In 2006 the International Astronomical Union (IAU) removed Pluto from the list of planets and classified it as a dwarf planet because of its small size, icy composition, and anomalous orbital characteristics. The IAU adopted this category to recognize the larger and more massive members with similar compositions and origins occupying the same orbital “neighborhood.”

Pluto

Table of Contents

References & Edit History Related Topics

Images & Videos

For Students

Pluto summary

Pluto

Q: Who discovered Pluto?

Amateur American astronomer Clyde Tombaugh found Pluto in the constellation Gemini on February 18, 1930.

Q: How far is Pluto from Sun?

Pluto’s mean distance from the Sun is about 5.9 billion km (3.7 billion miles or 39.5 astronomical units).

Q: Is Pluto's orbit circular or eccentric?

Pluto's orbit is more elongated, or eccentric, than any of the planetary orbits and more inclined (at 17.1°) to the ecliptic. In traveling its eccentric path around the Sun, Pluto varies in distance from 29.7 astronomical units, at its closest point to the Sun (perihelion), to 49.5 AU, at its farthest point (aphelion).

Q: Does Pluto have any moons?

Pluto possesses five known moons. Charon is by far the largest, being fully half the size of Pluto. Pluto’s other four moons—Hydra, Nix, Kerberos, and Styx—are much smaller than Charon. All four are elongated and revolve around Pluto outside Charon’s path in nearly circular orbits and in the same orbital plane as Charon.

dwarf planet

Written by Tobias Chant Owen

Fact-checked by The Editors of Encyclopaedia Britannica

Last Updated: Apr 14, 2024 • Article History

Recent News

Apr. 11, 2024, 4:54 AM ET (EarthSky)

New Horizons finds evidence for 2nd Kuiper Belt

Apr. 6, 2024, 3:18 AM ET (NPR)

Pluto was discovered at an Arizona observatory. It might be named the state planet

Is Pluto a planet?

Who discovered Pluto?

How far is Pluto from Sun?

Is Pluto's orbit circular or eccentric?

Does Pluto have any moons?

Pluto, large, distant member of the solar system that formerly was regarded as the outermost and smallest planet. It also was considered the most recently discovered planet, having been found in 1930. In August 2006 the International Astronomical Union (IAU), the organization charged by the scientific community with classifying astronomical objects, voted to remove Pluto from the list of planets and give it the new classification of dwarf planet. The change reflects astronomers’ realization that Pluto is a large member of the Kuiper belt, a collection of debris of ice and rock left over from the formation of the solar ...(100 of 3946 words)

Access the full article

Help support true facts by becoming a member.