fue introducido en la resolución de la UAI del 24 de agosto de 2006 sobre la definición de planeta para los cuerpos del sistema solar. Según la UAI, un planeta enano es aquel cuerpo celeste que: \n* Está en órbita alrededor del Sol. \n* Tiene suficiente masa para que su propia gravedad haya superado la fuerza de cuerpo rígido, de manera que adquiera un equilibrio hidrostático (forma casi esférica). \n* No es un satélite de un planeta u otro cuerpo no estelar. \n* No ha limpiado la vecindad de su órbita. Según estas características, la diferencia entre los planetas y los planetas enanos es que estos últimos no han limpiado la vecindad de su órbita. Esta característica sugiere un origen distinto para los dos tipos de cuerpos. De acuerdo con la definición de la UAI, aquellos objetos que respecto del Sol están más allá de la órbita de Neptuno reciben el nombre de objetos trasneptunianos. Si un objeto celeste cumple con la definición de planeta enano y pertenece también al grupo de los transneptunianos (si está en la intersección de esos conjuntos) se denomina plutoide. Las consecuencias más inmediatas de esta nueva definición fueron la pérdida de Plutón del estatus de planeta, su clasificación como planeta enano y el aumento de categoría de Ceres, antes considerado un asteroide, y de Eris, conocido como Xena de manera informal o 2003 UB313, su denominación provisional.

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A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. That is, it is in direct orbit of the Sun, and is massive enough for its gravity to crush it into a hydrostatic equilibrium shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit. The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of objects farther away from the Sun than Neptune that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by astronomer Mike Brown, who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern, who had coined the term dwarf planet in April 1991. The International Astronomical Union (IAU) currently recognizes five dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. Brown criticizes this official recognition: "A reasonable person might think that this means that there are five known objects in the solar system which fit the IAU definition of dwarf planet, but this reasonable person would be nowhere close to correct." It is suspected that another hundred or so known objects in the Solar System are dwarf planets. Estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered. Individual astronomers recognize several of these, and in August 2011 Mike Brown published a list of 390 candidate objects, ranging from "nearly certain" to "possible" dwarf planets. Brown currently identifies eleven known objects—the five accepted by the IAU plus 2007 OR10, Quaoar, Sedna, Orcus, (307261) 2002 MS4 and Salacia—as "virtually certain", with another dozen highly likely. Stern states that there are more than a dozen known dwarf planets. However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they actually fit the IAU's definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto. They subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1) are to be named under the assumption that they are dwarf planets. The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets. The question of whether other likely objects are dwarf planets has never been addressed by the IAU. The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed.

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