noun \ˈplü-(ˌ)tō\

: the object in our solar system that in the past was thought to be the planet farthest from the sun but that is no longer considered to be a planet

Full Definition of PLUTO

:  the Greek god of the underworld — compare dis
[New Latin] :  a dwarf planet occupying an orbit that crosses the orbit of Neptune

Usage Discussion of PLUTO

In 2006 the International Astronomical Union defined planet in such a way as to exclude Pluto, reclassifying it instead as a dwarf planet. Although discussion of the matter continues, the change has been widely accepted.

Origin of PLUTO

Latin Pluton-, Pluto, from Greek Ploutōn
First Known Use: 14th century

Rhymes with PLUTO


noun    (Concise Encyclopedia)

Solar system body, regarded as the ninth planet from the Sun until struck from the list of planets and reclassified as a dwarf planet in August 2006. It was discovered in 1930 by Clyde W. Tombaugh (1906–97) and named after the Greek god of the underworld. Its average distance from the Sun is about 3.7 billion mi (5.9 billion km)—it is located within the Kuiper belt—but its eccentric orbit brings it closer to the Sun than Neptune for 22 years during its 248-year orbit. Its axis is tipped 120°, so it rotates nearly on its side and “backward” (see retrograde motion) once every 6.39 days, locked synchronously with the orbit of its largest moon, Charon, discovered in 1978. Two additional, small moons were discovered in 2005. Pluto has a diameter of about 1,455 mi (2,340 km), less than 1% of Earth's mass, and only about 6% of Earth's surface gravity. Its estimated average surface temperature is near 390 °F (235 °C). Its thin atmosphere contains nitrogen, methane, and perhaps other heavier gases. Pluto is thought to be made of frozen gases with a significant fraction of rocky material. Its size, composition, and orbital location in the Kuiper belt sparked a long debate over its classification as a major planet, which culminated in a decision by the International Astronomical Union to drop it from the planetary ranks.


Next Word in the Dictionary: plutocracy
Previous Word in the Dictionary: pluteus
All Words Near: Pluto

Seen & Heard

What made you want to look up Pluto? Please tell us where you read or heard it (including the quote, if possible).