smallplanet

A small planet is a celestial body that orbits a star but does not possess sufficient mass to clear its orbital neighborhood of other debris. This definition, established by the International Astronomical Union (IAU) in 2006, distinguishes small planets from planets. To be classified as a planet, a celestial body must orbit the Sun, have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and have cleared the neighborhood around its orbit.

The term "small planet" often refers to objects that meet the first two criteria for planethood but fail the third. This category includes several dwarf planets within our solar system, such as Pluto, Ceres, Eris, Makemake, and Haumea. These bodies are massive enough to be spherical but share their orbits with other objects. For instance, Pluto resides in the Kuiper Belt, a region populated by many icy bodies.

The concept of a small planet is a consequence of reclassifying Pluto from a planet to a dwarf planet. This reclassification led to the need for a new category to describe celestial bodies with planetary characteristics that do not fit the full definition of a planet. While the term "dwarf planet" is the official IAU designation for most of these objects, "small planet" can be used informally to describe them, emphasizing their substantial size and round shape compared to smaller, irregular asteroids or comets. The study of small planets is crucial for understanding the formation and evolution of planetary systems, including our own solar system and potentially exoplanetary systems.