Dwarf planets are now one of three categories of non-satellite, non-stellar objects orbiting the sun, the other two being planets and small solar system bodies. As defined by the International Astronomical Union (IAU) in 2006, a dwarf planet is a celestial body which:
1. Has sufficient mass for its own gravity to overcome rigid body forces and assume hydrostatic equilibrium (giving it an approximately spherical shape); and
2. Has not cleared its own orbital region of other planets, dwarf planets, or planetesimals.
This definition distinguishes dwarf planets from regular planets, which have cleared their own orbital region and are spherical; and from small solar system bodies, which have not cleared their own orbital region and are not spherical.
In a previous communication, the IAU also distinguishes dwarf planets and all other planetary-type objects orbiting the sun from possible orbiting stellar objects which are massive enough to sustain thermonuclear fusion. A distant stellar companion star has not yet been discovered, but has not conclusively been ruled out.
The 2006 IAU decision effectively removed Pluto from the list of solar system planets by reclassifying it as a dwarf planet, leaving the solar system with only eight currently known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Nothing in these definitions makes it impossible for a ninth or even tenth planet to be discovered in future: yet most current gravitational perturbations are accounted for, making it unlikely that we will ever again have a chance at a tenth planet which meets the required criteria, barring something unexpected that is captured from interstellar space.
Besides Pluto, four other dwarf planets are currently recognised by the IAU: Ceres, Haumea, Makemake, and Eris. Ceres lies in the asteroid belt, Eris is in the scattered disc, the others are Kuiper belt objects.
Only Pluto and Ceres have been confirmed to fit the required criteria of a dwarf planet. The others are too far away to observe in sufficient detail at the present time. They have been given a preliminary designation as dwarf planets on the basis of having an absolute magnitude greater than +1, which implies a minimum diameter of 838 kilometres. (Ceres has an equatorial diameter of 974 km, with all other known dwarf planets believed to be larger.)
The IAU definitions apply only to objects within our own solar system. For now, the classification of planetary objects in other solar systems has been left open by the IAU.